1. 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”.

2. 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

3. 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

4. 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)

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

6. 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

7. 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

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