Question

On seeing a board of speed limit, you apply brakes that brings the car from speed of $108\text{km/h}$ to a speed of $72\text{km/h}$, covering a distance of $100\text{m}$ at a constant deceleration. How much time is required for the given decrease in speed?

• A. $3\text{sec}$
• B. $4\text{sec}$
• C. $2\text{sec}$
• D. $5\text{sec}$

Answer 1

The correct option is B $4\text{sec}$

Initial speed, $u=108\text{km/h}=30\text{m/s}$
Final speed, $v=72\text{km/h}=20\text{m/s}$
Using third equation of motion, $v^2=u^2+2as$
$a=\frac{(v^2-u^2)}{2s}=\frac{{20}^2-{30}^2}{2\times 100}$
$a=-2.5{\text{m/s}}^2$
Using first equation of motion, $v=u+at$
$t=\frac{v-u}{a}$
$t=\frac{20-30}{-2.5}=4\text{sec}$