Energy can be defined as the capacity to perform work. Thus, the unit of energy is the same as that of work. That is, the s.i unit of energy is joule. We have various types of energy; kinetic energy (Such as, the energy from a moving car or a falling stone), chemical energy, heat energy, electrical energy, light energy, light energy, sound energy, nuclear energy.

Conservation law

All the forms of energy
listed above can be changed from on form to another by means of suitable
machine or apparatus, for example; the energy in the nucleus of atoms produces
heat energy, which in turn can be used to generate electrical energy. The mechanical
energy that moves a car comes from heat energy
derived from the volume of fuel which is stored as chemical energy.

The principle of conservation of energy states that although
energy can be changed from form to another, the total energy of given system
remains unchanged i.e energy can neither be created nor destroyed during
transformation.

Mechanical Energy

A body can possess mechanical energy or have the ability to
work for two reasons either by virtue of its position or because of its motion.
The two kinds of energy are called kinetic energy and potential energy
respectively.

__Kinetic energy__

It is energy a body
possesses because it is in motion.

Mathematical formula,

KE = 1/2 mv

^{2 }Where m = mass, v = velocity.__Potential energy__

It is the energy a body possesses because of its position. A body of
mass (m) raising to a height (h) above the ground is said to possess potential
energy.

Mathematical
formula,

Power: This is defined as the rate of
doing work or the rate of transfer of energy.

Average power = (work done or energy
extended)/(time taken)

Power = (mgh)/t

The S.I unit of power is watt (W),former unit of power was horse-power.

Examples

1) A boy of mass 30kg is running with a
speed of 4 m/s. What is his kinetic energy?

Solution

KE = 1/2mv

^{2 }
M = 30kg, v = 4m/s

KE = 240J

2) A bullet of mass 4g is moving with a
speed of 216 km/hr. Calculate its kinetic energy.

**Solution**

M = 0.04 kg

V = 216km/hr

1hr = 60min

1000m = 1km

V = 60m/s

KE =72J

3) Calculate the power of a pump which lifts
500kg of water through a vertical height of 4m in 5 seconds.

**Solution**

P = mgh/t

m = 500kg

h=4m

g=10m/ S

^{2}^{}

Power = 4000J

4) A
bullet of mass 0.05kg has a speed of 400m/s. What is the kinetic energy, if it
hits a wall of which of the average resistance is 10000N. Calculate the
distance penetrated by the bullet.

**Solution**

KE = 1/2mv

^{2 }
KE = 0.5 x 0.05 x 400 x 400

KE = 4000J

Energy =work done = Force x distance

4000J = 10000 x d

Distance penetrated by the bullet = 0.4m

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