Question

The speed of a 1200 kg car is 90 km h–1 on a highway. The driver applies brakes to stop the car. The car comes to rest in 3 seconds. Calculate the average power of the brakes.

Answer 1

initial speed of car = 90 km/h =25 m/s
finally body at rest
e.g final speed of car =0
now,
V=u + at
0 =25 +a3

a = -25/3
hence retardation in applying break =25/3 m/s^2

displacement covered during this time ,
S =25 x 3 -1/2 x 25/3 x 9
=75 -75/2 =75/2 m
now ,
retarding F applying by break =1200 x 25/3
= 400 x 25 =10^4 N

now,
energy = F.S = 75/2 x 10^4 Joule


power = energy/time
=75 x 10^4/2 x 3 =12.5 x 10^4 w