Force Definition in Physics

An Interaction That Causes a Change in an Object's Motion

Newtons cradle

Force is a quantitative description of an interaction that causes a change in an object's motion. An object may speed up, slow down, or change direction in response to a force. Put another way, force is "any action that tends to maintain or alter the motion of a body or to distort it," says Encyclopaedia Britannica. Objects are pushed or pulled by forces acting on them

Contact force is defined as the force exerted when two physical objects come in direct contact with each other. Other forces, such as gravitation and electromagnetic forces, can exert themselves even across the empty vacuum of space.

Units of Force

Force is a vector; it has both direction and magnitude. The SI unit for force is the newton (N). One newton of force is equal to 1 kg * m/s2, where the "*" symbol stands for "times." Force is also represented by the symbol F.

Force is proportional to acceleration, which is defined as the rate of change of velocity. In calculus terms, force is the derivative of momentum with respect to time.

Force and Newton's Laws of Motion

The concept of force was originally defined by Sir Isaac Newton in his three laws of motion. He explained gravity as an attractive force between bodies that possessed mass. However, gravity within Einstein's general relativity doesn't require force.

Newton's First Law of Motion says that an object will continue to move at a constant velocity unless it is acted upon by an external force. Objects in motion remain in motion until a force acts on them. This is inertia. They won't speed up, slow down, or change direction until something acts on them.

Newton's Second Law of Motion says that force is directly proportional to acceleration (the rate of change of momentum) for a constant mass. Meanwhile, acceleration is inversely proportional to mass. This law is useful for measuring forces. If you know two of the factors, you can calculate the third. You also know that if an object is accelerating, there must be a force acting on it.

Newton's Third Law of Motion relates to interactions between two objects. It says that for every action there is an equal and opposite reaction. When a force is applied to one object, it has the same effect on the object that produced the force but in the opposite direction. The action and reaction forces happen at the same time. If you pull on a rope, the rope is pulling back on you.

Fundamental Forces

There are four fundamental forces that govern the interactions of physical systems. Scientists continue to pursue a unified theory of these forces. The four forces are:

  • Gravitation, which is the force that acts between masses. All particles experience the force of gravity. While the graviton has been proposed as the particle mediating gravity, it has not yet been observed.
  • The electromagnetic force, which acts between electrical charges. The mediating particle is the photon.
  • The strong nuclear force, which holds the nucleus of the atom together, mediated by gluons acting on quarks, antiquarks, and the gluons themselves.

Additionally, the weak nuclear force is mediated by exchanging W and Z bosons and is seen in beta decay of neutrons in the nucleus. (A boson is a type of particle that obeys the rules of Bose-Einstein statistics.) At very high temperatures, the weak force and the electromagnetic force are indistinguishable.