Chapter 5 5.2 and 5.3

5.1 Work

Work is done only when a constant force applied on an object,
causes the object to move in the same direction as the force

applied.

Work Done:

● D: the product of the component of the force parallel to the displacement times the displacement of a body.
● D: the scalar (dot) product between force and displacement of a body.
● It is a scalar quantity
● S.I. Unit is joule (J) / Kg m2 s-2

If more than 1 force acts on an object, the total work ( or net

.work ) is the sum of the work done by each force separately

or

Applications of work’s equation

• Work done, W depends on the direction of

force,F and displacement,s too.

• Examples as below:

EXAMPLE 5.1.1

You push your physics reference book 1.50 m along a horizontal
table with a horizontal force of 5.00 N. The frictional force is 1.60
N. Calculate
(a) the work done by the 5.00 N force,
(b)the work done by the frictional force,
(c) the total work done on the book.

Work Done By a
Variable Force

W = Area under ( force - displacement graph )

EXAMPLE 5.1.2

A horizontal force F is applied to a 2.0 kg radio-controlled car as it moves
along a straight track. The force varies with the displacement of the car as
shown in figure. Calculate the work done by the force F when the
car moves from 0 to 7 m.

5.3Power

Power tells you how fast work is being done or how fast energy is
transferred.

Average power, Pav

• D: the rate at which work is done.
• D: the rate at which energy is transferred
If an amount of work, W is done in an amount of time t by a force, the average power, Pav due

to force during that time interval is

• average amount of work done or energy converted per unit of time
• scalar quantity

• S.I. Unit = Kgm2s-3 / Js-1 / watt (w)

• Unit conversion between watt (W) and horsepower (hp)

Instantaneous power, P

Consider an object that is moving at a constant velocity v along a frictionless
horizontal surface and is acted by a constant force, F directed at angle  above the
horizontal as shown in figure. The object undergoes a displacement of ds.

Or

EXAMPLE 5.3.1

A 70 kg person runs up a staircase 3.0 m high in 3.5 s. How
much power does he develop in climbing the steps?