# Homework Help: Conservation of momentum - Brakes

1. Jan 20, 2010

### TSail

Not sure where to post this as it isn't actually homework, all variables are made up.

1. The problem statement, all variables and given/known data

A car, weighing 1000kg and moving at 3m/s puts it's breaks on and slows down to a stop. What happens to the momentum?

2. Relevant equations

p=mv

3. The attempt at a solution

Momentum is transfered into brakes as "Heat energy" where the momentum of each molecule (as it moves because of the heat energy) balances the system.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jan 20, 2010

### cepheid

Staff Emeritus
Hi TSail, welcome to PF!

The momentum of a given "system" is always conserved, but how you choose to define that system determines which forces are classified as "internal" and which forces are classified as "external." In this case if you take the system to be just the body of the car (for simplicity), then: without worrying about the mechanical details of how the braking force is applied, you can represent this braking force as an external force that acts on the car body in the direction opposite to its motion. Since the car body has a net external force acting on it, its momentum is not conserved.

It certainly does make sense to talk about the car's kinetic energy being transformed into heat energy for the reasons you described.

I suppose if you really did want to worry about the details, then you could describe the system as consisting of two parts: the wheels of the car, and the brake pads. The brake pads apply a braking torque to the wheels, reducing their angular momentum in the process. There should be an equal and opposite torque applied by the wheels to the brake pads. Of course, the brake pads are rigidly attached to other things, meaning that other external torques will arise that will prevent the brake pads (or indeed, the entire body of the car) from developing a non-zero angular momentum and starting to spin!

3. Jan 20, 2010

### Staff: Mentor

Depending on whether the car was moving east or west, day became longer or shorter.