Work and Energy



Tanusri Gururaj, Academic content writer of Physics at Edumarz

  • A body having the ability to do work possesses energy.

Unit of energy: Joule (J)

  • The body which does work loses energy whereas, the body on which work is done gains energy. 

  • The energy required to do 1 joule of work is 1 joule.

1 kilo joule = 1000 joule 

The energy possessed by bodies in motion.

Kinetic energy increases with speed.

According to the third equation of motion,

2as = v2 – u2

Work done = F s = m a s

W = ms (v2 – u2)/2s

W ½ m (v2 – u2)

Work done is equal to change in kinetic energy and taking initial velocity as 0 we get,

Ek = ½ x mv2

The energy possessed by bodies by virtue of position or configuration.

  • Gravitational potential energy:

It is the work done to raise a body from the ground to a certain height against gravity.  

Ep = mgh

  • Work done by gravity depends on the initial and final heights of the body and not on the path. 

  • Law of conservation of energy:

Energy can neither be created nor be destroyed and can only be transformed from one form to another. 

The total energy of a system remains constant.

Let us take an example of a ball of mass ‘m’ falling freely from a height ‘h.’

Initially, the kinetic energy is zero as velocity is zero. The potential energy is mgh.

Total energy = mgh

As the ball continues falling, the kinetic energy keeps increasing whereas, the potential energy keeps decreasing.  

When the ball reaches the ground and, the height is zero, kinetic energy is maximum (½ mv2) whereas, potential energy is minimum (0).

Total energy = Kinetic energy + Potential energy = constant 

Where kinetic energy = ½ mv2

Potential energy = mgh

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