Question

A car of mass 900 kg is traveling at 20 m/s when the brakes are applied. The car then comes to a complete stop in 5 s. What is the average power that the brakes produce in stopping the car?
A. 36,000 W
B. 7200 W
C. 3600 W
D. 1800 W

Answer 1

Answer:

A. 36,000 W

Explanation:

$m$ = mass of the car = 900 kg

$v_{o}$ = Initial speed of the car = 20 m/s

$v_{f}$ = Final speed of the car = 0 m/s

$W$ = Work done by the brakes on the car

Magnitude of work done on the car by the brakes is same as the change in kinetic energy of the car.hence

$W = (0.5) m (v_{o}^{2} - v_{f}^{2})\\W = (0.5) (900) ((20)^{2} - (0)^{2})\\W = 180000 J$

$t$ = time taken by the car to come to stop = 5 s

$P$ = Average power produced by the car

Average power produced by the car is given as

$P = \frac{W}{t} =\frac{180000}{5} \\P = 36000 W$