A car of 1200 kg is travelling at 20 m/s...

Question

A car of 1200 kg is travelling at 20 m/s when it slams on the brakes.
a. How much kinetic energy does the car have before it hits the brakes?

b. Where does this energy go when the driver brakes to a stop?

c. What is the work done by the car’s brakes?

 

d. If the car brakes over a distance of 50 m before coming to rest, what is the force of friction provided by the brakes on the car?

 

Answers: 2.4x105J, -2.4x105 J, 4800N

Answer 1

(a)

 

KE = m v^2 / 2 = (1200 kg)(20 m/s)^2 / 2 = 240,000 J

 

 

(b)

 

The energy is entirely dissipated by the force of friction in the brake system.

 

 

(c)

 

W = delta KE = KEf - KEi = (0 - 240,000) J = -240,000 J

 

 

 

(d)

 

Fd = delta KE

 

F = (delta KE) / d = (-240,000 J) / (50 m) = -4800 N

 

The magnitude of the friction force is 4800 N.