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In physics, a contact force is a force between two objects (or an object and a surface) that are in contact with each other. This is distinct from a non-contact force, or action-at-a-distance force (a force that acts over a distance), such as gravity or magnetic attraction/repulsion. Contact forces always exist in equal and opposite pairs, by Newton's third law.
A contact force has two components. The part of the force that lies within the plane of contact is friction, which must be overcome for the two objects to slide relative to one another along that plane. The part of the force that is perpendicular to the plane of contact is called the normal force. Friction is proportional to the normal contact force, and the constant of proportionality is denoted by μ (mu).
Strictly speaking, contact forces are only a useful simplification for introductory physics classes and other applications of classical mechanics. Everyday objects on Earth do not actually touch each other; rather contact forces are the result of the interactions of the electrons at or near the surfaces of the objects (exchange interaction).
Example
An example of contact force commonly encountered in college-level physics is the force between two masses A and B which are lying next to each other and a force F is being applied on one of the masses, for example A. In such a case, the contact force will be proportional to the mass of B.
See also
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