High blood pressure, or hypertension, is a condition when the blood pressure of a person is higher than normal. The higher the blood pressure levels of a person, the higher the risk for other health problems, such as heart disease, heart attack, and stroke and can be fatal if left untreated. Hypertension can often be controlled with some lifestyle changes or medication. Hypertension When the coronary arteries of the heart become narrower, the blood flow through them is restricted. This means that insufficient food and oxygen reach the muscles of the heart, causing angina (severe chest pain). A completely clogged blood vessel in the heart can cause a heart attack (myocardial infarction). Heart attack A stroke occurs when a blood vessel in the brain ruptures or when there is a blockage in the blood supply to the brain. Oxygen does not reach the brain cells and the brain cells die. The functions controlled by that part of the brain will be affected. Some of the symptoms of a stroke are slurred speech and numbness in the face, arm and leg, especially on one side of the body. Risk factors for stroke include old age, high blood pressure, diabetes, high cholesterol and smoking. The best way to lower the risk of stroke is to have blood pressure in the normal range. Stroke 2.3 Respiratory System Our respiratory system enables gas exchange between the blood in our body and the environment. Breathing is the external respiration process where air moves into and out of the lungs, the organ in which gas exchange takes place between the atmosphere and the body. Breathing involves inhalation and exhalation. When we inhale, we take in air or more specifically oxygen which is used by the body cells. When we exhale, we let the air out to get rid of the carbon dioxide in it. Chapter 2 Structures and Functions of the Human Body Systems 45 ©Praxis Publishing_Focus On Science
Structure of the Respiratory System When we breathe in, air enters the respiratory system through the nasal cavity in the nose. The inside of your nose is warm, wet and hairy. As it passes the nasal cavity, air is warmed and moistened whereas the dust gets trapped in the hair. After the air leaves the nasal cavity, it goes down the trachea, also known as the windpipe. The wall of the trachea contains C-shaped rings of cartilage which support and prevent the trachea from collapsing during inhalation. The trachea is also lined with cilia (tiny hairs) which remove dust, dirt and bacteria from the air. In the chest, the trachea branches into two bronchi (singular: bronchus) with each entering the right and left lungs respectively. The lungs are in a space called the thoracic cavity. The lungs are protected inside the ribcage that consists of the ribs and intercostal muscles. Thoracic cavity is separated from the abdominal cavity by a muscular sheet known as the diaphragm. Each bronchus splits into smaller tubes called bronchioles. The bronchioles end in tiny air sacs called alveoli (singular: alveolus). The alveolus is the site of gas exchange where oxygen comes into the blood and carbon dioxide is removed from the blood. When you breathe out, the air leaves the body through the nose and also through the mouth when you speak. Activity 6 Counting our breaths 1 Count how many breaths you take in a minute by using a stopwatch. Repeat this three times. Breathing in and out again counts as one breath. 2 Find the average number of breaths you take in one minute. 3 Repeat steps 1 to 2 but this time walk around the classroom for a few minutes before counting your breaths. 4 Make a comparison and explain the difference between the breathing rate of when you are at rest and after exercising. Resource Bronchiole Intercostal muscles Larynx Nasal cavity Nostril Pharynx Epiglottis Trachea Right lung Diaphragm Left lung Alveolus Bronchus 46 ©Praxis Publishing_Focus On Science
The amount of oxygen a person requires varies with his activity. When a person breathes more deeply, a bigger volume of air goes into the lungs. When a person takes shallow breaths more quickly, the lungs will be filled and emptied more often. Adaptation of the Alveoli for Efficient Gas Exchange There are millions of alveoli in the lungs that make it look like a sponge. The alveoli are adapted to make gas exchange in the lungs occur easily and efficiently. The adaptations are as shown in the chart below. Inhaled air Alveolus wall Alveolus Exhaled air Deoxygenated blood Capillary Red blood cell Oxygenated blood Oxygen Carbon dioxide A network of blood capillaries surrounds the alveolus to speed up the gas exchange to and from the alveolus. The inner surface of the alveolus which is moist enables oxygen to dissolve in the moisture before being transported through its wall. The alveolus wall is one-cell thick allowing the air and the blood to be close enough to one another for efficient gas exchange. There are millions of alveoli which gives a greater surface area for gas exchange. Mechanism of Breathing In and Out The breathing mechanism consists of inhalation and exhalation. The taking in of air is called inhalation. The letting out of air is called exhalation. There are two sets of muscles that aid in breathing: the external and internal intercostal muscles between the ribs and the diaphragm under the lungs. 1 External intercostal muscles contract and pull the rib cage upwards and outwards 2 Diaphragm muscles contract and flatten 3 Volume of thoracic cavity increases causing air pressure in the lungs to decrease 4 The higher air pressure outside forces air to enter the lungs Rib cage Air enters Thoracic cavity Diaphragm 1 4 3 2 Inhalation Chapter 2 Structures and Functions of the Human Body Systems 47 ©Praxis Publishing_Focus On Science
1 Internal intercostal muscles contract and rib cage moves downwards and inwards 2 Diaphragm muscles relaxes and arches upwards 3 Volume of thoracic cavity decreases causing air pressure in the lungs to increase 4 The higher air pressure inside the lungs pushes air out Diaphragm Rib cage Air exits Thoracic cavity 1 4 3 2 Exhalation Activity 7 Understand the breathing mechanism Aim: Investigate the relationship between the air pressure in the thoracic cavity and the process of inhalation and exhalation Materials and apparatus: Thin rubber sheet, balloons, glass jar, Y-shaped glass tube and cork Procedure: 1 Set up a model of the chest as shown in the diagram. 2 Pull down the rubber sheet and observe if there are any changes to the size of the balloons. 3 Push up the rubber sheet and observe if there are any changes to the size of the balloons. Observation: Record your observations. Discussion: 1 What do the glass jar, thin rubber sheet, Y-shaped glass tube, and balloons represent in the human respiratory system? 2 When the thin rubber sheet is pulled down, what breathing process is shown? 3 When the thin rubber sheet is pushed up, what breathing process is shown? Conclusion: Write down the conclusion. Balloon Glass jar Thin rubber sheet Y-shaped glass tube Cork Smoking is harmful to the respiratory system of smoker as well as people who are in the vicinity of the smoker. This is because they breathe in either the smoke emitted from the burning cigarette or the air exhaled by the smokers. These people are known as passive smokers. Science Facts 48 ©Praxis Publishing_Focus On Science
Diseases of the Respiratory System Cigarette smoke, air pollutants such as exhaust fumes from vehicles, gases from factories and haze contain harmful substances that cause various diseases of the respiratory system. Asthma is a chronic disorder in which the air passages of the lungs periodically become too narrow due to mucus being produced excessively, arising from the sensitivity of the patient towards air pollutants. The patient may suffer wheezing, chest tightness, coughing, and breathing difficulties. Asthma Bronchitis Bronchitis is the inflammation of the bronchi that may be caused by tar and irritants in cigarette smoke. A patient with chronic bronchitis will have persistent coughing, shortness of breath and chest tightness daily for months or years. Bronchitis Inflamed bronchus and excess mucus production Alveolar membranes break down Normal Bronchitis Normal Emphysema Emphysema is a condition due to the damage of alveoli in the lungs caused by harmful substances such as irritants in cigarette smoke. Thus, the surface area for gas exchange is reduced. A person with emphysema is constantly short of breath and feels tired even from just doing light work. Smoking is the leading cause of this disease. There is no cure for this disease, but the symptoms of this disease can be controlled. Emphysema Lung cancer is a disease in which there is uncontrolled growth of cells in the lungs mainly due to cigarette smoke. Cigarette smoke contains chemical substances such as tar that is carcinogenic. Some of the symptoms of lung cancer include persistent coughing, blood in the phlegm, and pain when breathing. Lung cancer It is important that we take good care of our respiratory system for it to function properly. Some of the good habits that we can practice to protect our respiratory system are as follows. • Stay away from smoking • Ensure there is a supply of fresh air in the place you are in • Keep floors and carpets dust free • Avoid open burning • Turn off the engine of the vehicle you are in while waiting Chapter 2 Structures and Functions of the Human Body Systems 49 ©Praxis Publishing_Focus On Science
