Question

A motorcar of mass $1200kg$ 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 magnitude of the force required.

• A. 4000 N
• B. - 4000 N
• C. -6000 N
• D. 6000 N

Answer 1

The correct option is C -6000 N

Initial velocity of the car, $u=90km/h=25m{s}^{-1}$;

Final velocity of the car, $v=18km/h=5m{s}^{-1}$;

Time, $t=4s$;
$v=u+at$
$5=25+(a\times 4)$
$\therefore -a\times 4=20$ or $a=\frac{-20}{4}=-5m{s}^{-2}$

Change in momentum, $\Delta p=m(v-u)$

$=1200(5-25)=1200\times (-20)=-24000Ns$

Magnitude of force
Therefore, the force required, $F=\frac{m(v-u)}{t}=\frac{-24000}{4}=-6000N$