Page 218 - Physics

Physics for Secondary Schools
has been converted into heat energy. The
energy changes are as follows:
PE → KE → KE + sound energy + heat.
Note that; although energy is transformed
from one form to another, it is never used
up or lost.
Other examples of situations where
energy is transformed are as follows:
1. Water at the top of a dam has
potential energy. This is transformed
into kinetic energy as the waterfalls.
2. The PE of a stretched bow can be
transformed into KE of an arrow.
3. Consider a footballer kicking a ball.
The chemical energy in his muscles
is converted into the kinetic energy
of the ball which partly changes into
heat energy.
Principle of conservation of energy
Suppose a 3 kg object is dropped from
a height of 5 m above the ground as
in Figure 9.20. Ignoring the effects of
air resistance, the object will fall to
the ground with an acceleration due to
gravity, g.
Choosing the ground as the reference level,
the PE of the object when released is:
PE =mgH
i destroyed but can be transformed
=3 kg × 10 N/kg × 5 m
= 150 J
= 150 J
212
Physics Form 1 Final.indd 212
3 kg
A
h=2.5 m
B H = 5 m
C
Figure 9.20
Since its initial velocity was zero, it had
no KE when released, KE = 0.
i
Upon reaching the ground, it had no PE.
Since h = 0, PE = 0. It does however has
f
f
kinetic energy. Therefore, kinetic energy,
KE = 150 J.
f
Note: KE = PE = 150 J. Since no energy
is lost, the fi nal velocity can always
be calculated.
This means that while falling under the
infl uence of gravity, the initial PE of
the object is completely converted into
kinetic energy as it falls. This illustrates
the principle of conservation of energy
which states that:
“Energy is neither created nor
from one form to another.”
One can verify this principle by
determining the total mechanical energy
at each stage of falling objects.
Student’s Book Form One
16/10/2024 20:58