Mass of the motor car, m = 1200 kg

Initial velocity of the motor car, u = 90 km/h = 25 m/s

Final velocity of the motor car, v = 18 km/h = 5 m/s

Time taken, t = 4 s

According to the first equation of motion: v = u + at

5 = 25 + a (4)

a = – 5 m/s^{2}

Negative sign indicates that its a retarding motion i.e. velocity is decreasing.

Change in momentum = mv – mu = m (v – u)

= 1200 (5 – 25) = – 24000 kg m s ^{-1}

Force = Mass × Acceleration = 1200 × – 5 = – 6000 N

Acceleration of the motor car = – 5 m/s^{2 }

Change in momentum of the motor car = – 24000 kg m s ^{-1 }

Hence, the force required to decrease the velocity is 6000 N. (Negative sign indicates retardation, decrease in momentum and retarding force)