# 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,

Initial velocity, u = 90 km/h

= = 25 m/s

Final velocity, v = 18 km/h

= = 5 m/s

Acceleration, a =?

Time, t = 4 s

Now, v = u + at

5 = 25 + a × 4

4a = 5 – 25

4a = - 20

a = - a = - 5 m/s2

Therefore, the acceleration of the motorcar is -5 m/s2.

Now we will calculate change in momentum:

Change in momentum = mv – mu

= 1200 × 5 - 1200 × 25

= 6000 – 30000

= - 24000 kg m/s

Therefore, change in momentum is 24000 kg m/s

Now we will calculate the magnitude of force:

Force, F = m × a

= 1200 × (-5)

= - 6000 N

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