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.

Answer 1

Given : Mass of the motorcar, $M=1200kg$

Initial velocity of the car, $u=90km/h=25m/s$ $(\because 1km/h=\frac{5}{18}m/s)$

Final velocity of the car, $v=18km/h=5m/s$

Time taken $t=4s$

$\Delta P=mv-mu$

$\Delta P=1200\times 5-1200\times 25=-24000$ $kg$ $m/s$

$v=u+at$

$5=25+a\times 4$

$⟹a=-5$ $m/s^2$ (-ve shows retardation)

$|F|=m|a|=1200\times 5=6000N$