Science: Work and Energy: Different Forms of Energy: Mechanical and Potential Energy (For CBSE, ICSE, IAS, NET, NRA 2022)

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  • Work is said to be done when force is applied on a body and the body moves through some distance in the direction of force.
  • If a force is applied on a body and the body does not move, then no work is done at all.
  • E. g. When you try to push a wall, you do not do any work as distance moved by the wall is zero.
  • If no force is applied on a body and the body is either at rest or moving with a constant velocity, then again, no work is done.
  • Work is measured as the product of force and the displacement in the direction of the force.
  • work = force × displacement in the direction of the force.
  • Work is a scalar quantity as it specifies only magnitude not direction.
  • The SI Unit of work is newton-meter (Nm) or joule (J) .


  • The capacity of a body to do work is determined by the energy possessed by it.
  • For example, if you need to ride a bicycle, you need to paddle it and for paddling you need to do work and for it you need strength which comes from food.
  • The strength here refers to energy.
  • Energy possessed by a body = Total work that the body can do
  • Energy has the same unit as work, i.e.. , joule denoted by J.
  • conversion of 100 % of energy may not always be practicable, because, in the process of conversion of energy into work some energy may remain unused or may be wasted.

Different Forms of Energy

Energy is available to us in many different forms like mechanical, thermal, light, electrical, magnetic, sound and nuclear.

Mechanical Energy

Mechanical Energy

This is the capacity of doing work that a body possesses by virtue of its position (potential energy) or by virtue of its motion (kinetic energy) .

Potential Energy

  • A body (say hammer) raised to a certain height above the ground when left to itself, falls down. If it is allowed to fall on a piece of dried clay it may break it into pieces.
  • A body raised above the ground has thus ability to do work i.e.. it has energy. This energy possessed by a body raised above the ground is called its potential energy.
  • When two bodies one lighter and another heavier are dropped from the same height on a pit of sand it will be found that the heavier body penetrates more in sand than the lighter body.
  • Hence a heavier body possesses more potential energy.
  • If same body is dropped from different heights, we find that the body dropped from a greater height penetrates more, hence it has more potential energy. Potential energy of a body, thus depends on Weight of the body (W = mg)
  • Height of the body (h) above the ground
  • It is found that the relation between Potential energy PE (Ep) , weight (W) , and height (h) is Ep = W × h = mgh.

Kinetic Energy

  • Kinetic energy is the capacity of doing work that a body has by virtue of its motion.
  • To understand the factors on which the kinetic energy of a moving body depends perform the following activity Make a stack of two thick hard bound books (about 10 cm) .
  • When a body comes down from a height its potential energy decreases whereas its kinetic energy increases.
  • The kinetic energy (KE) of a moving body depends on:
  • its mass (m) – more the mass (for same velocity) more is its kinetic energy. its velocity (v) – more the velocity (for same mass) more is its kinetic energy.

Thermal Energy

This is a form of energy which flows into our body to give us sensation of hotness and out of our body to give us sensation of coldness.

Light Energy

The form of energy which enables us to see things is called light energy.

Electrical Energy

You may be familiar with the energy that lights our bulbs, runs our fans, operates our pumps, heats our rooms, turns on our TV and radio and runs the refrigerator in our homes. The electrical energy is generated due to movement of charged particles.

Magnetic Energy

You know that a magnet can attract a piece of iron. Thus, magnets have an ability to do work. The energy involved in the functioning of a magnet is called magnetic energy.

Sound Energy

The form of energy which enables us to hear is called sound. Sound originates when a body vibrates giving out waves which travel to our ear through a material medium.

Nuclear Energy

The nuclear energy is a non-conventional form of energy which is released in nuclear reactions by conversion of mass into energy.

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