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

Given:

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

Solution:

ΔP = mv − mu
ΔP = 1200 × 5−1200×25 = −24000 kg m/s

v = u + at
5 = 25 + a × 4
⟹ a = −5 m/s2
(-ve shows retardation)

∣F∣ = m∣a∣ =1 200 × 5 = 6000 N

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