Question

A car moving along a straight highway with a speed of 108 km/h, is brought to rest within a distance of 100 m. How long does it take for the car to stop?

• A. $\frac{14}{2}s$
• B. $\frac{20}{3}s$
• C. $\frac{25}{3}s$
• D. $\frac{32}{3}s$

Answer 1

The correct option is B $\frac{20}{3}s$

Given,

$u=108km/h=\frac{108\times 1000}{60\times 60}=30m/s$

$v=0m/s$

$S=100m$

From 3rd equation of motion,

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

$2\times 100\times a=0^2-30\times 30$

$a=-\frac{30\times 30}{2\times 100}=-4.5m/s^2$

From 1st equation of motion,

$v=u+at$

$0=30-4.5t$

$t=\frac{30}{4.5}=\frac{20}{3}sec$

The correct option is B.