Friction

What is friction? It is a force that opposes the motion of and object, such as one sliding across a surface. It is directed parallel to the surface of contact. It works against the direction of movement

Friction can be good or bad Friction allows you to strike a match. Friction keeps your vehicle on the road. Friction keeps you walking. Friction is helpful when climbing.

Friction can be bad inside an engine. Not enough lubrication of moving parts can lead to catastrophic engine failure.

Friction is bad for spacecraft reentering Earth’s atmosphere A Spacecraft needs heat shields to guard against atmospheric drag at high velocity.

Static Friction Force Static Friction happens when an object at rest does not move along a surface even though a force is exerted to make it slide.

Maximum friction force depends on: 1) relative roughness of the two surfaces indicated by the coefficient of static friction of the surfaces (us). 2) The magnitude of the contact force between the object and the surface on which it rests. The larger the contact force (normal force), that harder it is to make the object start sliding.

Kinetic Friction Force One the object is sliding, the less force is needed to keep it sliding. Once an object is sliding, irregularities in surfaces have no time to settle into each other completely. Less force is required to keep it moving.

Kinetic Coefficient of Friction uk = coefficient of kinetic friction has been determined experimentally for many surfaces. For example: the coefficient of friction between rubber and dry concrete is 0.7 – 1.0, while the coefficient of friction between rubber and wet concrete drops to 0.5 – 0.9.

Calculation with Kinetic Coefficient of Friction F = ukn (n = normal force = weight) How much force is needed to keep a suitcase moving which weighs 164 N on a marble floor, uk is 0.2 ? F = (0.2)(165N) = 33 N

What is the coefficient of friction between a 65Kg chair and a floor, if the chair can be pushed at a constant speed with a force of 220 N? w = mg n = w n = 65 kg x 9.81 m/s2 = 638 N uk = F = 220 N = .35 n 638 N