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Work and Energy

Chapter: Work and Energy


Work is said to be done when a force produces motion, against the direction of the object. It is equal to the product of force and displacement.

Work = force x displacement

W = F s

The SI unit of work is Joule.



Without energy, life is impossible. We all get energy from the sun. Sun is the biggest natural source of energy to us. Let us consider some examples which can explain energy much better. Most of you must be aware of the cricket game.  In cricket, when a fast moving cricket ball hits a wicket, the wicket is thrown away. In the same way, an object gets the capability to do work when it’s raised to a certain height. Fill a balloon with air and press it. We can see a gradual change in its shape. It keeps on coming to its original shape till it is pressed gently. But if the balloon is pressed hard, it will burst and produce a blasting sound. These examples conclude that the objects achieve the capacity of performing work. This capability of doing work is called energy. The object gains energy when the work is done on it and it loses energy when the work is done by the object.

An object which contains energy can exert a force on any other object. And also we can see an energy transfer from the former to the latter when this happens. Also the second object may start moving as it gains energy and therefore, do some work. Hence it can be concluded that the first object had some capacity to do work. This implies that any object that possesses energy can do work.

Energy is defined as the capacity to do work. Therefore, the unit of energy is same as work i.e. Joule (J). So, we can say 1 J is the energy required to do 1 Joule of work..

Forms of energy

The various forms of energy are:

  1. Mechanical Energy
  2. Potential Energy
  3. Kinetic Energy
  4. Thermal Energy
  5. Chemical Energy
  6. Electrical Energy
  7. Light Energy
  8. Nuclear Energy

Following is a brief explanation of the various forms of energy:

1. Mechanical Energy:

Mechanical Energy is classified into potential and kinetic energy.

2. Potential Energy:

Potential Energy is defined as the work done against a given force in changing the position of an object with respect to a reference position. It also can be simply defined as the amount of energy stored due to the work done in an object.
If an object of mass m is placed at height h, its potential energy is given by:-

P.E. = mgh

where g is acceleration due to gravity

3. Kinetic Energy

We can say that any moving object can do work. An object, which is moving faster, can do more work than the same object that moves relatively slower. A bullet moving at a high speed, blowing wind, a rotating wheel, these all can do work.  Did you ever observe how can the wind move the blades of a windmill? All these objects in motion possess energy. This is called Kinetic Energy. Thus the energy possessed by a moving object is called its kinetic energy.

The kinetic energy of an object of mass m moving with velocity v is given by:-

K.E. = mv2/2

4. Thermal energy

Thermal energy is defined as the internal energy present in a system in a state of thermodynamic equilibrium by virtue of its temperature. The thermal energy includes both the kinetic energy and potential energy of a system's constituent particles such as atoms, molecules, or particles. It actually originates from the individually random or disordered motion of particles in a huge assembly. Thermal energy is said to be entirely the kinetic energy in some ideal gases. In some other cases, thermal energy is stored in vibrations. Thus the thermal energy can be equally separated as potential and kinetic energy.

5. Chemical Energy

Chemical energy is the amount of energy that is available for release from chemical reactions.

6. Electrical Energy

Electrical energy is defined as that amount of energy which is being converted from electrical potential energy. An electric circuit delivers both electric current and electric potential. This combination supplies the electrical potential energy. This electrical potential energy is getting converted to another type of energy at some point and thus stops behaving as electrical potential energy. So electrical energy is called as potential energy until it is being delivered to the end-usage. Electrical energy is always called as another type of energy after it is converted from potential energy. For example, it is called as heat energy, light energy, etc.

7. Light Energy

Light energy refers to the electromagnetic radiation of any wavelength, whether it is visible or not. The Primary properties of light are intensity, propagation direction, frequency and polarization, while the speed of the light in a vacuum, 299,792,458 meters per second, and are one of the basic fundamental constants of nature.

8. Nuclear Energy

Nuclear energy is the actual use of exothermic nuclear processes to generate useful heat and electricity.

Law of Conservation of Energy

We observed that the energy could be changed from one form to the other. Have you ever wondered what happens to the total energy of a system during or after the process? When energy gets transformed, the total energy is said to be changed. This is called the law of conservation of energy. According to this law, it is said that energy can only be converted from one form to another; it can neither be created nor destroyed. Also, the total energy before and after the transformation remains to be the same. The law of conservation of energy is found to be valid in all situations and for all kinds of transformations.

Unit of Energy

The unit joule is very small and hence it is not convenient to express large quantities of energy. So we use a bigger unit of energy called kilowatt hour (kW h). What is 1 kW h? Let us take an example of a machine that is using 1000 J of energy every second. If this machine is being used continuously for one hour, it is going to consume 1 kW h of energy. Thus, 1 kW h is the energy used in one hour at the rate of 1000 J s.


Power is defined as rate of doing work.

Power = work/ time

S.I. unit of power is watt (W)

Commercial unit of Energy

Commercial unit of energy is kilowatt hour (kW h)

1 kW h = 3.6 X 10 6 J