Question

A motor car running at the rate of 7 m $s^{-1}$ is Stopped by applying the brakes after it travels 10 m. Find the total resistance to the motion when the brakes are on.

• A. It is one-fourth of the weight of the car.
• B. It is two-thirds of the weight of the car.
• C. It is one-eighth of the weight of the car.
• D. It is four-fifths of the weight of the car.

Answer 1

The correct option is A It is one-fourth of the weight of the car.

using of eq. $v^2=u^2+2as$

The stopping by brake $F=F_u$

$F=-\mu mg$

$a=-ug$

$-u^2=2\mu gs⟹49=2\mu \ast 9.8\ast 10$

$\mu =\frac{1}{4}$

$F=\mu mg=\frac{1}{4}mg=\frac{1}{4}$Weight of car.