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.
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 agiven system remains unchanged i.e energy can neither be created nor destroyed during transformation.
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.
It is energy a body possesses because it is in motion.
KE = 1/2 mv2 Where m = mass, v = velocity.
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.
KE = mgh Where m = mass, g = accelerationdue to gravity, h = distance.
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.
1) A boy of mass 30kg is running with a speed of 4 m/s. What is his kinetic energy?
KE = 1/2mv2
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.
M = 0.04 kg
V = 216km/hr
1hr = 60min
1000m = 1km
V = 60m/s
3) Calculate the power of a pump which lifts 500kg of water through a vertical height of 4m in 5 seconds.
P = mgh/t
m = 500kg
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.
KE = 1/2mv2
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