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In physics, a contact force, is a force that acts at the point of contact between two objects, in contrast to body forces. Contact forces are described by Newton's laws of motion, as with all other forces in dynamics.
Contact force is the force in which an object comes in contact with another object. Contact forces are also direct forces. Contact forces are ubiquitous (present, appearing, or found everywhere) and are responsible for most visible interactions between macroscopic (visible to the naked eye; not microscopic) collections of matter. Pushing a car up a hill or kicking a ball or pushing a desk across a room are some of the everyday examples where contact forces are at work. In the first case the force is continuously applied by the person on the car, while in the second case the force is delivered in a short impulse. Certain contact forces describe specific phenomena and are important enough to have been given unique names. The most common instances of this include friction, normal force, and tension. According to forces, contact force may also be described as the push experienced when two objects are pressed together.
In the Standard Model of modern physics, the four fundamental forces of nature are known to be non-contact forces. The strong and weak interaction primarily deal with forces within atoms, while gravitational effects are only obvious on a macroscopic scale. Molecular and quantum physics show that the electromagnetic force is the fundamental interaction responsible for contact forces. The interaction between macroscopic objects can be roughly described as resulting from the electromagnetic interactions between protons and electrons of the atomic constituents of these objects. Everyday objects do not actually touch each other; rather, contact forces are result of the interactions of the electrons at or near the surfaces of the objects (exchange force). This includes friction.
- Knight, Randa ll (2008), Physics for Scientists and Engineers: A Strategic Approach (2 ed.), California: Pearson Addison-Wesley, ISBN 0-321-51671-0 p. 127