Final answer:
Friction always works opposite to the direction of velocity, meaning it acts against the motion or attempted motion of an object. Depending on whether the object is moving or stationary, it is subject to kinetic or static friction, respectively.
Step-by-step explanation:
Direction of Frictional Force
Friction always works opposite to the direction of velocity. This fundamental property means that if an object is moving in one direction, the force of friction is exerting a force in the direct opposite direction to this motion. When an object attempts to move or is in motion, the frictional force runs parallel to the surface between the objects and perpendicular to the normal force. In situations where there is motion, the friction involved is termed kinetic friction, and it serves to slow down moving objects, such as a hockey puck sliding on ice. When objects are at rest, static friction comes into play, which is typically greater than kinetic friction and prevents the motion until a certain threshold is exceeded.
The magnitude of the frictional force depends on two factors: the coefficient of friction between the two surfaces and the normal force exerted by the surface. An interesting point to note is that when there is no friction, objects would accelerate down an incline at a rate of a = g sine, which is independent of mass, illustrating how friction can affect the acceleration of an object.