The **normal force** is a force which acts perpendicular to a surface and prevents objects from being pushed through it. On a microscopic level, the normal force is caused by electrostatic repulsion.

If the surface is level and no external forces other than gravity act on the object, the normal force on the object is equal to

- $ F_N = mg $

where *m* is the mass of the object and *g* is the strength of the gravitational field (measured in N/kg or m/s^{2}; on earth it is 9.81 m/s^{2}).

If the surface is slanted *θ* degrees past horizontal, and normal force will be equal to the force of gravity perpendicular to the surface, which is

- $ F_N = mg \cos \theta $

Unless the object penetrates the surface, the normal force will always be equal to the force pushing the object into the surface.