# Fundamentals of Physics/Circular Motion and Other Applications of Newton's Laws

In the last chapter, five types of forces were mentioned. Those forces are: `normal force, friction, tension, weight, and drag. In this chapter, we will focus on three types of forces: friction, drag, and centripetal acceleration. These forces are all forces that we encounter in our daily lives, and there are plenty of examples highlighting the importance of these forces. For example, a race car must be designed so that it encounters minimal air resistance as it moves along the track. In addition, the tires must be specially manufactured so that there is minimal friction between the tires and the track.

## Friction

Friction is a force that always opposes all motion; it always acts in a direction exactly opposite to that in which an object is moving (or trying to move), and typically comes when the object rubs or drags against another object as it moves. It is defined as ${\displaystyle {\vec {f}}=\mu {\vec {N}}}$ where ${\displaystyle \mu }$ is the coefficient of friction, and ${\displaystyle {\vec {N}}}$ is the normal force acting on the object. There are two types of friction, static and kinetic, also known as sliding friction. Static friction is the amount of initial force required to move an object, and kinetic friction is the amount of force required to sustain motion in an object. Because of this, static friction is always greater than or equal to kinetic friction.

## Drag

Drag is similar to friction, in the sense that it always acts in the direction opposite of motion, but unlike friction, drag occurs from air or water. Thus, the air resistance experienced by a race car is a type of drag.