2.4 Excretory System The metabolic reactions in our body produce some waste products that our body must remove. If waste products accumulate in the body, the toxicity may affect our health or damage the organs in our body, which can be fatal. Therefore, waste products must be disposed of. The disposal of metabolic waste products from the body is called excretion. This is carried out by the excretory organs: skin, lungs, kidneys and liver. Skin Skin, the largest organ in the body, is made up of three layers: epidermis, dermis and subcutaneous layer. Apart from acting as an excretory organ, it is also a heat insulator, a sensory organ and a barrier that protects the body from harmful microbes. Blood vessels Hair Sweat pore Sweat duct Sweat gland Adipose tissue Dermis Epidermis Subcutaneous layer Structure of human skin The dermis layer is where the sweat glands are found. There are many sweat glands throughout the body especially under the armpits, palms of the hands and soles of the feet. A sweat gland is made up of a coiled tube connected to a sweat duct that opens on the surface of the epidermis as a sweat pore. Excess water from blood capillaries is absorbed by sweat glands and released from the skin as sweat. The sweat is made up of 99% water and 1% waste products from the blood such as sodium chloride and urea. Lungs Carbon dioxide and water are produced during cell respiration as waste products. Carbon dioxide and water diffuses out of the blood capillaries into the alveoli. Carbon dioxide and water are expelled from the lungs via exhalation. 50 ©Praxis Publishing_Focus On Science
Kidneys We have two kidneys situated at either side of our backbone, in the abdomen, and just above the waist. Each kidney is connected to a renal artery, a renal vein and a ureter. Renal arteries transport blood that contains waste products to the kidneys whereas renal veins carry filtered blood from the kidneys. The main function of the kidneys is to filter waste products and excess water from the blood and excrete them from the body as urine. Urine is a yellowish liquid that contains water, mineral salts and urea. The urine from the kidney are channelled to the urinary bladder by the ureter. The urinary bladder stores urine. When the bladder is full, we will feel the urge to urinate. The muscles at the urinary bladder contract to expel the urine through the urethra to the exterior. Livers The liver detoxifies blood by getting rid of toxic substances from the food we consume. These toxic substances are broken down and converted into harmless materials by the liver and then transported to the kidneys to be eliminated through our urine. Excess amino acids that cannot be stored in our body will also be converted into urea to be excreted through our urine. Our excretory system plays a major role in the disposal of toxic or poisonous wastes from our body. How do we keep our excretory system healthy? Think About It Left lobe of liver Falciform ligament Aorta Hepatic vein Right lobe of liver Gall bladder We should take good care of our kidneys as they help to filter the blood. They remove toxic waste materials from our body and regulate the balance of fluid and mineral salts in our body. Humans can still lead a healthy life with only one healthy kidney. However, if both kidneys fail, the patient may die if he does not go for dialysis regularly. Left renal artery Aorta Left renal vein Left kidney Right renal vein Ureter Urethra Right renal artery Right kidney The urinary system Urinary bladder Vena cava Chapter 2 Structures and Functions of the Human Body Systems 51 ©Praxis Publishing_Focus On Science
Health Effects of Smoking, Alcohol Consumption and Drug Addiction Smoking, alcohol consumption and drug addiction may have short-term and long-term effects on the human body. Smoking Drug addiction Alcohol consumption Smoking has many negative effects on the body. This is due to the harmful substances in the cigarette smoke that the smokers breathe in. Some of the harmful substances and their effects are: • Nicotine causes addiction, damages brain tissues, hardens artery walls and causes blood to clot easily, thus increasing the risk of heart diseases. • Carbon monoxide gas prevents the red blood cells from transporting oxygen to the body cells. • Tar particles stick to and kill cells in the air passages and increase the production of phlegm in the lungs. • Carcinogens cause the growth of abnormal cells in the body which are cancerous. Other problems or conditions that can be caused by smoking: • Shortness of breath, wheezing, chest pain and tightness • Higher risk of stomach cancer and ulcers • Higher risk of stroke, heart attack and other heart diseases • Higher risk of respiratory illnesses such as asthma, emphysema and bronchitis According to World Health Organisation (WHO), alcohol consumption is responsible for various illnesses and disabilities of millions of people globally each year, and deaths as well. Short-term effects from drinking alcohol which is reversible once the alcohol has been processed and excreted from the body: • Slowing down of the reflexes and may affect balance • Difficulty in thinking and concentrating • Slurred speech • Blurred vision • Slower breathing and heart rate • May result in abdominal pain, indigestion and bloating Drunk driving increases the risk of accident that may cause serious injuries, or even death. Long-term effects on heavy drinkers (alcoholics): • Difficult for the body to digest food and absorb vital nutrients • Higher risk of cancers associated with the digestive system • Higher risk of cardiovascular diseases • Higher risk of liver diseases such as cirrhosis, fibrosis and liver cancer Drugs are chemical substances that are used to treat diseases but when taken without a doctor’s supervision, they can lead to addiction. There are several categories of drugs such as stimulants that increase alertness, depressants that reduce anxiety, and hallucinogens that cause hallucination. Over time, drug misuse affects the health of drug addicts. Stimulants such as methamphetamines can lead to heart disease or heart failure. Drugs injected through needles can cause bacterial infections in blood vessels or collapsed veins from repeated use. Certain drugs cause damage to the stomach, liver and kidney. 52 ©Praxis Publishing_Focus On Science
1 Carbohydrates are organic compounds that contain the elements carbon, hydrogen and . 2 Dietary fibre is an example of a carbohydrate. 3 such as glucose, fructose and galactose are examples of simple carbohydrates. 4 The molecules that make up protein are called . 5 Most animal fat are fats. 6 High levels of low-density lipoproteins contribute to which is the fatty build-up in the walls of arteries making the arteries narrower. 7 The deficiency of vitamin may cause scurvy, a disease where the bleeding of the gum occurs. 8 Iron is required in the formation of in red blood cells. 9 makes up about 60% to 70% of our body mass. 10 The amount of heat produced from the complete combustion of one gram of food is known as the of the food. 11 Boric acid is a food additive that is used in noodles as a . 12 Chemical digestion involves the breakdown of complex molecules into simple molecules with the aid of . 13 kills bacteria in the food that enters the stomach. 14 The pancreatic juice contains digestive enzymes such as trypsin, and pancreatic amylase. 15 In the duodenum emulsifies fats into small droplets. 16 The final breakdown of food occurs in the last section of the small intestine called the . 17 The main components of the circulatory system are the , blood vessels and blood. 18 are blood vessels that have valves and carry blood back to the heart. 19 arteries provide nutrients and oxygen to the cardiac muscles. 20 In the lungs, bronchioles end in tiny air sacs called . 21 During inhalation, the rib cage is pulled upwards and outwards due to the of external intercostal muscles. 22 is a condition due to the damage of alveoli in the lungs that can be caused by irritants in cigarette smoke. 23 The urine from the kidney is channelled to the by the ureter. RECALL Fill in the missing words. Chapter 2 Structures and Functions of the Human Body Systems 53 ©Praxis Publishing_Focus On Science
THINKING CAP Put on your 1 In your opinion, why does a 15-year-old boy need more energy than a 15-year-old girl of the same body size? 2 Why is it not advisable to cook vegetables too long? 3 Zaki felt dizzy after donating blood. Why is that so? How can he overcome it? 54 ©Praxis Publishing_Focus On Science
