When two bodies are in contact, each exerts a contact force on the other, the two contact forces being equal and opposite. The contact force is the sum of the frictional force, which is tangential to the surfaces at the point of contact, and the normal reaction, which is normal to the surfaces at the point of contact. If the frictional force is zero, the contact is said to be smooth, and otherwise the contact is said to be rough. If the two bodies are moving, or tending to move, relative to each other, the frictional forces will act to oppose the motion. See also friction.