In order for a set of of concurrent forces acting on a body to be in equilibrium , the sum of the forces in two perpendicular directions must both be zero. This is Newton's first law of motion. The force of gravity acts vertically downwards towards the center of the earth. The contact force between two surfaces acts perpendicular to the surfaces. The amount of stretching, x, is directly proportional to the applied force, F a .
Frictional forces are surface forces and act parallel to the surfaces.
There are two types of frictional forces, static forces, F fs between surfaces at rest relative to each other and kinetic forces, F fk between surfaces that are moving. The maximum value of F fs and the value of F fk are both directly proportional to the contact force between the surfaces. The value of m s is almost always greater than m k . Any number of vectors may be added by this method. The resultnt vector is the vector drawn from the origin to the tail of the last vector.