Project Risk Factors that Impact Heart Health Activity objective: To identify the risk factors that impact the heart health and raise awareness about cardiovascular diseases. Problem statement: One of the main causes of death in Indonesia is cardiovascular diseases. Cardiovascular diseases are all types of diseases that affect the heart or blood vessels. How to raise awareness among the population about cardiovascular diseases and ways to maintain a healthy heart? Concept applied: Healthy heart Procedure: 1 Divide the class into groups of four to carry out this task. 2 Appoint a leader in each of the groups to delegate the subtasks to the group members. 3 Have a discussion on how to present the information gathered. Solution: The information gathered must at least include the following aspects and you are encouraged to add more. (a) What are the main types of cardiovascular diseases? (b) What are the symptoms of each disease? (c) What are the risk factors for cardiovascular diseases? (d) How does smoking or consuming alcohol affect the heart health? (e) How to create awareness among the community on how to maintain a healthy heart so that cardiovascular diseases can be prevented? Presentation: Write a comprehensive report and present it to the class as creatively as possible. Chapter 2 Structures and Functions of the Human Body Systems 55 ©Praxis Publishing_Focus On Science
Work, Energy and Simple Machines CHAPTER 3 What will you learn? Define work and power Calculate work and power using equations and proper units Explain different forms of energy Describe the relationship between mechanical energy with potential energy and kinetic energy Differentiate between the types of simple machines used in daily life Describe how different types of simple machines work and their benefits in daily life Understand the concept of mechanical advantage of simple machines and how to calculate it Identify the simple machines in the human body A delivery man is holding a tray of fresh food while waiting for his customer. No work is being done by the man at that moment. Do you know why? What do you think must happen for work to be done? ©Praxis Publishing_Focus On Science
3.1 Work Who do you think is doing work – a boy who is playing video games or a boy who is playing football? Work is said to be done when a force is used to move an object through a distance in the direction of the force. The force must be exerted in the same direction as the moving object. Work is defined as the force exerted on an object multiplied by the distance the object moves in the direction of the force. Work done, W (in joules) = Force, F (in newton) × Distance, d (in metres) Example 1 A car is pushed with a force of 600 N. The car moves 100 m horizontally. What is the work done? Solution: Work done, W = force × distance = 600 × 100 = 60 000 J The SI unit of work is joule (J), named in honour of James Prescott Joule, a nineteenth century English physicist. One joule of work is done when 1 N of force is exerted on an object to move it 1 m in the direction of the force. There is no work done when there is no force or no displacement. Example 2 A man exerted a force of 8000 N on a stationary car but the car did not move. How much work was done? Solution: No work was done. Example 3 A boy who weighs 550 N climbs up a flight of stairs. How much work does the boy have to do to climb the stairs? Solution: The boy exerts an upward force to overcome his own weight or the pull of gravity. Work done, W = force x distance = 550 × 3 = 1650 J 3 m 7 m A boy playing video games on his smartphone A boy playing football Chapter 3 Work, Energy and Simple Machines 57 ©Praxis Publishing_Focus On Science
3.2 Power When three athletes run the same distance, they are doing the same amount of work but the amount of time taken by each of them to cover the distance is different. The athlete who took the shortest time has the highest power rating than the other athletes. What is power? Power is defined as the rate at which work is done, or how quickly work is done. It can be calculated by dividing work done by time. What happens to the power produced if more work is done in the same amount of time? Think About It The SI unit of power is watt, named in honour of James Watt, a nineteenth century British scientist who developed the steam engine. Watt can be defined as the power needed to do one joule of work in one second. Power, P (in watt) = Work, W (in joules) Time, t (in seconds) Example 5 Calculate the time taken for a crane that uses 750 W to lift a load of 2000 N to a height of 6 m. Solution: Power, P = Work done Time taken 750 = 2000 × 6 Time taken Time taken = 2000 × 6 750 = 16 s Example 4 A weightlifter lifts 500 N through a distance of 2 m in 4 s. How much power did he use? Solution: We need to calculate the work done before determining the power used. Work done, W = force x distance = 500 × 2 = 1000 J Power, P = Work done Time taken = 1000 4 = 250 watts Example 6 A boy who weighs 50 kg takes 40 s to go up a long flight of stairs with a vertical height of 8 m. Calculate the power of the boy. [Take gravitational field strength, g = 9.8 m s−2] Solution: The force exerted is equal to the weight of the boy (mg). Weight of boy, F = mg = (50.0 kg) (9.8 m s−2) = 490 N Work done, W = Fd = 490 × 8 = 3920 J Power, P = W t = 3920 40 = 98 watts 58 ©Praxis Publishing_Focus On Science
3.3 Sources and Forms of Energy Energy is defined as the ability to do work. Living things need energy to grow and perform life processes and activities. Which activity do you think requires more energy, walking or swimming? On the other hand, non-living things such as electrical appliances need energy to work. As science and technology advance, more energy is required to power equipment, automobiles and buildings. We get this energy from a number of sources which we can classify into non-renewable energy sources and renewable energy sources. In 2020, renewable energy in Indonesia made up 11.2% of the country’s energy mix, with hydro and geothermal power plants making up the majority of this contribution. By the year 2025, and 2050, respectively, Indonesia wants 23% and 31% of its energy to come from renewable sources. Let’s take a look at the pie chart below which shows the world’s total consumption by source in 2018. According to the pie chart, oil, coal and natural gas make up the bulk of usage. They are examples of fossil fuels. They are formed from the remains of extinct species from millions of years ago. Considering how long it takes for fossil fuels to form and how much of them are consumed globally, these resources cannot be quickly restored once used up. They are therefore referred to as non-renewable sources. Source: IEA (2020), Global share of total final consumption by source, 2018. https://www.iea.org/data-and-statistics/charts/global-share-of-total-final-consumption-by-source-2018. All rights reserved; as modified by Praxis Publishing Singapore Pte. Ltd. World’s total consumption by source, 2018 Electricity 19.3% Others 3.5% Coal 10% Oil Biofuels 40.8% and waste 10.2% Natural gas 16.2% Chapter 3 Work, Energy and Simple Machines 59 ©Praxis Publishing_Focus On Science
Coal comes in different forms, from hard black rocks to soft brown dirt. Some forms burn hotter and cleaner than others. Coal is used to produce electricity. Many coal beds are near the ground’s surface. We get coal by mining. Most coal is used by power plants where it is burned to make steam. The steam turns turbines to produce electricity. Coal is also used in steel manufacturing. It is refined to produce a substance called coke which burns at very high temperatures to smelt iron into steel. Coke is better as a fuel than coal because coke burns with no smoke whereas coal burns with smoke. Non-renewable Energy Sources Coal, petroleum and natural gas are the types of fossil fuels currently in use. They are made into fuels for different kinds of equipment. The majority of non-renewable energy sources are fossil fuels. Nuclear energy, is also considered non-renewable because it uses uranium extracted from the Earth, but the amount of uranium reserve is limited. The coal we use today was formed millions of years ago in swampy areas where plentiful huge plants grew. When the plants fell and decayed in the swamps, they were covered by mud, soil and other plants. Layers upon layers of sediments piled up. Over millions of years, intense heat and pressure of the Earth converted the plant matter into a mixture of carbon and hydrocarbon compounds, known as coal. Petroleum is formed over millions of years from the decay of algae and tiny ocean animals known as plankton. The tiny animals and plants were buried in the sediments on the ocean floor. These sediments and organic matter are buried deep in the Earth, under pressure and heat, eventually becoming oil-bearing shale and finally, crude oil. Gasoline and diesel are used as fuels in cars. Petroleum, also known as crude oil, is a yellowish-black liquid mixture of mostly hydrocarbons that occur naturally. Gasoline, diesel and kerosene, are the refined products of crude oil. Gasoline and diesel are major sources of energy for transportation. Mining activities to obtain coal Coal Petroleum 60 ©Praxis Publishing_Focus On Science
