NCERT Class 9 Science Work and Energy Work Revision Notes


Work is done when a force produces motion in a body.

Work done in moving a body is equal to the product of force exerted on the body and the distance moved by the body in the direction of the force.

  • Work = Force X Distance
  • W=FS                 
  • Work is a scalar quantity. It has only magnitude and no direction.
  • Its SI unit is joule (J).
  • One joule of work is said to be done on an object when a force of 1 N displaces the object by 1 m along the line of action of force.
  • Work done by a force can be positive, negative or zero.
  • It is positive when a force acts in the direction of motion of the body.
  • It is negative when a force acts opposite to the direction of motion of the body.
  • It is zero when a force acts at right angles to the direction of motion of the body.


  • We define energy as the ability to do work.
  • The amount of energy possessed by a body is the amount of work it can do when that energy is released.
  • Energy is a scalar quantity. 
  • Its SI unit is joule (J). 1 kJ = 1000 J
  • The main forms of energy are kinetic energy, potential energy, chemical energy, heat energy, light energy, sound energy, electrical energy and nuclear energy.
Types of energey

Kinetic Energy

  • The energy possessed by a body on account of its motion is known as kinetic energy.
  • Kinetic energy possessed by a body of mass ‘m’ moving with a velocity ‘v’ is given by the formula:

Kinetic Energy = 1/2mv2

  • Kinetic energy of a body is directly proportional to o the mass of the body
  • the square of the velocity of the body

Potential Energy

  • The energy possessed by a body because of its position or configuration is known as potential energy.
  • The work done in raising an object from the ground to a point against gravity is called gravitational potential energy.  
  • Potential energy of a body of mass ‘m’ raised to a height ‘h’ above the surface of the Earth is given by the formula:

Potential Energy = m g h

where ‘g’ is the acceleration due to gravity.

Law of conservation of energy:

Energy can neither be created nor destroyed; it can only be converted from one form to the other.

  • Sum of kinetic energy and potential energy of an object is its total mechanical energy.


Power is defined as the rate of doing work or the rate of transfer of energy.

  • Power is a scalar quantity. Its SI unit is watt (W).
  • It is said to be one watt when the rate of consumption of energy is 1 Js−1.
  • Larger units of power are kilowatt (kW), megawatt (MW) and horsepower (hp).
    • 1 kW = 1000 W
    • 1 MW = 106 W
    • 1 hp = 746 W

Commercial Unit of Energy

  • The commercial unit of energy is kilowatt hour (kWh).
  • One kilowatt hour is the amount of electrical energy consumed when an electrical appliance with a power rating of 1 kilowatt is used for 1 hour.
  • 1 kWh is equal to 3.6 × 106 J of energy.  
  • 1 kilowatt hour of electrical energy is commonly known as 1 unit.