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

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Here, `m=1200kg, u=90km//h=(90xx1000m)/(60xx60)=25m//s`
`v=18km//h=(18xx1000m)/(60xx60s)=5m//s, t=4s`
Acceleration, `a=(v-u)/t=(5-25)/4= -5m//s^(2)`
change in momentum `= m(v-u)=1200(5-25)= -24000kg m//s`
Change in momentum `= m(v-u)=1200(5-25)= -24000kg m//s`
Force required, `F=ma=1200(-5)= -6000N`
Magnitude of force required `=6000N`
Negative sign shows that force is opposing the motion.

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