Similar to coal and petroleum, natural gas is a fossil fuel that was formed when dead plants and bacteria settled at the bottom of marshes and oceans and began to degrade over time. These organisms were turned into gas after millions of years of exposure to heat and pressure. A rock traps natural gas underground, where it remains until it is extracted. Natural gas, like the previously mentioned fossil fuels, coal and petroleum, is a source of energy, but it has an advantage over them in terms of the amount of usable energy it can generate. However, it is a non-renewable fossil fuel, just like the others. Natural gas is delivered to our homes to use for heating. It is used in stoves to boil water. Nuclear energy is produced from uranium, a non-renewable energy source. Uranium is extracted and mined to produce nuclear energy. Just like coal and other fossil fuels, viable uranium supplies are limited. Nuclear energy is not burned, as compared to fossil fuels. As a result, carbon dioxide and other harmful gases are not produced when using nuclear power. Compared to energy from fossil sources, nuclear energy is significantly cleaner. About 10% of the world’s electricity is generated from uranium inside the reactor of a nuclear power plant. Natural Gas Nuclear Energy Chapter 3 Work, Energy and Simple Machines 61 ©Praxis Publishing_Focus On Science
Renewable Energy Sources Renewable energy sources are energy sources that can be replaced after being used. Examples of renewable energy sources are wind, the Sun, moving water, biomass, waves and heat from inside the Earth. Wind Wind is moving air. Wind is used to spin the blades of wind turbines. A wind turbine converts the kinetic energy of wind into electrical energy. It is a clean source of energy, free from pollution. The Sun gives out heat and light energy. We use it to keep warm and to dry ourfood.Now, we have devices to collect solar energy for water heating. Some devices such as solar panels convert it directly into electricity. This energy can be used to heat water and power homes, buildings and even cars. The advantages of using solar energy are that it does not cause pollution and it incurs low maintenance costs. However, it is weather dependent and expensive in energy storage. In ancient times, wind was used to turn the blades of windmills. A windmill converts wind energy directly into mechanical energy to grind grains or pump water. Science Facts Wind turbines are constructed in areas with consistent and strong wind. Solar panels are used to convert solar energy into heat energy and electrical energy to power home appliances. Sun 62 ©Praxis Publishing_Focus On Science
Moving water in streams and waterfalls has energy. The energy from water can be used as a source of energy in different ways. Hydroelectric energy is the energy released when water rushes from a dam into a turbine. The fast-moving water turns the turbine to generate electricity. This energy source is reliable and efficient. It is also flexible as we can control the flow of water. However, there are limited places to build the hydroelectric power stations. They can only work in hilly areas. Biomass is the organic materials that come from plants and animals. Wood, unwanted agricultural waste such as dried plants and husks, animal manure, vegetable oils and food waste are examples of biomass. When biomass is burned directly, or converted to liquid biofuels or biogas that can be burned as fuels, biomass energy is released. Biomass is cheaper than fossil fuels and reduces landfills. However, it is not entirely clean and there is a risk of deforestation. Biofuels obtained from plants such as corn and beans are used as alternative sources of energy. Alcohol of the type known as ethanol is produced by fermenting corn, sugarcane or other plants. Ethanol can be used to power vehicles. It is a renewable energy source because we can always grow more plants. Moving Water Biomass Chapter 3 Work, Energy and Simple Machines 63 ©Praxis Publishing_Focus On Science
Steam Turbine Cooling tower Generator Injection well Hot water Geothermal energy is the heat energy beneath the surface of the Earth. This energy can be drawn from the hot water below the Earth’s surface or by pumping cold water onto the hot rocks and returning the heated water to the surface. This can drive steam turbines to produce electricity. It is free of pollution and the supply is constant. However, this type of energy can be obtained only in specific locations and these sites are usually prone to earthquakes. Geothermal energy is released to the surface of the Earth by geyser. The steam rises from the geothermal power station above a rocky lake coastline Cold water is pumped onto the hot rocks and the heated water is returned to the surface. Wind blowing over the ocean produces the vertical movement of the surface water known as waves. We can capture the wave energy by using special floating devices called Salter’s ducks which are connected in a chain on the water. By bobbing up and down on the sea, the generators inside the Salter’s ducks convert wave energy into electricity. Waves Heat from Inside the Earth Indonesia is one of the countries in the world that has the highest geothermal energy resources. Give a reason. Think About It 64 ©Praxis Publishing_Focus On Science
Different Forms of Energy There are different forms of energy: chemical energy, light energy, thermal energy, sound energy, electrical energy, potential energy, kinetic energy and mechanical energy. Chemical energy is the energy stored in food and chemical compounds such as batteries, natural gas and coal. It is released in other forms of energy during chemical reactions. Wood contains chemical energy because when we burn it, it gives out heat and light energy. Science Facts The food we eat is an example of stored chemical energy, the energy is released during digestion where the molecules in the food are broken down into small pieces. Fuel stores chemical energy. When it is burned, the chemical energy is released and converted into heat and light energy. The chemical energy stored in a car battery is released in the form of electrical energy when we turn on the ignition switch. Light energy is produced by hot objects such as bulbs and the Sun, and can be seen by human eyes. Without light, we cannot see in the dark. The sun gives out light, it travels at a speed of 300 000 km/s to reach us. We can see illuminating objects as these objects give out light, for example fireflies. Non-illuminating objects such as trees do not give out light. We can see them because they reflect the light that falls on them into our eyes. A firefly has an abdomen that illuminates when its body undergoes a chemical reaction that makes it glow. This type of light production is called bioluminescence. Chemical Energy Light Energy Chapter 3 Work, Energy and Simple Machines 65 ©Praxis Publishing_Focus On Science
Thermal energy is the total energy in an object due to the movement of particles within the object. The faster the particles in an object move, the greater the thermal energy and the higher the temperature of the object. The movement of water particles becomes faster when the water is heated. Have you ever experienced an electrical outage? The flow of electric charges is what generates electrical energy. Electrical energy is produced when electric charges flow through a conductor such as an electric wire. The faster the electric charges move, the more electrical energy they carry. Electrical energy is widely used in our home appliances. For example, batteries are used to power the remote control that we use to turn on/off the television. Batteries convert chemical energy into electrical energy. Electrical energy is used to power up the lights in our homes. Lightning is a natural phenomenon and it causes thunder. Would lightning produce electrical energy? What energy does a television produce when it is turned on? Sound is a form of energy produced by vibrations. Sound cannot travel through vacuum. However, for sound to travel from one place to another, a medium like a solid, liquid or gas is required. People can hear what we talk because sound energy move from our vocal chords to them in the form of vibrations. Thermal Energy Sound Energy Electrical Energy When a drum is struck, the skin vibrates and produces sound. Some musical instruments produce sound when we tap them, pluck their strings or blow the columns and so on. The girl enjoys listening to music with her headphones. Headphones are basically speakers that use an electromagnet to vibrate air to create sound. Are electricity and electrical energy the same? Electricity is the moving of electrical charges, either mechanically in the case of static electricity, or by flowing in a closed loop in the case of current electricity. Electrical energy is a form of energy resulting from the moving of the electric charges. It has the ability to do work, such as lighting up a bulb. Science Facts 66 ©Praxis Publishing_Focus On Science
