force, commonly, a "push" or "pull," more properly defined in physics as a quantity that changes the motion, size, or shape of a body. Force is a vector quantity, having both magnitude and direction. The magnitude of a force is measured in units such as the pound, dyne, and newton, depending upon the system of measurement being used. An unbalanced force acting on a body free to move will change the motion of the body. The quantity of motion of a body is measured by its momentum, the product of its mass and its velocity. According to Newton's second law of motion (see motion), the change in momentum is directly proportional to the applied force. Since mass is constant at ordinary velocities, the result of the force is a change in velocity, or an acceleration, which may be a change either in the speed or in the direction of the velocity.
Two or more forces acting on a body in different directions may balance, producing a state of equilibrium. For example, the downward force of gravity (see gravitation) on a person weighing 200 lb (91 km) when standing on the ground is balanced by an equivalent upward force exerted by the earth on the person's feet. If the person were to fall into a deep hole, then the upward force would no longer be acting and the person would be accelerated downward by the unbalanced force of gravity. If a body is not completely rigid, then a force acting on it may change its size or shape. Scientists study the strength of materials to anticipate how a given material may behave under the influence of various types of force.
There are four basic types of force in nature. Two of these are easily observed; the other two are detectable only at the atomic level. Although the weakest of the four forces is the gravitational force, it is the most easily observed because it affects all matter, is always attractive and because its range is theoretically infinite, i.e., the force decreases with distance but remains measurable at the largest separations. Thus, a very large mass, such as the sun, can exert over a distance of many millions of miles a force sufficient to keep a planet in orbit. The electromagnetic force, which can be observed between electric charges, is stronger than the gravitational force and also has infinite range. Both electric and magnetic forces are ultimately based on the electrical properties of matter; they are propagated together through space as an electromagnetic field of force (see electromagnetic radiation). At the atomic level, two additional types of force exist, both having extremely short range. The strong nuclear force, or strong interaction, is associated with certain reactions between elementary particles and is responsible for holding the atomic nucleus together. The weak nuclear force, or weak interaction, is associated with beta particle emission and particle decay; it is weaker than the electromagnetic force but stronger than the gravitational force.