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A force that arises when one macroscopic body presses against another.
When one 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.
Because contact forces are so common in mechanics, there are several important special cases of contact forces that are commonly encountered.
- Applied Force: When a person, animal or machine purposely pushes or pulls an object, the resulting contact force is often called an applied force.
- Collision Forces: Contact forces occuring between objects involved in a collision. During a collision between relatively rigid bodies the collision forces will often far exceed other forces on the objects involved in the collision. In the limit that only collision forces are relevant the momentum of the system composed of all the colliding objects is conserved during the collision.
- Friction: The component of the contact force from a surface that is parallel to the plane of the surface. Friction forces will arise when (1.) an object is sliding along the surface or (2.) an object resting on the surface is subject to forces that would cause it to slide in the absence of friction. The force of friction will always resist the existing or intended sliding motion.
- 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 which guarantees that the object will not pass through the surface. The name is derived from the mathematical meaning of normal (perpendicular).
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