Potential energy is the energy stored in an object due to its position or condition. Examples of potential energy are gravitational potential energy and elastic potential energy. Gravitational potential energy is the energy stored in the object due to its vertical position or height from the ground. For example, the fruit on a tree positioned well above the ground stores gravitational potential energy due to its position and the Earth’s gravitational force acting on the fruit. The higher the fruit is above the ground, the more gravitational potential energy it possesses. Gravitational potential energy can be calculated using the formula: Gravitational potential energy = m × g × h where, m = mass of the object in kg g = gravitational field strength (9.8 N/kg on Earth) h = the height of the object in m and the SI unit of energy is Joule (J). Fruit A possesses greater gravitational potential energy than fruit B because its position is higher above the ground. What are other examples or situations that show gravitational potential energy? Think About It A B Potential Energy Activity 1 1 Carry out the following activities with additional activities of your own in groups of four: (a) Push a hockey ball on the table to the floor. (b) Walk up the steps to the stage in the school hall. (c) Lift a basketball from the floor and hold it over your head. 2 Calculate the gravitational potential energy involved in each activity using the formula ‘Gravitational potential energy = mgh’. Calculating the gravitational potential energy The bird on an electric pole possesses gravitational potential energy. Chapter 3 Work, Energy and Simple Machines 67 ©Praxis Publishing_Focus On Science
Example 7 An object that has a mass of 40 kg is lifted to the height of 5.2 m using a pulley. What is the gravitational potential energy gained by the object? Solution: Gravitational potential energy = mgh = 40 × 9.8 × 5.2 = 2038.4 J The amount of gravitational potential energy increases when the: • mass of the object increases. • height of the object from the surface of the Earth increases. • gravitional strength increases. The gravitational potential energy for an object on the surface of the Earth is zero. Elastic potential energy is the energy stored in an object due to it being stretched or compressed. An elastic band needs a force to be stretched. The elastic band gains energy when a stretching force is applied. When the elastic band is stretched by a force to a particular length, work is said to be done, affected by the force applied and the length stretched. The work that is done is transferred as energy and stored in the elastic band as elastic potential energy. Elastic potential energy is calculated with this formula: Elastic potential energy = 1 2 Fx where, F = force applied in N x = displacement in m where displacement is the length stretched in the direction of the force applied. A force is used to stretch the elastic band to a particular length. The work done is transferred as elastic potential energy stored in the elastic band. The pole vaulter is doing work by applying his body weight to bend the pole. The elastic potential energy is stored in the pole. 68 ©Praxis Publishing_Focus On Science
Kinetic Energy 0.5 m 25 N Example 8 The archer pulls an arrow on an elastic bow string. The bow string is pulled back a distance of 0.5 m with a force that increases uniformly from zero to 25 N. Calculate the elastic potential energy that is stored in the bow string. Solution: Elastic potential energy = 1 2 Fx = 1 2 × 25 × 0.5 = 6.25 J Kinetic energy is the energy of a moving object. It depends on its mass and velocity. If both the mass of the object and its velocity increase, the kinetic energy increases too. The kinetic energy of an object can be calculated using the formula: Two moving cars may collide due to the kinetic energy each of them possesses. Kinetic energy = 1 2 mv2 where, m = mass of the moving object in kg v = the velocity of object in metre per second (m/s) and the SI unit of kinetic energy is Joule (J). Example 9 Calculate the kinetic energy of a ball with a mass of 200 g that moves at 12.2 m s−1. Solution: Kinetic energy of the ball = 1 2 × 0.2 × 12.22 = 14.88 J Which has more kinetic energy - an airplane during takeoff or a rocket during launch? Give a reason. Think About It Chapter 3 Work, Energy and Simple Machines 69 ©Praxis Publishing_Focus On Science
Mechanical energy is the sum of the kinetic energy and the potential energy in an object used to do work. It is energy in an object due to its motion or position, or both. Mechanical energy = potential energy + kinetic energy When a person lifts his hand to push the door open, he possesses potential chemical energy (energy stored in him) and kinetic energy (energy in the motion of his hand). Both are transferred into mechanical energy which causes work to be done (door opens). Kinetic energy Mechanical energy Potential energy When a hammer is lifted, the potential energy increases because of its high position. Then, when the hammer moves at great speed towards the nail, it possesses kinetic energy. The mechanical energy which is the combination of kinetic energy and potential energy in the hammer, will drive the nail into the wall (work is done). A barbell lifted above this man’s head possesses mechanical energy due to its vertical position above the ground (gravitational potential energy). The moving ball possesses mechanical energy due to both its speed (kinetic energy) and its position above the ground (gravitational potential energy). A compressed spring possesses mechanical energy due to its condition (elastic potential energy). Mechanical Energy 70 ©Praxis Publishing_Focus On Science
When an object falls, its height from the ground decreases and its velocity increases. Its potential energy changes into kinetic energy. Thus, the potential energy of a falling object decreases while its kinetic energy increases. However, the mechanical energy of the falling object is constant (provided there is no loss of energy due to friction) because it is the sum of the potential energy and the kinetic energy of the object. The decrease in the potential energy of the object during falling equals the increase in its kinetic energy. Although there is an increase in its kinetic energy as the object falls, the value of the kinetic energy does not exceed the value of mechanical energy because the mechanical energy of an object is the sum of the potential energy and the kinetic energy of the object. Activity 2 1 Work in groups. 2 Identify three examples of situations whereby mechanical energy is involved. State the sources of the mechanical energy in each situation. 3 Each group is to prepare a poster to present their findings. 4 Discuss all the sources with the members of other groups. Identifying the sources of mechanical energy KE = 0, PE = mgH So, ME = mgH C If velocity of object = v: PE = mgh, KE = – mv2 So, ME = mgh + – mv2 B A h H 1 2 1 2 If velocity of object = V: PE = 0 (because h = 0), KE = – mV2 So, ME = – mV2 1 2 1 2 The kinetic energy, KE at the maximum height before falling equals to zero (because velocity of object = 0), therefore the mechanical energy, ME at the maximum height equals to the potential energy, PE only. The potential energy, PE just before the object hits the ground equals to zero (because the height = 0), therefore the mechanical energy, ME equals to the kinetic energy, KE of the object only. Chapter 3 Work, Energy and Simple Machines 71 ©Praxis Publishing_Focus On Science
Energy Transformation Energy can be changed from one form into another in a system, but it cannot be destroyed or created. This is known as the law of conservation of energy. Energy changes take place all around us. When energy is used, it often converts from one form to another. Let us look at these situations. Do your palms get warm when rubbed against one another, and can you hear the rubbing sound? What are the energy changes involved? A little girl is on a swing in the playground. What is the energy change involved? Think About It A hair dryer produces sound and hot air when we blow our hair. We can say that electrical energy has changed into heat energy and sound energy. Electrical energy → heat energy + sound energy Resource Activity 3 1 Work in groups of five. 2 Set up an electric circuit as shown. 3 Observe what happens when the switch is turned on to complete the circuit. Touch the bulb. 4 Describe the energy transformation that occurs. Identifying the energy transformation 72 ©Praxis Publishing_Focus On Science
3.4 Simple Machines Do you know that the staircase that helps you move between floors in your house or a building, is a machine? How is a staircase a machine? A machine is a device that makes work easier to be carried out. When we use a machine, we apply a force over some distance. The force applied on the machine is called the input force. The work we do on the machine is called the input work. The machine also does work by applying a force to move an object over some distance. The force that the machine applies is called the output force. The work that the machine does is called the output work. In an ideal machine, the input work equals to the output work. However, in actual machines, the output work is lesser than the input work because some of the input work is used to overcome friction. The mechanical advantage of a machine is the measure of its performance. The actual mechanical advantage is the ratio of the output force to the input force of the machine. Mechanical advantage (MA) = Output force (N) Input force (N) Generally, it is difficult to measure the input and output forces because an unknown amount of input force is used to overcome friction. Therefore, the distance measurements are used to calculate the ideal mechanical advantage. Mechanical advantage (MA) = Input distance Output distance Let’s say the ideal mechanical advantage of a machine is 3 (in the absence of friction), this means that the machine multiplies the input force by a factor of 3. For example, if we apply 100 N as input force, the machine will multiply 100 N by 3 to generate an output force of 300 N. The trade-off is that the input force must be applied over a greater distance than the distance the object is moved. A simple machine is a device or tool that helps us to do work easier. There are six types of simple machines: levers, inclined planes, wedges, screws, wheels and axles and pulleys. Levers A lever is a bar that rests or rotates around a fixed point called the pivot or fulcrum. Effort is the force applied at a certain point on the bar. A lever is used to lift a weight, known as the load. Effort Bar Fulcrum Load When the fulcrum is placed closer to the load, the effort used is lesser, but the effort must move through a greater distance. The mechanical advantage of a lever is calculated by dividing the input arm (the distance from the fulcrum to the input force) by the output arm (the distance from the fulcrum to the output force). Output force Input force Input arm = 20 cm Output arm = 5 cm Mechanical advantage = 20 cm 5 cm = 4 The mechanical advantage is greater than one if the fulcrum is nearer to the output force compared to the input force. Chapter 3 Work, Energy and Simple Machines 73 ©Praxis Publishing_Focus On Science
