Calculate the work done by the brakes of a car of mass 1000 kg when its speed is reduced from 20 m /s to 10 m /s?

Mass of the car, m = 1000kg

Initial velocity (v1) = 20m/s

Final velocity (v2) = 10m/s

Initial kinetic energy = 1/2 mv1²

= 1/2 x 1000 x (20)²

= 200,000J

Final kinetic energy=1/2 mv2²

= 1/2 x 1000 x (10)²

= 50,000 J

Work done by the brakes = change in kinetic energy

= (K.E) 2 - (K.E) 1

= 50,000 - 200,000

= - 150000J

= - 150KJ

(The negative sign shows that the force applied by the breaks is in the opposite direction to the direction of motion of the body)