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Question
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.
Solution
Mass of the motor car, m = 1200 kg
The initial velocity of the motor car, u = 90 km/h-1
= `90 xx 5/18`
= 25 ms-1
The final velocity of the motor car, v = 18 km/h-1
= `18 xx 5/18`
= 5ms-1
Time taken, t = 4 s
(i) Acceleration, a = `"Change in velocity"/ "Time"`
= `(v - u)/t`
= `(5 - 25)/4`
= -5 ms-2
(ii) Change in momentum
= mv = mu = m (v - u)
= 1200 (5 - 25)
= -24000 kg ms-1
The magnitude of change in momentum
= 24000 kg ms-1
(iii) Force, F = ma
= 1200 (-5)
= -6000 N
∴ Magnitude of force = 6000 N
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