Published on: February 05, 2024


Energy is required for all life activities. Different sources of energy are:

  • Sun (the biggest natural source of energy)

  • Tides

  • Wind

  • Coal, petroleum, natural gas, biomass, etc.



The sun is our primary source of energy. Wind and tides are directly made by the sun. Sunlight is used by the leaves of trees to make food. Animals use it. The trunks of trees are used as biomass. The leaves, fruits, and trunk can be changed into biogas. The trees that fell into the earth's crust long ago changed into coal. The animals that fell into the earth long ago changed into petroleum and natural gas. Thereby, we can understand that almost all the sources of energy are directly or indirectly related to the sun.

Energy: In science, energy can be defined as the capacity to do work.


How does an object with energy work?

If an object possesses energy, then it can apply force to another object. Here, the transfer of energy is taking place between the objects. The object that applies force loses energy. The object that undergoes motion gains energy and does some work. Thus, an object that possesses some energy can do work.

Example situations are given below:

1) Moving a cricket ball means having energy. It hits the stump, and the stump moves. The ball does work on the stump.

2) A raised hammer has energy. When it hits the nail, work is done on it.

3) Water in a dam has energy. This is due to its position.

4) When we release the pressure on a balloon, it regains its shape because of the energy gained by the strain.

5) A bow has its energy due to the bending, i.e., strain.

6) A wound spring has its energy due to the winding, i.e., strain.


Unit of Energy

Joule (J) - 1J is the energy needed to do 1J of work.

Unit of the energy and the work are Joule.

1kJ = 1000J

James Prescott Joule

• Well-known British physicist.

• He stated the law of the heating effect of electric current.

 P = I2R, whereP - heat produced, I - current flowing through a wire, R - Resistance of the wire

• He invented electrical welding.

• He verified the law of conservation of energy.

• The unit of work and energy, the joule, is named after him.

• He discovered the value of the mechanical equivalent of heat, which is a universal constant equal to 4.2 J.


Different forms of energy 

They are:

  • electrical energy

  • sound energy

  • light energy

  • heat energy

  • chemical energy

  • tidal energy

  • nuclear energy

  • wind energy

  • mechanical energy

These forms of energy are classified into types.

  1. Kinetic Energy

  2. Potential energy


Kinetic Energy

Definition: The energy possessed by a body due to its motion is called kinetic energy.

Unit: Joule(J)

Consider a toy car on a floor.

  • If a tin comes and hits a toy car, it will start moving, showing that it has acquired energy. It is the energy the car got due to its motion. This type of energy is called kinetic energy.

  • If the tin we used is filled with sand and is made to hit the toy car, the car would move faster, showing that it has greater kinetic energy. This reveals that mass is a factor determining kinetic energy.

  • If this sand is made to hit the toy car with greater velocity, then the toy car will move very fast, indicating its greater kinetic energy. It reveals that velocity is a factor that affects kinetic energy.

From the above activity, w­e can infer that kinetic energy depends on the following two factors:

  • Mass

  • Velocity


Equation for kinetic energy

Consider a moving object with mass and uniform velocity, u.

Kinetic energy = energy possessed by an object due to its motion = work done on the object to make it move.

W=F×s     ...........(1),

because - force applied on direction of force, - distance moved by the object on the direction of force, i.e., displacement.


When the force act on it, its velocity changes from to v. 

The relation connecting the initial velocity  and final velocity of an object moving with an uniform acceleration a, and the displacement s, is

v2 - u2 = 2as


This gives,

s = v2 - u22a      ............(2)


we know that F = ma  .................(3)


Substitute (2) and (3) in (1)


So, we can write the work done by the force, as 

W = ma × v2-u22a


W = 12m( v2-u2)

If the object is starting from its stationary position, that is u = 0, then

W = 12mv2

It is clear that the work done is equal to the change in the kinetic energy of an object.

If = 0, the work done will be 12mv2

Thus, the kinetic energy possessed by an object of mass m, and moving with a uniform velocit v is,

Ek = 12mv2


Problem 1

Calculate the kinetic energy of an object of mass 10 kg moving with a velocity 2 m/s.


