Mass of the motor car, m = 1200 kg
Initial velocity of the motor car, u = 90 kmph = 25 ms−1
Final velocity of the motor car, v = 18 kmph = 5 ms−1
Time taken, t = 4 s
According to the first equation of motion:
v = u + at
5 = 25 + a (4)
a = - 5 ms−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 kgms−1
Force = Mass × Acceleration = 1200 × - 5 = - 6000 N
Acceleration of the motor car = - 5 ms−2
Change in momentum of the motor car = - 24000 kg ms−1
Hence, the force required to decrease the velocity is 6000 N.
(Negative sign indicates retardation, decrease in momentum and retarding force.)