Friction is the force that opposes the motion of objects in contact. There are four main types of frictional forces: static friction, sliding friction, rolling friction, and fluid friction. Static friction prevents motionless objects from starting to move, while sliding friction acts on objects moving across surfaces. Rolling friction and fluid friction generally oppose motion less than sliding friction. Lubricants like oil, grease, and wax can reduce sliding friction by changing it to fluid friction.
