What are the two things that determine friction force?

The friction force between two surfaces is primarily determined by the coefficient of friction and the normal reaction force. The coefficient of friction is a dimensionless value that represents the frictional properties of the surfaces in contact. It varies depending on the materials involved; different materials have different coefficients that dictate how easily they can slide over each other.

The normal reaction force is the perpendicular force exerted by a surface on an object resting on it. This force is influenced by the weight of the object due to gravity, as well as any additional forces that may be acting vertically on the object. The greater the normal force, the greater the frictional force can be, assuming the coefficient of friction remains constant.

In summary, friction force is a product of both how much force is pushing the two surfaces together (the normal force) and how "sticky" the surfaces are with respect to each other (the coefficient of friction). Understanding both these factors is crucial in engineering applications, such as determining braking forces, the stability of structures, or the performance of machinery.