Question

A motor car running at the rate of 7 m$s^{-1}$ can be stopped by applying brakes in 10 m. Show that total resistance to the motion , when brakes are on, is one fourth of the weight of the car.

Answer 1

Given,

$u=7m/s$

$s=10m$

$v=0m/s$

3rd equation of motion,

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

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

$a=-\frac{7\times 7}{2\times 10}$

$a=-2.45m/s^2$

If, we consider $g=10m/s^2$, then we get

$a=-\frac{g}{4}$

Resistive force, $f=ma$

$f=-\frac{mg}{4}$