Question

A car moving with a speed of $54\text{km/h}$ is brought to rest in $10\text{seconds}$ by applying brakes. Find the magnitude of average retardation produced by the brakes.

• A. $2.0{\text{m/s}}^2$
• B. $1.5{\text{m/s}}^2$
• C. $2.5{\text{m/s}}^2$
• D. $3.0{\text{m/s}}^2$

Answer 1

The correct option is B $1.5{\text{m/s}}^2$

Initial velocity $\left(u\right)=54\text{km/h}$
Final velocity $\left(v\right)=0\text{m/s}$
Time interval $t=10\text{s}$
We have,
Initial velocity $\left(u\right)=54\text{km/h}$
$=54\times \frac{5}{18}=15\text{m/s}$
Final velocity $\left(v\right)=0\text{m/s}$
Time interval $t=10\text{s}$
We have, $a=\frac{v-u}{t}=\frac{0-15}{10}=-1.5{\text{m/s}}^2$
Here, negative signs show retardation
$\therefore$ The magnitude of average retardation $=1.5{\text{m/s}}^2$