Claw hammer The see-saw, a pair of scissors, and a claw hammer are examples of first-class levers. Can you identify the fulcrum, effort and load of each lever? Scissors • In a first-class lever, the fulcrum is positioned between the load and the effort. • Based on the position of the fulcrum, load or effort, a lever can magnify the force applied and make it easier to do work. • The input force is the force that a person applies to a lever to make it move while the output force is the force that the lever applies on the load. • The direction of the output force and that of the input force are always different. In other words, if the effort is “down”, the load moves “up”. • The mechanical advantage can be more or less than one and this depends on the location of the fulcrum relative to the load and effort. → When the fulcrum is near the load and further away from the effort, as in a claw hammer that is used to pull out nail, the lever provides a mechanical advantage that is more than 1. The lever with a mechanical advantage more than 1 usually makes tasks easier to be carried out with little effort. → When the fulcrum is near the effort and further away from the load, as in a pair of scissors with its blades longer than its handle, the mechanical advantage is less than 1. The lever provides no mechanical advantage but increases the speed with which the load can move. Fulcrum Load Effort Based on the positions of the effort, fulcrum and load which are interchangeable, levers are grouped into three categories: first-class lever, second-class lever and third-class lever. First-class Lever 74 See-saw ©Praxis Publishing_Focus On Science
Fulcrum Load Effort Nutcracker • In a second-class lever, the load is positioned between the fulcrum and the effort. • The input force (effort) is always further away from the fulcrum compared to the distance of the output force (load) from the fulcrum. The output force will always be greater than the input force, thus the mechanical advantage is always greater than one. The wheelbarrow, nutcracker and bottle opener are examples of second-class levers. Can you identify the fulcrum, effort and load of each lever? Bottle opener Second-class Lever Wheelbarrow Chapter 3 Work, Energy and Simple Machines 75 ©Praxis Publishing_Focus On Science
• In the third-class lever, the effort is positioned between the fulcrum and the load. • The input force is always closer to the fulcrum compared to the distance of the output force from the fulcrum, so the output arm always moves farther than the input arm. The output force will always be smaller than the input force, thus the mechanical advantage is always less than one. • Since third-class levers do not give a mechanical advantage, they are used mainly to increase the speed (or distance covered per unit time) of the load. Fulcrum Load Effort Activity 4 1 Work in groups. 2 Identify levers used in daily life and determine the classes of the levers. 3 Prepare a poster on the different classes of levers with the position of the load, effort and fulcrum labelled on every lever. Describe the benefits of using these levers. 4 Each group will present their findings. Identifying different classes of levers used in daily life The shovel, broom, tong and fishing rod are examples of third-class levers. Can you identify the fulcrum, effort and load of each lever? Tongs Fishing rod Shovel Third-class Lever Broom 76 ©Praxis Publishing_Focus On Science
Inclined Planes An inclined plane is a flat, sloping surface without any moving parts. It is used to move an object from a lower level to a higher level. The longer the inclined plane, the lesser the force needed to move the object. An example of an inclined plane is a ramp. The ramp reduces the force needed to move an object from a lower to a higher level. Comparing the force needed when lifting a box straight into the back of the lorry to the force needed when a ramp is used to move the box. Weight = 1200 N Force = 360 N Force = 1200 N h = 1.2 m 4 m P Q Without using a ramp A box weighing 1200 N is lifted to the back of the lorry that is 1.2 m off the ground. The force exerted which is 1200 N (the weight of the box), over a distance of 1.2 m, equals to 1440 J of work. Using a ramp with a length of 4 m The amount of work that needs to be done is the same, it does not change. However, the distance, over which force is exerted, becomes 4 m. Therefore, the force that needs to be exerted is 360 N only. Force = work distance = 1440 4 = 360 N Work = force × distance = 1200 × 1.2 = 1440 J When the load is being pushed up on an inclined plane, the mechanical advantage of the machine is the ratio of the length of the inclined plane (PQ) to its height, h. In this situation, the ramp has a mechanical advantage of about 3.3. Which ramp has a greater mechanical advantage if both have the same height? Think About It Mechanical advantage = length of inclined plane height of inclined plane = 4 1.2 = 3.3 Another way to calculate mechanical advantage of the ramp is as follows: Mechanical advantage = output force or load input force or effort = 1200 360 = 3.3 Chapter 3 Work, Energy and Simple Machines 77 ©Praxis Publishing_Focus On Science
Front teeth Wedges A wedge is an inclined plane that moves, and it has one or more sloping sides. A wedge can be used to split things apart. The mechanical advantage of a wedge increases as it becomes longer with a thinner tip. Input force Output force Output force Wedge The output force is in a different direction from the input force due to the shape of the wedge. Which wedge requires lesser input force to do the same amount of work? Think About It The teeth of carnivores (meat eaters) are more wedge shaped compared to herbivores (plant eaters). This is because carnivores use their teeth to cut and tear meat while herbivores use their teeth to grind plant tissues. Axe Knife P Q In humans, the front teeth are wedge shaped. When we bite a fruit, our teeth is pushed into the fruit. The teeth change the downward effort force into sideways force which pushes the skin of the fruit apart. The teeth of a herbivore The teeth of a carnivore Some of the examples of wedges 78 ©Praxis Publishing_Focus On Science
Screws A screw is an inclined plane wrapped around a cylinder. A screwdriver is used to drive the screw into an object or flat surface. When the screw is rotated many times, it moves downward into the object following the threads on the screw. The force applied (input force) is changed by the threads to a force that pulls the screw into the object (output force). Only a small effort is applied on the screw to produce a large force downwards. The mechanical advantage of a screw can be obtained by dividing the output force by the input force. Threads on the screw The force applied by the screw (output force) is always greater than the force applied to the screw (input force). Therefore, the mechanical advantage of a screw is always greater than one. The mechanical advantage of a screw can also be calculated by taking the ratio of the circumference of the screw head to the distance between the screw thread, that is the distance travelled by the thread after each revolution. Mechanical advantage (MA) = output force (N) input force (N) Why is it harder to turn a screw when the threads on the screw are more widely spaced? Think About It Bolt and nut Jar lid Drill bits Light bulb Guitar tuning keys Cork opener Chapter 3 Work, Energy and Simple Machines 79 ©Praxis Publishing_Focus On Science
Wheels and Axles A wheel and axle consists of a large circular object called the wheel which is connected to a smaller rod-shaped object called the axle. The axle allows the wheel to rotate around it and they rotate together. The wheel and axle rotate together because the force applied to the wheel is transferred to the axle, and vice versa. Wheels and axles can be found in a pizza cutter, a rolling pin, a water tap and a bicycle wheel. Pizza cutter Rolling pin Water tap Bicycle wheel Axle Wheel 80 ©Praxis Publishing_Focus On Science
