Question

When a car driver travelling at a speed of $20\text{m/s}$ applies the brakes and brings the car to rest in $4\text{sec}$, the acceleration of car will be :

• A. $2{\text{m/s}}^2$
• B. $3{\text{m/s}}^2$
• C. $5{\text{m/s}}^2$
• D. $-5{\text{m/s}}^2$

Answer 1

The correct option is D $-5{\text{m/s}}^2$

The car is travelling in a straight line.
Given,
Initial velocity, $u=20\text{m/s}$
When the brakes are applied, the car decelerates and finally stops.
$v=0$
Applying $1^{\text{st}}$ equation of motion:
$v=u+at$
($t=4\text{s}$ being the time interval of motion)
$\Rightarrow 0=20+4a$
or, $a=-\frac{20}{4}$
$\therefore a=-5{\text{m/s}}^2$
The negative sign of acceleration represents the retardation of car.