Question

A $20kg$ body is moving with a velocity of $7.5m{s}^{-1}$. What is the force required to make it come to rest in three seconds.

• A. $45N$
• B. $50N$
• C. $55N$
• D. $50N$

Answer 1

The correct option is B $50N$

Given: mass of object $m=20kg$, initial velocity $u=7.5m{s}^{-1}$ , final velocity $v=0$ , and time $t=3s$
Let $a$ be the acceleration required to bring the car to rest, and $F$ be the magnitude of the force.

Using first equation of motion:
$v=u+at$
Acceleration, $a=\frac{v-u}{t}=\frac{0-7.5}{3}=-2.5m{s}^{-2}$

Using Newton's second law of motion:
Force,
$F=m\times a=20\times -2.5\Rightarrow F=-50N$

Therefore the magnitude of force required to bring the car to rest is $50N$. The negative sign indicates that the force required is opposite in direction to that of motion of body.