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# A motorcar of mass 1200 kg is moving along a straight line with a uniform velocity of 90 kmph. Its velocity is slowed down to 18 kmph 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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Solution

## 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.)  Suggest Corrections  0      Similar questions  Explore more