Ground reaction force

In physics, and in particular in biomechanics, the ground reaction force (GRF) is the force exerted by the ground on a body in contact with it. For example, a person standing on the ground exerts a contact force on it (equal to the person's weight) and at the same time an equal and opposite ground reaction force is exerted by the ground on the person.

In the mentioned example, the ground reaction force coincides with the notion of normal force. However, in a more general case, the GRF will also have a component parallel to the ground, for example when the person is walking, which requires exchanging horizontal forces with the ground in order to achieve motion.

The use of the word reaction derives from Newton's third law, which essentially states that if a force, called action, acts upon a body, then an equal and opposite force, called reaction, must act upon another body. The force exerted by the ground is commonly referred to as the reaction, although, since the distinction between action and reaction is completely arbitrary, the expression ground action would be, in principle, equally acceptable.

Ground Reaction Force is not equivalent to friction, however the normal component of the ground reaction force (GRF) can contribute to friction: the maximum force of friction between two surfaces equals the product of normal force, or the component of force applied perpendicular to the contact surface, and the coefficient of friction.