Question

A car of mass $1000kg$ is moving at a speed of $20m{s}^{-1}$. It is brought to rest at a distance of $50m$. Calculate the net force acting on the car.

• A. $4000N$
• B. $3000N$
• C. $2000N$
• D. $1000N$

Answer 1

The correct option is A $4000N$

Given: Initial velocity $u=20m{s}^{-1}$, displacement $s=50m$, mass $m=1000kg$ and final velocity $v=0$.
Let $a$ be the acceleration of the car.
Let the force acting on the car be $F$

From the third equation of motion,
$v^2-u^2=2as\Rightarrow 0-{20}^2=2\times a\times 50$
$\Rightarrow a=-4m{s}^{-2}$

From Newton's second law,
$F=ma$
$\Rightarrow F=1000\times (-4)=-4000\text{N}$
Hence, the net force is $-4000N$. The negative sign indicates that the force is acting opposite to the direction of initial velocity and causes deceleration.