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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. 

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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/s2 

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

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)

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