Force is an influence on an object that causes it to accelerate. Its units are given in kg·m/s2, or Newtons (N). Force is given by the formula

$ \vec{F} = m \vec{a} $

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.

$ p_\mathrm{avg} = \frac{\Delta F}{\Delta t}\, $
$ p = \lim _{\Delta t\rightarrow 0} \frac{\Delta F}{\Delta t} = \frac{\mathrm{d}F}{\mathrm{d}t}\, $

If force is variable and given as a function $ \vec{F} = f(x) $ (with x being the position), and b - a is the interval over which the force acts, work is given by

$ W = \int^b_a f(x)dx $

If the force is constant, this simplifies to

$ W = \vec{F} d $

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