A motorcar is moving with a velocity of 108 km/h and it takes 4 s to stop after the brakes are applied. Calculate the force exerted by the brakes on the motorcar if its mass along with the passengers is 1000 kg.

The initial velocity of the motorcar

u = 108 km/h

   = 108 × 1000 m/ (60 × 60 s)

   = 30 m s-1

Final velocity of the motorcar v = 0 m s^-1.

The total mass of the motorcar along with its passengers = 1000 kg and the time is taken to stop the motorcar, t = 4 s.

We have the magnitude of the force (F) applied by the brakes as m (v – u)/t.

On substituting the values, we get

F = 1000 kg × (0 – 30) m s^-1/4 s = – 7500 kg m s^-2 or – 7500 N.

The negative sign tells us that the force exerted by the brakes is opposite to the direction of motion of the motorcar.