A motorcar of mass 1200kg is moving along a straight line with a uniform velocity of 90km/h. Its velocity is slowed down to 18km/h in 4s by an unbalanced external force. Calculate the acceleration and change in momentum. Also calculate the magnitude of the force required.

Given : Mass of the motorcar, M = 1200kg
Initial velocity of the car, u = 90 km/h = 25 m/s (≈ 1km/h = 5/18 m/s)
Final velocity of the car, v = 18 km/h = 5 m/s
Time taken t = 4 s
ΔP = mv - mu
ΔP = 1200 × 5 - 1200 × 25 = -24000 kgm/s
v = u + at
5 = 25 + a × 4
⇒ a = -5 m/s² (-ve shows retardation)
|F| = m|a| = 1200 × 5 = 6000 N