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)1800 W(B)3600 W(C)7200 W(D)36,000 W

Answer 1

(D) 36,000 W

Step-by-step explanation:

mass (m) = 900 kg

initial velocity (u) = 20 m/s

final velocity (v) = 0 m/s

time (t) = 5 s

What is the average power that the brakes produce in stopping the car?

average power =
`(work)/(time)` =
`(force x distance)/(time)` =
`(mass x acceleration x distance)/(time)`

we need to find the acceleration and the distance it took before the car stopped before we can apply the formula above.

• we can get the acceleration from v = u + at

0 = 20 + (a x 5)

-20 = 5t

t = -4 m/s^{2} ( the negative sign shows the car is decelerating)

• we can get the distance from ut +
  `0.5at^(2)`

= (20 x 5) + (
`0.5 x (-4) x 5^(2)`)

= 100 - 50 = 50 m

now that we have found the acceleration and distance we can apply the formula for the average power

=
`(900 x 4 x 50)/(5)` = 36,000 W