A force that arises when one macroscopic body presses against another.
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Motivation for Concept
When macroscopic body encounters another, the bodies cannot simply pass through each other. Instead, their contact results in a change in the motion of each body.
Special Cases
Applied Force
When a person purposefully presses against another object, the resulting contact force is often called an applied force.
Normal Force
An object in contact with a surface will always be subject to a contact force that has a component perpendicular to the surface. That perpendicular component guarantees that the object will not pass through the surface. It receives the special name of normal force, derived from the mathematical meaning of normal (perpendicular).
Friction
An object in contact with a surface will sometimes experience a contact force that has a component parallel to the surface. Since the perpendicular component receives a special label as the normal force, the parallel component is also generally given a name: the force of friction. This breaking up of the contact force is useful, since in many cases the friction component of the contact force can be related to the normal component.