Page 217 - Physics

Work, energy and power
Transformation of energy
Energy provides a capacity to do work.
Different types of work require different
forms of energy. Therefore, energy can
be transformed from one form to another.
The transformation of energy is referred
as energy conversion. Transformation
of energy occurs everywhere and every
second of the day. However, during the
energy transformation, the total amount
of energy remains constant.
Consider the example of a raised hammer
striking a nail described in Figure 9.4. As
the hammer strikes the nail it does work,
energy is converted from potential energy
to kinetic energy. When the brick hits the
nail, parts of the energy is used to drive the
nail while some energy are converted to
heat and sound energies. Figure 9.17 shows
the relationship between work and energy.
Work
Kinetic Potential
energy energy
Figure 9.17: Work and energy are
interchangeable
Note that; for any useful work to be done,
energy is required. Cooking gas, petrol,
and food store chemical energy which
can only be used after changing it into
other forms of energy through a chemical
reaction. For example, a car is fuelled at
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a petrol station as shown in Figure 9.18
before it can be driven. Chemical energy
stored in petrol is eventually converted
into mechanical energy, thus enabling the
car to move.
Figure 9.18: Fuelling a car at a fi lling station
Consider a locomotive diesel engine such
as a train shown in Figure 9.19. The fuel is
burnt to release chemical energy which is
in turn converted to mechanical rotational
energy. This rotation is transferred to the
wheels and makes the train move.
Figure 9.19: Train that uses diesel engine
When a stone drops from a cliff of height,
h, its PE decreases while its KE increases
as it falls. On hitting the ground, some
dust may fl y off (KE) and some sound
may be heard (sound energy). The point
of impact may also become warmer as a
result of the impact. This shows that, PE
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