= 10 kg

= 2 m/s


Ek = 12mv2      = 12 ×10×2×2      = 20 J


Problem 2

Calculate the work done to increase the velocity of a car of mass 1500 kg from 8 m/s to 10 m/s.


 Initial Ek of car = 12 × 1500 × 8 × 8 = 48000 JFinal Ek of the car = 12 × 1500 ×10 × 10 = 75000 J


The work done for the change in E of the car = 75000 J  - 48000 J = 27000 J


This problem can be solved in another way as well. Take a look 

W = 12m (v2 - u2) = 12 × 1500(10×10-8×8) = 27000 J


Potential Energy


Definition: Energy possessed by an object due to its change in position or height is called potential energy. A body can gain potential energy due to position or strain.

Unit: Joule(J)­





Projecting a stone using a catapult

Work is done to stretch a rubber band. The work done to stretch is stored in it as potential energy. Using this potential energy, work is done.

A paper remains on the top of a compressed spring.

A wound spring can do the work. The work done to compress a spring is stored in it as its potential energy. Using this potential energy, the spring does work.

Winding the spring of a toy car and then releasing.

The work done to wind the spring is stored in it as its potential energy. Using this potential energy, the spring does work to help the car move forward.

A stone from a height falling on a glass

On raising the stone, the work done to raise the stone is stored in the stone as its potential energy on reaching the top. On release, the stone does work on the glass using the stored energy.


A stretched bow releasing an arrow

The work done to bend the bow is stored in it as potential energy. When we release the rubber string, it sends the arrow forward using this stored energy.

In all the above situations, when one object is released, its shape changes and causes a movement in another object. I.e., the potential energy is changed into kinetic energy.


Potential energy of an object at a height

The potential energy of an object at a height is called the gravitational potential energy of the object because the work done on it is against the gravitational force. As the height of the object increases, more work has to be done against gravity, and thus, the potential energy increases.


Equation for potential energy at a height

Consider a body of mass m being raised to a height ℎ.

The force to be applied = mg

i.e., F = mg

W = Fs

s = h

Hence, W=Fs = mg × h =mgh

This work mgh is stored in the body as its potential energy.

i.e., E= mgh

Potential energy does not depend on the path on which the object is moved. It only depends on the initial and final positions of the object.


Problem 1 

An object of mass 10 kg remains at a height 6 m  above the ground. How much is its potential energy?


Mass, = 10 kg


g = 9.8 m/s2 


Ep = 10 × 9.8 × 6 = 588 J       

The unit of potential energy is the same as that of work. That is, the unit of potential energy is the joule.


Problem 2

Take a look at the figure and calculate the potential energy with respect to A and with respect to B when the object is allowed to fall freely.


Mass, = 20 kg


With respect to A,

h = 5 m 


Ep = mgh = 20 × 9.8 × 5 = 980 J


Ep with respect to A is 980 J.


With respect to B,

h = m


Ep = mgh = 20 × 9.8 × 0 = 0 J


Ewith respect to B is 0.


Problem 3

A body of mass 5 kg is on the first floor of a building which is at a height of 4 m from the ground. Then it is lifted to the second floor of height 3 m.

g = 9.8 m/s2

Mass, = 5 kg


1. Calculate the potential energy of the body with respect to the first floor.


F = ma = mg = 5 × 9.8 = 49     

h = 3 m

Ep = mgh = 49 × 3 = 147 J

2. Calculate the potential energy of the body with respect to the ground.


F = ma = 5 × 9.8 = 49h = 4 + 3 = 7 mEp = mgh = 49 × 7 = 343 J


Problem 4 

A body of mass 10kg has a potential energy of 750 J.Calculate the height at which it remains. (= 9.8m/s2)


Ep = 750 J m  = 10 kgEp  = mgh  750 = 10 ×  9.8 × hh = 75098 = 7.65 m


Are various forms of energy interconvertible?


Energy gets converted into other forms of energy. In other words, different forms of energy are interconvertible. For example,

  • The light energy of the sun gets converted into chemical energy for photosynthesis.

  • Friction is caused by the conversion of kinetic energy into heat energy.

  • The vehicle is moving because chemical energy is converted into kinetic energy.

  • Water boils in an electric kettle because electrical energy gets converted into heat energy.

  • Ball falling from a height to the ground because potential energy gets transformed into kinetic energy.


