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

Explanation

Mass, m = 1200 kg
Initial velocity, u = 90 km/h
`= (90 xx 1000)/(3600) = 25 ms^(-1)`
Final velocity, v = 18 km/h
`= (18 xx 1000)/(3600) = 5ms^(-1)`
Time, t = 4s
Acceleration, `a = (v-u)/(t)`
`a = ((5-25)ms^(-1))/(4s) = (-20 ms^(-1))/(4s)`
`= -5 ms^(-2)`
Change in momentum = mv - m u
`= m(v-u)`
`= 1200 kg xx (-20 ms^(-1))`
`= 24000 kg ms^(-1)`
Force `= mxx a`
`= 1200 kg xx (-5 ms^(-2))`
`= -6000 N`