Question

A car of mass $1,000kg$ 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. $4,000N$
• B. $3,000N$
• C. $2,000N$
• D. $1,000N$

Answer 1

The correct option is A

$4,000N$

For calculating force, we need to calculate deceleration first. From the third equation of motion,
$v^2-u^2=2aS$ where, v- final velocity, u- initial velocity, a- acceleration and S- distance travelled.
Given, u=$20m{s}^{-1}$, S$=50m$, mass $m=1000kg$ and v$=0$.
Therefore, $0-{20}^2=2\times a\times 50$
$\Rightarrow a=-4m{s}^{-2}$
Let the force acting on the car be $F$.
From Newton's second law,
$F=ma$
$\Rightarrow F=1000\times (-4)=-4000\text{N}$
Hence, the net force is$-4,000N$. The negative sign indicates that the force is acting opposite to the direction of initial velocity and causes deceleration. This net force is actually the frictional force acting between the ground and the tyre of the car.