7.1 Energy and Power

CHAPTER 7 ENERGY AND
POWER

7.1 WORK,ENERGY AND POWER

WORK

u Product of force,F and displacements in the direction of the force

W = Fs

u S.I Unit for Work is Joule (J)

1 Newton (N) x 1 metre (m) = 1J

u 1 kg = 10 N

Marie lifts a 5 kg mass from the floor and EXAMPLE 1
puts it on a table 1 metre high. Calculate
the work done.

Solution

F = 50 N
s=1m

W= Fs
= 50 N x 1 m
= 50 J

The energy used to pull a large rock 40 metre along EXAMPLE 2
the rough floor is 6 kJ. Find the tension in the rope
used to pull the rock, if the rope is parallel to the
floor.

Solution

s = 40 m
Work done = 6000 J

F=?
F = Work done,,,

Displacement
= 6000 J

40 m
= 150 N

The work done to pull up a bucket of 1 kg EXAMPLE 3
filled with 23 kg of water is 4.08 kJ. How far up
is the bucket pulled ?
Solution

Work done = 4080 J
F = 230 N + 10 N
s=?

s = work done
force

= 4080 J
240 N

= 17 m

ENERGY AND POWER

Energy Power

Definition The ability to do The rate of doing
S.I Unit work work,W

Joule (J) Watt (W)
1 W = 1 Js-1

Power, P = Work done, W
Time taken, t

A tired squirrel ( mass of approximately 1 kg) does a push- EXAMPLE 1
ups by applying a force to elevate its center-of-mass by 5
cm in order to do a mere 0.5 J of work. If the tired squirrel
does all this work in 2 sec, determine its power.

Solution

Work done = 0.5J
Time taken = 2 sec

P = 0.5 J
2s

= 0.25 W

When doing a chin-up, a physics student lifts her 42 EXAMPLE 2
kg body a distance of 0.25 metre in 2 seconds.
What is the power delivered by the student’s
biceps ?

Solution

F = 420 N
s = 0.25 m
t = 2 seconds

P = 420 N x 0.25 m
2 seconds

= 52.5 W