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.

Solution:
The initial velocity of the motorcar
u = 108 km/h
= 108 × 1000 m/(60 × 60 s) = 30 m s-1
and the final velocity of the motor car
v = 0 m s-1
.The total mass of the motorcar along with its passengers = 1000 kg and the
time taken to stop the motorcar, t = 4 s.
we have the magnitude of the force (F) applied by the brakes as
F = 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.