Question

A car travelling at a speed of $30km/h$ is brought to rest in a distance of $8m$ by applying brakes. If the same car is moving at a speed of $60km/h$ then it can be brought to rest with same brakes in

• A. $64m$
• B. $32m$
• C. $16m$
• D. $4m$

Answer 1

Answer: (B)

$32m$

Given,

$u=30km/h=\frac{30\times 1000}{60\times 60}=8.33m/s$

$s=8m$

$v=0m/s$

From 3rd equation of motion,

$2as=v^2-u^2$

$a=\frac{v^2-u^2}{2s}$

$a=\frac{0^2-8.33\times 8.33}{2\times 8}$

$a=-4.336m/s^2$

Now, $u=60km/h=\frac{60\times 1000}{60\times 60}=16.67m/s$

$v=0m/s$

From 3rd equation of motion,

$2as=v^2-u^2$

$s=\frac{v^2-u^2}{2a}$

$s=\frac{0^2-16.67\times 16.67}{-2\times 4.336}$

$s=32m$

The correct option is B.