Correct Answer - `5 m s ^(-2); 2400 kg ms ^(-1)`;6000 N

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

`implies5 = 25 + a (4)`

` impliesa = - 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 ms^(-1) `

`:.` Force = Mass `xx` Acceleration

`= 1200 xx - 5 = - 6000 N `

Acceleration of the motor car `= - 5 m//s^(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)