Q. How do green plants produce food? Where do they get energy from?


The light energy from the sun helps the plants prepare food. The primary users, like the human being, certain animals, like elephants, cows, etc., eat them directly. The lion, tiger, etc. eat the primary users. Somehow or another, some plants and animals fell inside the earth. Inside the earth, they underwent some transformations due to temperature and pressure from the earth. The plants changed into coal. The animals changed into petroleum.


Q. Why does the air move from place to place?


Earth’s atmosphere is called air. It has mass and weight so that it can exert a pressure called atmospheric pressure. As the sun’s rays warm the air, atmospheric pressure decreases. Since the earth’s surface is non-uniform, the sun’s rays heat the earth unevenly. This causes changes in atmospheric pressure. Thus, air moves from higher pressure to lower pressure. i.e., air acquires kinetic energy.

In short, air moves from one place to another due to the conversion of heat energy from the sun to kinetic energy.


Q. How are fuels, such as coal and petroleum, formed?

The primary users, like humans, and certain animals, like elephants, cows, etc., eat plants directly. The lion, tiger, etc. eat the primary users. Somehow or another, some plants and animals fell inside the earth. Inside the earth, they underwent some transformations due to temperature and pressure from the earth. Thus, plants and animals are changed into fuels like coal, petroleum, etc.

Thus, we can say that coal and petroleum were formed due to the conversion of light energy from the sun to chemical energy.


Q. What kinds of energy conversions sustain the water cycle?

The four main stages in the water cycle are:

Evaporation: heat energy from the sun warms the earth’s surfaces and causes the water to change into water vapor and form clouds. Here, the heat energy of the sun is converted into kinetic energy. When they rise, the water vapor exerts pressure. Thus, the kinetic energy gets converted into potential energy.

Condensation: water vapor cools down and turns into liquid.

Precipitation: water falls from the clouds in the form of rain, hailstones, snow, etc.

Collection: water formed through precipitation gets collected into bodies of water such as the ocean, lakes, rivers, etc. Here, the water gains potential energy.

Thus, the water cycle involves the conversion of solar energy into kinetic and potential energy.


Hence, it can be considered that almost all the sources of energy on earth are due to solar energy.


Energy conversions

Energy conversions in daily life activities are listed below:




Law of Conservation of Energy


Do you know how much energy is converted from one form to another?

Consider a body of mass resting at a height ℎ from the ground level.


Eof the ball = mgh

E of the ball = 0 (since the ball is at rest, its velocity is zero)

Total Energy = mgh + 0 = mgh


When it is just above the ground,

Ep = mg × 0 = 0


Ek = 12 mv2 v2 = u2 +2asv2 = 2gh

Ek = 12mv2 = 12 × m × 2gh = mgh


Total energy = E+ E= mgh + 0 = mgh 


Isn’t this the total energy at the beginning as well as just before touching the ground?

  • When a body is falling down, its loss of gravitational potential energy is equal to the gain in kinetic energy.

  • When a ball is thrown upwards, its loss in kinetic energy is equal to its gain in gravitational potential energy.

This can be stated as a law called the law of conservation of energy.

Energy can neither be created nor destroyed. But one form of energy can be converted into another form.


Life could not have been possible without the transformation of energy. Do you agree?

Yes. Of course. If the chemical energy in a log of wood never gets transformed into heat energy, we will not be able to use it as a fuel. We can’t do any work if the food we eat does not change into energy.



Frequently Asked Questions


Q1. What is kinetic energy ?

Ans: Kinetic energy is the energy associated with the motion of an object. It depends on both the mass and the velocity of the object. The formula for kinetic energy (KE) is given by:

Ek=  12×m×v2


  • Ek is the kinetic energy,
  • m is the mass of the object,
  • v is the velocity of the object.


Q2. What is potential energy?

Ans: Potential energy is the energy that an object possesses due to its position or state, and it can be converted into kinetic energy or other forms of energy as the object's position changes.


Q3. Explain interconversion among forms of energy.

Ans: Interconversion of energy refers to the process by which one form of energy can be transformed into another form while maintaining the total amount of energy in a closed system. This concept is based on the law of conservation of energy, which states that energy cannot be created or destroyed; it can only change from one form to another.