**Force** is an influence on an object that causes it to accelerate. Its units are given in kg·m/s^{2}, or Newtons (N). Force is given by the formula

Average force is equal to a change in linear momentum over time for a given interval, and instantaneous force is equal to the derivative of linear momentum with respect to time.

If force is variable and given as a function (with x being the position), and *b* - *a* is the interval over which the force acts, work is given by

If the force is constant, this simplifies to

Some common forces include the force of gravity, the normal force, and the force of friction.