# Homework Help: Find torque in disc brakes given normal force and coefficient of friction

1. Dec 23, 2011

### bling-bling

1. The problem statement, all variables and given/known data
In the disc brakes that slow down a car, a pair of brake pads squeezes a spinning rotor; friction between the pads and the rotor provides the torque that slows down the car. If the normal force that each pad exerts on a rotor is 85 N, and the coefficient of friction is 0.62, what is the frictional force on the rotor due to each of the pads? If this force acts 8.0 cm from the rotational axis, what is the magnitude of the torque on the rotor due to the pair of brake pads?

2. Relevant equations
Ffr = (mu)N
t = Fd

3. The attempt at a solution
For the frictional force i used the formula, friction force = 0.62(85) = 52.7 N each
For the torque, i used the force form above and multiplied it by 0.08 to get 4.216 Nm

2. Dec 23, 2011

### briantaekim

Your solution looks right. Except that at the end the problem states "torque on the rotor due to the pair of brake pads." So I'm guessing he meant to multiply the value by two.