Friction

Friction always opposes motion or the potential for motion. Friction is a resistive force. If an object is moving, friction will resist that motion with a force in the opposing direction. If an object is at rest and a force is applied to it, friction will oppose the potential motion caused by the applied force.

Normal Force F N Fg The normal force is the force perpendicular to a surface that supports the weight of an object. On a horizontal surface the normal force will be equal and opposite to the weight.

Static Friction Applied Force Ff Static friction occurs between objects at rest and some other object with which they are in contact. The friction force will cancel out an applied force up to a point. As the applied force increases the friction force increases. This means static friction is a variable force – there is no one value for the static friction.

Maximum Static Friction The maximum value of the static friction is given by the equation above. Note that the static friction value is only as great as the applied force, however, so the actual static friction force may be less than the maximum value.

Kinetic Friction Motion Ff Kinetic friction is a constant force for a given object in motion. As long as the object is moving the force of friction will be the same. Remember that the force of friction always opposes motion.

Kinetic Friction Kinetic friction is a constant value.

General Formula For Friction