• The input force is applied to turn the wheel and the output force is exerted by the axle. • Let’s consider the wheel and axle on a doorknob. When force is applied to the knob (wheel), the central shaft (axle) rotates in a smaller distance and retracts the latch with greater output force causing the door to be opened. • Themechanical advantageis calculated by dividing the radius of the wheel that is the applied force (input force) by the radius of the axle that applies the force (output force). The mechanical advantage is greater than one. • A doorknob, a steering wheel and a screwdriver are examples of wheel and axle where force is applied to the wheel. There are two ways the wheel and axle work together: force is applied to the wheel or force is applied to the axle. Force is applied to the wheel • The input force is applied to turn the axle and the output force is exerted by the wheel. • The mechanical advantage is calculated by dividing the radius of the axle that is the applied force (input force) by the radius of the wheel that applies the force (output force). The mechanical advantage is less than one, that means there is no increase in output force applied by the wheel when the input force is applied to the axle. However, the rotation of the axle over a short distance can cause the wheel to rotate at a greater speed over a longer distance. • Let’s consider the wheel and axle on a Ferris wheel. When force is applied to the axle, the wheel moves a greater distance and turns faster with less output force. • An electric fan and ferris wheel are examples of wheel and axle where force is applied to the axle. Force is applied to the axle Screwdriver Ferris wheel Doorknob Steering wheel Electric fan Chapter 3 Work, Energy and Simple Machines 81 ©Praxis Publishing_Focus On Science
Pulleys A pulley is a wheel on an axle. It is designed to rotate with a rope moving along a groove at its circumference. The groove helps to keep the rope in place. The object to be lifted is tied to one end of the rope and force is exerted to the other end by pulling the rope downwards. The downward force turns the wheel with the rope and lifts the object at the other end. There are three types of pulleys: fixed pulley, movable pulley and combined pulley. The mechanical advantage of a pulley is equal to the number of rope segments pulling up on an object. Effort (input force) Load • It consists of a single pulley. • The rope moves along the groove of the pulley, but the wheel is fixed to a spot. • This type of pulley will not reduce the effort (input force) needed to lift the load; it only reverses the direction of the effort. • As a downward force is applied to one side of the pulley, the other side of the pulley is pulled upwards. • In this single fixed pulley, only one rope segment pulls up on the load, so the mechanical advantage is 1. Moreover, the effort (input force) needed to lift the load is equal to the weight of the load. To lift the load, the length of rope pulled downwards must be equal to the height the load is to be lifted. • Pulleys on a sail, a window blind and a flagpole are fixed pulleys. Movable pulley on a crane Effort (input force) Load • This pulley is attached to the object that is to be lifted. • Both the pulley and load attached to it can move from one place to another. • One end of the rope is attached to a fixed point that does not move. • In this movable pulley, two rope segments pull up on the load, so the mechanical advantage is 2. The pulley multiplies the input force by a factor of 2, in other words the load is twice the input force. For example, if the load weighs 30 kg, the input force required to pull up the load would be 15 kg, but twice as much rope must be pulled to lift the load. • Pulleys on cranes, elevators and weightlifting machines are examples of movable pulleys. Movable Pulley Fixed Pulley Fixed pulley on a window blind 82 ©Praxis Publishing_Focus On Science
Simple Machines in the Human Body Simple machines such as levers, wedges and inclined planes can be found in a human body. The simple machines help humans accomplish various tasks throughout the day. Block and tackle pulley system on sailing ships • This type of pulley is a combination of fixed and movable pulleys that work together. • The mechanical advantage depends on the number of rope segments pulling up on the load, usually 2 or greater than 2. • In this type of pulley, the amount of force required to lift an object is greatly reduced. • An example of combined pulley is the block and tackle pulley system. A fixed pulley block and a movable pulley block are used in the block and tackle pulley system to reduce the amount of force required to lift a heavy object by however many pulleys incorporated into the system. Effort (input force) Load Load Fulcrum Effort F E L Nodding is an example of a first-class lever. The joint where the skull meets the top of the spine is the fulcrum, the muscles at the back of the neck contract to provide the effort force to tilt the head up, and the weight of the head is the load. Combined Pulley / Compound Pulley First-class Lever Chapter 3 Work, Energy and Simple Machines 83 ©Praxis Publishing_Focus On Science
Load Fulcrum Effort F L E Standing on tiptoes is an example of a second-class lever. The ball of the foot is the fulcrum, the calf muscles contract to provide the effort to lift the body, and the weight of the body is the load. Second-class lever is rarely found in the human body. Load Fulcrum Effort F L E Bending of the arm is an example of a third-class lever. The elbow joint is the fulcrum, the biceps contract to provide the effort to lift the arm, and the weight of the forearm or any weight that it is holding is the load. Third-class lever is the most common type of lever in the human body. Can you give other examples? The wedge-shaped front teeth in the mouth, called the incisors, are used to bite food. When we eat, the amount of force produced by both the top and bottom front teeth will break the food apart Second-class Lever Third-class Lever Wedges 84 ©Praxis Publishing_Focus On Science
Activity 5 1 Work in groups. 2 Research on other examples of simple machines in the human body and how each one works (other than those mentioned in the text). 3 Each group prepare a poster to present their findings. Identifying the simple machines in human body Activity 6 1 Work in groups. 2 Browse the Internet to get more information about Rube Goldberg machines. 3 Build your own Rube Goldberg machine based on a simple task to be solved such as switching on a lamp. 4 Collect items such as cardboard, string, cardboard tubes, straw, dominoes, marbles, spoons, needles, toy cars and boxes. 5 By using the concept of the chain of action and reaction, build your machine. 6 Test out your machine to see whether it is working before presenting to the class. Designing a Rube Goldberg machine Rube Goldberg who is an American inventor, engineer and cartoonist, was well known for his creation of the caricature scientist, Professor Butts. The cartoon character he illustrated used complicated machines to perform very simple tasks in the most hilarious ways and eventually the inventions were called the Rube Goldberg Machines. One of his most popular illustrations was the self-operating napkin as shown here. It involves a continuous chain of actions and reactions that uses simple machines like pulleys and levers. Can you decipher all the steps involved from A to M? Up to today, a global Rube Goldberg Machine contest is held annually to challenge students to use their creativity to build their own Rube Goldberg machine that solves a simple task using the most complex process. Science Facts A B C D E G H I J K L M F Chapter 3 Work, Energy and Simple Machines 85 ©Praxis Publishing_Focus On Science
1 is done when a force is used to move an object through a distance in the direction of the force. 2 The S.I. unit of work is . 3 is defined as the rate at which work is done. 4 Coal, , nuclear energy and natural gas are the types of fossil fuels currently in use; they are non-renewable energy sources. 5 Energy sources such as wind, running water, biomass, waves and heat from inside the Earth are energy sources that can be replaced after being used. 6 potential energy is the energy stored in an object due to its vertical position or height from ground while potential energy is the energy stored due to its stretching or compressing condition. 7 Kinetic energy is the energy of a moving object that depends on its and velocity 8 The sum of the potential energy and the kinetic energy is called energy. 9 A simple is a device or tool that helps us to do work easier. 10 The force applied on a machine is called the force. 11 The force that a machine applies to move an object over some distance is called the force. 12 The mechanical advantage of a machine is the measure of its . 13 The mechanical advantage is the ratio of the force to the force of the machine. 14 A lever is a bar that rotates around a fixed point called the . 15 In the class lever, the fulcrum is positioned between the load and the effort. 16 For the second-class lever, the mechanical advantage is always than 1. 17 The third-class levers do not give a mechanical advantage, but they are used to increase the of the load. 18 The longer an inclined plane, the the force needed to move an object from a lower to a higher level. 19 A is a movable inclined plane used to split things apart. 20 It is to turn a screw when the threads on the screw are more widely spaced. 21 A pulley is a wheel on an axle designed to rotate with a rope moving along a groove at its . 22 In a wheel and axle, the axle allows the wheel to rotate around it and they together. 23 In the bending of the arm, the joint is the fulcrum. RECALL Fill in the missing words. 86 ©Praxis Publishing_Focus On Science
