Page 214 - Physics

Physics for Secondary Schools

The displacement of the object is equal PE depends on mass, acceleration due to
to the change in its height as shown in gravity and height of an object above a
Figure 9.15. Thus,
reference level as opposed to KE which is
d = h − h dependent on mass and velocity. When a
f i
Substituting this expression on the right force lifts an object at a constant velocity,
side of the above equation gives: the work done by the force equals the
f (
Fd = mg h – h i) change in the object’s gravitational
potential energy. That is,
PE = W = mgh.
∴ Fd = mgh − mgh .
f i
The quantity on the left side of the Example 9.5
equation is the work done. The quantities, A stone of 2 kg mass falls from a height

mgh and mgh are the respective potential of 25 m above the ground. Calculate
i f
energy values at the heights h and h . The the PE initially possessed by the stone.
i f
difference between mgh and mgh is Solution
i f
the change in the object’s PE. It follows m = 2 kg, g = 10 m/s 2
that, change in potential energy is equal PE = mgh
to work done (W) in lifting the object that
2
is ΔPE = W. = 2 kg × 10 m/s × 25 m
 500 kgm /s  2 2 500 J
If h = 0, that is the ground level is the
i
reference level, then Therefore, PE of the stone is 500 J.
PE = mgh
where h is the height from the surface. Example 9.6
1 1 A 2 kg object is at rest on a table 1.2 m
mgh.
PE =
Thus, PE = mgh.
2 2 above the fl oor. The ceiling in the room
When determining the PE of an object, a is 2.8 m above the fl oor as shown in
reference level from which h is measured Figure 9.16.
must be chosen. The choice is arbitrary
and the reference level can be located Ceiling
anywhere. For convenience, the reference
level h is considered to have zero PE.
i
Note that, while the PE of an object Table 2 kg 2.8 m
depends on the location of the reference
level, the change in PE does not. The SI 1.2 m
unit of PE is Joules which is equivalent to Floor
(kgm /s ) just like KE. Figure 9.16
2
2

208
Student’s Book Form One

Physics Form 1 Final.indd 208 16/10/2024 20:58