In science, force is the push or pull on an object with mass that causes it to change velocity (to accelerate). Force represents as a vector, which means it has both magnitude and direction.

In equations and diagrams, a force is usually denoted by the symbol F. An example is an equation from Newton's second law:

F = m·a

where F = force, m = mass, and a = acceleration.

## Units of Force

The SI unit of force is the newton (N). Other units of force include

- dyne
- kilogram-force (kilopond)
- poundal
- pound-force

Galileo Galilei and Sir Isaac Newton described how force works mathematically. Galileo's two-part presentation of the inclined-plane experiment (1638) established two mathematical relationships of naturally-accelerated motion under his definition, strongly influencing how we measure force to this day.

Newton's Laws of Motion (1687) predict the action of forces under normal conditions as well as in response to change, thus laying the foundation for classical mechanics.

## Examples of Forces

In nature, the fundamental forces are

- gravity
- weak nuclear force
- strong nuclear force
- electromagnetic force
- residual force

The strong nuclear force holds protons and neutrons together in the atomic nucleus. The electromagnetic force is responsible for the attraction of opposite electric charge, repulsion of like electric charges, and the pull of magnets.

Non-fundamental forces are also encountered in everyday life. The normal force acts in a direction normal to the surface interaction between objects. Friction is a force that opposes motion on surfaces. Other examples of non-fundamental forces include the elastic force, tension, and frame-dependent forces, such as centrifugal force and the Coriolis force.