Want!

Aw man, now I want a bazillion of these buttons/magnets. amavia, there's a line of piggies!

Car brakes and internal forces

It is worth noting that for all vehicles that travel on wheels (such as cars and bicycles), the sum of rolling resistance and static friction is what causes the vehicle to slow when the brakes are applied. The actual force applied in braking (for example, clamps applied to disk brakes) is internal, and by Newton's First Law cannot cause a change in the vehicle's motion. [Source]

I added that to the (English) Wikipedia page on rolling friction, and it's now been called into question on the Polish version of the page. kadath or anyone else got a good resource I can use to back me up on that here? My explanation is below, but I'd really like an external confirmation on it to fit the way Wikipedia works.

The issue is that you need to take the car as a whole, that is the body of the car, plus the brakes, plus the wheels. This is the body. Newton's Laws say that the sum of the external forces (also called the net force) causes a change in the body's motion (Law #1), and this sum of forces is equal to the mass times the acceleration (law #3, ΣF = ma). When you draw a Free Body Diagram, all you draw are the external forces, not what's going on inside.

Consider Newton's Second Law in this context, that every action has an equal and opposite reaction. In this case, we can think of the two bodies as the brake pads and the disk of the brake itself. Whatever force the brake pads are applying to the disk is equal and opposite of the force the disk is applying on the brake pads. These cancel out, and the center of mass of the two objects will be unaffected. Compare it to the Earth and the Moon - they apply equal and opposite forces of gravity upon each other, and so their center of mass will not accelerate due to those two forces alone. You need an external force to cause a change in motion, and in the case of the car it's friction between the tires and the road (rolling or kinetic, depending upon whether the wheel is rolling or sliding).