Most of the time we ignore or assume many things, for example, we ignore air
resistance, assume Fg is constant. However,
friction is too significant to completely ignore, and is a factor in many problems.
There are two types of friction, and for good reason, they are
different. For example, when you brake hard and the tires do not skid, friction
helps you to stop. However, if you brake hard and the tires skid, you still
eventually stop, even though it takes longer.
These are the two frictions: