Question

An automobile travelling with the speed of $72km{h}^{-1}$ can be stopped within a distance of $20m$, by applying brakes. Determine the retardation.

• A. $-100{ms}^{-2}$
• B. $-10{ms}^{-2}$
• C. $5{ms}^{-2}$
• D. $-5{ms}^{-2}$

Answer 1

The correct option is B $-10{ms}^{-2}$

Initial velocity of automobile $u=72km/h=72\times \frac{5}{18}=20m/s$

Final velocity $v=0m/s$

Distance covered to stop $S=20m$

Using $v^2-u^2=2aS$

$\therefore$ $0-{20}^2=2\times a\times 20$

$⟹a=-10m/s^2$