Question

A $1500kg$ car is initially moving with speed $30m/s$. Suddenly, a rock falls onto the road ahead.
The maximum coefficient of friction between the car tires and the roads is $\mu =0.7$.
Approximately how much distance will the car need to stop to avoid hitting the rock?

• A. $22m$
• B. $46m$
• C. $66m$
• D. $87m$
• E. $131m$

Answer 1

Answer: (C)

$66m$

Given : $u=30$ m/s $v=0$ m/s $\mu =0.7$ $m=1500$ kg

Frictional force $f=\mu mg$

$\therefore$ Acceleration of car $a=\frac{-f}{m}=-\mu g=-0.7\times 9.8=-6.86$ $m/s^2$

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

$\therefore$ $0-(30{)}^2=2\times -6.86\times S$ $⟹S\approx 66$ m