Work, Energy, and Power

What is meant by energy?

A calculated quantity (Joules) that is conserved during any change at which is transferred when a force does work.

Energy comes in various forms such as thermal (heat), electrical, sound, elastic, kinetic, gravitational etc.

Principle of conservation of energy: Energy cannot be either destroyed or created, only change from one form to another.

E.g.:  Light bulbs transform electrical energy into light, which illuminates dark spaces.

A cat sitting on the highest branch of a tree has what is known as potential energy. If the cat falls off the branch and falls to the ground, its potential energy is now being converted into kinetic energy.

Water can generate electricity. When water falls from the sky, it converts potential energy into kinetic energy. This energy is then used to spin the generator turbine to produce electricity. The potential energy of the water in the dam can be converted into kinetic energy.

What is meant by work done on a body?

Work done on a body is when a force moves (displaces) the body in the direction of the force. Energy is then transferred from one body to another.

Work done is defined as the product of force causing a motion and the distance travelled in the direction of motion.

Formula: Work done = Force x Displacement  

Potential energy: Potential energy is the energy stored in an object due to its position, arrangement, or state.

What is meant by Kinetic energy?

The energy an object possesses due to its motion.


It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity.

Derivation of kinetic energy:

Worked E.g.:

Elastic potential energy: Energy stored as a result of applying a force to deform an elastic object. The energy is stored until the force is removed and the object springs back to its original shape, doing work in the process.

E.g.: Balloons are elastic and store potential energy when they are filled with air. When the air is released, the potential energy is converted into the energy of motion, which is also known as kinetic energy.

What is meant by gravitational energy?

Energy an object possesses because of its position in a gravitational field.


Worked E.g.:

Gravitational potential energy is inversely proportional to kinetic energy.

The decrease in gravitational potential energy is equal to the increase in kinetic energy and vice versa.


Efficiency of energy:  Measurement of the proportion of energy which is wasted.


Worked E.g.:

What is meant by power?

Power is the amount of energy that is transferred per second.

It is defined as work done per unit time. Formulas:

Derivation of power:

Worked E.g.:

Efficiency of power:  The rate at which energy is usefully transferred.


Worked E.g.: