normalkraft

Normalkraft, in physics commonly called the normal force, is the contact force exerted by a surface on a body that presses against it. It acts perpendicular to the surface, along the surface normal, and arises from the microscopic interaction between the surface and the object. The normal force prevents interpenetration of the two bodies and adjusts in magnitude as other forces change.

In the simplest case, a block rests on a horizontal table with gravity acting downward. If there are no additional vertical forces, the normal force balances the weight, so N equals mg. On an inclined plane with angle theta to the horizontal, gravity can be split into a perpendicular component mg cos theta and a parallel component mg sin theta. With no motion perpendicular to the plane, the normal force equals mg cos theta.

The normal force can differ from the object's weight when other vertical forces are present or when the contact accelerates. For example, pushing down on the block or accelerating the surface upward increases N; accelerating the surface downward reduces N. In general, N is whatever perpendicular force is required to satisfy zero acceleration perpendicular to the contact surface (or to maintain contact without separation), given the other forces acting on the body.

Normalkraft is a constraint force and is closely related to friction, which depends on N. The frictional force is often proportional to N (f ≤ μN for static friction, f = μkN for kinetic friction). The concept extends to curved surfaces and three-dimensional contact, where N varies with local surface orientation and deformation.