The simple answer : they slow you down.
The complex answer : brakes are designed to slow down your vehicle but probably not by the means that you think. The common misconception is that brakes squeeze against a drum or disc, and the pressure of the squeezing action is what slows you down. This in fact is only part of the reason you slow down. Brakes are essentially amechanism to change energy types. When you're travelling at speed, your vehicle has kinetic energy. When you apply the brakes, the pads or shoes that press against the brake drum or rotor convert that energy into thermal energy via friction. The cooling of the brakes dissipates the heat and the vehicle slows down. This is all to do with The First Law of Thermodynamics, sometimes known as the law ofconservation of energy. This states that energy cannot be created nor destroyed, it can only be converted from one form to another. In the case of brakes, it is converted from kinetic energy to thermal energy.
Angular force. Because of the configuration of the brake pads and rotor in a disc brake, the location of the point of contact where the friction is generated also provides a mechanicalmoment to resist the turning motion of the rotor.
Thermodynamics, brake fade and drilled rotors.
If you ride a motorbike or drive a race car, you're probably familiar with the term brake fade which is used to describe what happens to brakes when they get too hot. A good example is coming down a mountain pass using your brakes rather than your engine to slow you down. By the First Law ofThermodynamics, as you start to come down the pass, the brakes on your vehicle heat up, slowing you down. But if you keep using the brakes, the drums or discs and brake pads will stay hot and get no chance to cool off. The next time you try to brake, because the brake components are already so hot, they cannot absorb much more heat. Once they get to this stage, you have to look at the brake pads themselves.In every brake pad there is the friction material which is held together with some sort of resin. Once this lot starts to get too hot, the resin holding the pad material together starts to vapourise, forming a gas. That gas has to have somewhere to go, because it can't stay between the pad and the rotor, so if forms a thin layer between the two trying to escape. The result is very similar tohydroplaning while going too fast in the rain; the pads lose contact with the rotor, thus reducing the amount of friction. Voila. Brake fade.
The typical symptom of this would be to get the vehicle to a stop and wait for a few minutes. As the brake components cool down, their ability to absorb heat returns, the pads cool off which means they have more chance to heat up again before the resinvapourises, hence the next time you use the brakes, they seem to work just fine. This type of brake fade was more common in older vehicles. Newer vehicles tend to have less outgassing from the brake pad compounds but they still suffer brake fade. So why? Well it is again to do with the pads getting too hot. With newer brake pad compounds where outgassing isn't so much of a problem, the pads transfer heatinto the calipers because the rotors are already too hot and the brake fluid starts to boil as a result. As this happens, bubbles form in the brake fluid. Air is compressible, brake fluid isn't, so you can put your foot on the brake pedal and get full travel but have no braking effect at the other end. This is because you're now compressing the gas bubbles and not actually forcing the pads againstthe rotors. Voila. Brake fade again.
So how do the engineers design brakes to reduce or eliminate brake fade? For older vehicles, you give that vapourised gas somewhere to go. For newer vehicles, you find some way to cool the rotors off more effectively. Either way you end up with cross-drilled or grooved brake rotors. While grooving the surface may reduce the specific heat capacity of the...