THINKING CAP Put on your 1 Explain how energy is transferred and transformed when an arrow is shot using a bow? 2 What can be done to increase the mechanical advantage of an inclined plane without changing its height? Give a reason. 3 A wheel and axle system has a mechanical advantage of 3 and the radius of the axle is 15 cm. What is the radius of the wheel? 4 Why are pulleys used to lift objects during the construction of buildings? Chapter 3 Work, Energy and Simple Machines 87 ©Praxis Publishing_Focus On Science
Project Making a Waterwheel Activity objective: To design and create a working waterwheel Problem statement: Waterwheels use the energy of flowing water to turn a wheel and the turning wheel can then power other machines to do work. During ancient times, waterwheels were used to grind grains into flour, drive pumps and run farm equipment. Today, the modern equivalents of waterwheels are the huge turbines of hydroelectric power plants, which generate electricity. Waterwheels do not pollute the environment. We can demonstrate the mechanics of a waterwheel by making our own waterwheel. Fact finding: • Pretend that you are an engineer who needs to build a model of a waterwheel to demonstrate its power and mechanics. • Refer to various sources on the Internet and books on the concept of obtaining power from flowing water. Concept applied: Generate power from flowing water Action plan: (a) According to the design of the model, prepare a variety of building materials. (Suggestion of materials: paper plates, paper cups or plastic spoons, tape, straw, wood skewer) (b) Sketch a few designs of waterwheels before building the model. (c) Build the model that you have sketched. (d) Test your model by pouring water onto it to make it spin. (e) Think about improvements that you can make to your model to make it spin faster. Make the necessary changes to your design and test it again. Solution: Design of the model of waterwheel and its features Presentation: Compare your own model of waterwheel with your friends. Share the information with the class about what you learned about how to obtain power from flowing water. 88 ©Praxis Publishing_Focus On Science
Wayang Kulit, or the leather puppet performance is an Indonesian form of shadow puppetry. Do you know how are the shadows of the leather puppets formed? Do you know what are the shadows used for? Vibrations, Waves and Light CHAPTER 4 What will you learn? Understand how vibration is produced Describe and categorise waves Understand how sound waves are produced and travel from one place to another Know how humans hear sounds Identify the frequency range of human hearing Know how sound is reflected Understand echolocation and sonar Differentiate audible sound, infrasound and ultrasound Know the characteristics of light Investigate reflection, refraction and dispersion of light Know the basic structure of the eye and how humans can see Construct a pinhole camera Compare a camera to the human eye ©Praxis Publishing_Focus On Science
Vibration or oscillation is a periodic motion in which an object moves back and forth repeatedly about the equilibrium position. Examples of objects that vibrate 4.1 Vibrations Look at the objects on the right. In the beating heart, the rocking chair and the swing, is a motion known as a periodic motion. The motion repeats itself after a certain period of time. Some other examples of periodic motion are a vibrating tuning fork, a swinging pendulum and the rotation of Earth around the Sun. The rocking motion of a rocking chair to-and-fro Beating of the heart where the heart contracts and relaxes to pump blood A swing in motion Piano string Tuning fork Guitar string Cone of the speaker Surface of the drum 90 ©Praxis Publishing_Focus On Science
A complete cycle occurs when the bob swings from point A-B-C-B-A upon the application of force. Displacement is the distance in any direction from the equilibrium position. Amplitude is the maximum displacement of an object from its equilibrium position. The SI unit for amplitude is metre (m). Period, T is the time taken for one complete cycle. Period, T is calculated using the equation: T = t n where n = number of oscillations t = time taken for n number of oscillations (s) A pendulum with a bob hanging on a string from fixed point, O and oscillating about the equilibrium position, B Fixed point B A C O Equilibrium position of the pendulum Frequency, f is the number of complete cycles in one second. The SI unit for frequency is Hertz (Hz). Frequency, f is related to period, T by the equation: f = 1 T where T = period (s) f = frequency (Hz) The equilibrium position of this weighted spring is O. A complete cycle occurs when the spring vibrates from point K-O-J-O-K upon the application of force. The amplitude is OK or OJ. A weighted spring vibrating from its equilibrium position, O J O K Example 1 A pendulum makes 120 oscillations in 30 s. Determine its period and frequency. Solution: Period, T = 30 120 = 0.25 s Frequency, f = 1 0.25 = 4 Hz When the frequency involved in a repeated motion is high, we call it vibration. When the frequency involved in a repeated motion is low, we call it oscillation. Science Facts Chapter 4 Vibrations, Waves and Light 91 ©Praxis Publishing_Focus On Science
Problem statement How does the length of a pendulum affect the period of the pendulum? Hypothesis As the length of the pendulum increases, the period of the pendulum increases. Manipulated variable Length of pendulum Responding variable Period of pendulum Constant variable Mass of the pendulum bob Materials and apparatus Thread, pendulum bob, retort stand and clamp, stopwatch and metre ruler Procedure 1 Prepare a pendulum with a 20 cm long thread. A B One complete oscillation: A B A Retort stand Thread Pendulum bob Diagram (a) Diagram (b) 2 Hang the pendulum on a retort stand clamp. 3 Pull the hanging pendulum to one side and release. 4 Record the time for the pendulum to make 20 complete oscillations. Repeat this step to obtain a second and third reading. 5 Repeat steps 1 to 4 using different lengths of thread (40 cm, 60 cm, 80 cm and 100 cm) for the same pendulum. Result Record your result. Length of pendulum (cm) Time taken for 20 oscillations (s) Period, T (s) Reading 1 Reading 2 Reading 3 Reading 4 20 40 60 80 100 Discussion What is the relationship between the length of the pendulum and the period of the pendulum? Conclusion Can the hypothesis be accepted? Write down your conclusion. Experiment 1 Investigating the effect of the length of a swinging pendulum on its period 92 ©Praxis Publishing_Focus On Science
4.2 Waves A wave is a travelling disturbance from a vibrating source. As a wave travels, it carries energy along with it in the direction of its propagation. Types of Waves Sound waves, light waves, radio waves and seismic waves are some examples of waves. Based on the direction of vibrations of the particles relative to the direction of propagation of waves, there are two types of waves: transverse wave and longitudinal wave. Activity 1 Aim: To study transverse and longitudinal waves Materials and apparatus: Slinky spring and ribbon Procedure: 1 Place a slinky spring on the floor. Tie one end of the slinky spring to the leg of a table. 2 Tie a short ribbon to any part of the spring. Ribbon 3 Move the spring up and down as shown in the diagram so that a set of transverse waves is produced. Record the movement of the spring. Ribbon Direction of vibrations Transverse and longitudinal waves Chapter 4 Vibrations, Waves and Light 93 ©Praxis Publishing_Focus On Science
4 Move the spring in a to-and-fro direction as shown in the diagram below so that a set of longitudinal waves is produced. Record the movement of the spring. Direction of vibrations Ribbon Observation: Write down what you observe. Discussion: 1 What does the ribbon tied to the spring represent? 2 What does Step 3 show? 3 What does Step 4 show? Conclusion: Write down your conclusion. In a transverse wave, the particles in the medium move perpendicular to the direction of propagation. Crests and troughs are formed in the medium. Examples of transverse waves are light waves and water waves. Direction of vibrations Wave direction Stretched slinky spring Crests Trough Crest forming The crest moves towards the wall For a transverse wave, the horizontal distance between two consecutive crests or between two consecutive troughs measured in metres (m) is known as the wavelength, λ. The amplitude of the wave is the height of the wave from its equilibrium position. Direction of propagation of wave Amplitude Crest Trough Wavelength Cycle The stages in the formation of transverse waves with the troughs and crests seen moving towards the wall Transverse wave 94 ©Praxis Publishing_Focus On Science