Question

Brakes are applied to a car moving with disengaged engine, bringing it to a halt after $2$ $s$. What is it's velocity at the moment when the breaks are applied if the coefficient of friction between the road and the tyre is $0.4$?

• A. $3.92m{s}^{-1}$
• B. $7.84m{s}^{-1}$
• C. $11.2m{s}^{-1}$
• D. $19.6m{s}^{-1}$

Answer 1

The correct option is B $7.84m{s}^{-1}$

We have:
Frictional force $=ma$
$\mu mg=ma=\frac{mv}{t}$
$\therefore v=\mu gt$
$=0.4\times 9.8\times 2=7.84m/s$