Forces and Friction

When solving questions regarding Force, one has to always be very aware of Newton's Laws. Friction between two objects is a force that resists motion. There are two types of friction that we consider, sliding and static. The magnitude of the frictional force (always parallel to the surface) divided by the magnitude of the normal reaction to the surface (always perpendicular to the surface) must be less than or equal to the coefficient of static friction. If the object is moving then the frictional force opposes motion and has magnitude of the coefficient of sliding friction multiplied by the normal reaction.