Question

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

• A. $10m$
• B. $15m$
• C. $25m$
• D. $35m$

Answer 1

Answer: (B)

$15m$

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

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

$⟹a=-10m/s^2$

Distance covered in first second, $S_1=ut+\frac{1}{2}a{t}^2$

where $t=1s$

$\therefore$ $S_1=20\times 1+\frac{1}{2}(-10)(1{)}^2=15m$