force

force, in physics, a quantity that produces a change in the size or shape or the motion of a body. Commonly experienced as a "push" or "pull," force is a vector quantity, having both magnitude and direction. The study of forces in equilibrium is statics; that of forces and motion is dynamics. Energy can be used to produce a force, and the energy used in applying a force over a distance is called work. Friction is a force that opposes motion. There are four fundamental forces of nature:

• the gravitational force is carried by the hypothetical graviton
• the electromagnetic force is carried by the photon
• the strong interaction which holds the atomic nucleus together is carried by gluons
• the weak interaction associated with radioactivity and particle decay is carried by the Z and W particles.

The gravitational force and the electromagnetic force have an infinite range. The other two forces are short range and only act on the atomic scale. All particles experience the gravitational force, but only charged particles such as the proton, the electron and the muon experience the electromagnetic force. In standard (SI) units, forces are measured in newtons (kg-m/s²). One newton of force will cause an acceleration of 1 m/sec² on a 1-kg mass. In english units of measurement, the pound (lb) is used. A 1-lb force equals 4.44822 newtons.