Figure 4 Ship, with gravitational and normal forces. Friction Related to the normal force is the frictional force . The two are related because they are both due to the surface in contact with the body. Whereas the normal force was perpendicular to the surface, the frictional force is parallel. Furthermore, friction opposes motion, and so its vector always points away from the direction of movement.
Friction is divided into two categories, static and kinetic.
It is the force which makes it difficult to start something moving. On the other hand, kinetic friction occurs when the body is in motion. This is the force which causes objects to slow down and eventually stop. Friction is usually approximated as being proportional to the normal force. Figure 5 Ship, with gravitational, normal, and frictional forces Push and Pull Another force which may act on an object could be any physical push or pull.