Certain force acting on a 20 kg mass changes its velocity from 5 m s-1 to 2 m s-1. Calculate the work done by the force.

Solution:

Given

Initial velocity u = 5 m/s

Mass of the body = 20kg

Final velocity v = 2 m/s

The initial kinetic energy

E_i = (1/2) mu² = (1/2) × 20 × (5)² 

= 10 × 25

= 250 J

Final kinetic energy 

E_f = (1/2) mv² = (1/2) × 20 × (2)² 

= 10 × 4

 = 40 J

Therefore,

Work done = Change in kinetic energy

Work done = E_f – E_i

Work done = 40 J – 250 J

Work done = -210 J