FACTS ABOUT FRICTION Friction is a force between two surfaces that opposes motion. It arises from irregularities in the surface of the objects, which can be thought of as microscopic “ridges” and “valleys”.

If two surfaces are pressed tightly together, air is squeezed out from between them. Electrons then become close to each other, repel, and increase the amount of friction. Air and fluids can act as lubricants between two surfaces to reduce friction. EX: Air hockey, ice skating

What three factors affect the force of friction? 1. Type of surface (texture of different surfaces) 2. Force pressing the objects together (Weight. . . or the normal force!) 3. Whether objects are moving or at rest

How do these factors relate to Ff? Ff = μ FN Ff = a measure of the repulsion of electrons between two surfaces (in Newtons) μ = the coefficient of friction is a constant, that depends upon surface texture (no units) FN = normal force that holds the objects together (in Newtons)

REFERED TO AS THE… “FUN EQUATION” Ff = μ FN But what happened to factor #3 (whether the objects are moving or at rest)?

TYPES OF FRICTION Kinetic Friction – results when two objects are sliding relative to one another Static Friction – results when two objects are at rest relative to one another

Static Friction Ridges and valleys have more time to sink into one another and interact More interaction (repulsion between electrons) means a higher force of friction Therefore, the coefficient of friction is higher for static objects than moving objects μs > μk

How could all this save your life? Consider: Which takes longer to stop, a skidding tire or a rotating tire?