CBSE – 9th Standard Science

Class 9 Chapter 11 – Work & Energy

  • Doing anything is termed as ‘Work’.
  • For work to be done a force should act on an object, and the object must be displaced.
  • We define work to be equal to the product of the force and displacement.
  • Work done by force acting on an object is equal to the magnitude of the force multiplied by the distance moved in the direction of the force
  • Work has only magnitude and no direction.
  • The unit of work is newton metre (N m) or joule (J).
  • 1 J is the amount of work done on an object when a force of 1 N displaces it by 1 m along the line of action of the force.
  • Work done is negative when the force acts opposite to the direction of displacement.
  • Work done is positive when the force is in the direction of displacement.


  • The Sun is the biggest natural source of energy to us. Many of our energy sources are derived from the Sun.
  • An object that possesses energy can exert a force on another object.
  • The energy possessed by an object is thus measured in terms of its capacity of doing work.
  • The energy of an object is its capacity for doing work.
  • The unit of energy is, therefore, the same as that of work, that is, joule (J).
  • 1 J is the energy required to do 1 joule of work.
  • Kilo joule (kJ) is the larger unit for energy.
  • 1 KJ equals 1000 J.  1000 J = 1 kilo joule(KJ)
  • There are different forms of energy. They are heat energy, light energy, electrical energy, chemical energy (potential energy+kinetic energy) etc.


  • Mechanical energy (potential energy + kinetic energy)
  • Heat energy
  • Chemical energy
  • Electrical energy
  • Light energy.


  • The kinetic energy of a body moving with a certain velocity is equal to the work done on it to make it acquire that velocity.
  • Kinetic energy is the energy possessed by an object due to its motion. The kinetic energy of an object increases with its speed.
  • The work done is equal to the change in the kinetic energy of an object


  • Potential energy is energy that is stored within a system
  • The force is often called a restoring force.
  • PE = W = F x d = m a h
  • Familiar examples of potential energy:
    • A wound-up spring
    • A stretched elastic band
    • An object as some height above the ground
  • The gravitational potential energy of an object at a point above the ground is defined as the work done in raising it from the ground to that point against gravity.
  • The potential energy of an object at a height depends on the ground level or the zero level you choose.
  • An object in a given position can have a certain potential energy with respect to one level and a different value of potential energy with respect to another level.


  • According to this law, energy can only be converted from one form to another; it can neither be created nor destroyed.
  • The total energy before and after the transformation remains the same.
  • The law of conservation of energy is valid in all situations and for all kinds of transformations.
  • During the free fall of the object, the decrease in potential energy, at any point in its path, appears as an equal amount of increase in kinetic energy.


  • Power measures the speed of work done, that is, how fast or slow work is done.
  • Power is defined as the rate of doing work or the rate of transfer of energy.
  • The unit of power is watt [in honour of James Watt (1736 – 1819)].