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CBSE Class 9 Physics Notes for Chapter 4 Work and Energy


Work done on an object is the magnitude of the force multiplied by the distance moved by the object in the direction of the applied force.

The unit of work is joule.

1 joule= 1 newton x 1 metre

If the force does not bring about any displacement in the object, the work done is considered to be zero.


Energy is the capability to do work.

An object in motion has kinetic energy. For an object with mass m moving with velocity v,

K.E=  0.5 mv2

The energy possessed by a body due to its change in position or shape is called potential energy. The gravitational potential energy of an object of mass m, at a height h is given by mgh.


Law of conservation of energy: This law states that energy can neither be created nor destroyed. It can only be transformed from one form to another.

The sum of the kinetic and potential energy of an object is called its mechanical energy.


Power is the rate of doing work. Its SI unit is watt. 1 W = 1 J/s

The energy used in one hour at the rate of 1kW is called 1 kWh.

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