Question

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

Answer 1

Mass of car = 1000 kg 

Initial velocity, u = 20 m/s 

Final velocity, v = 10 m/s 

Retardation = a 

Distance covered = s 

Using third equation of motion 

v² – u² = 2as 

as = -150 

Work done, 

W = Fs

F = ma 

W = mas 

as = -150 

W = (1000)(150) = -150 kJ 

Negative sign shows that the brakes are acting on the opposite direction of motion.