1. Friction

2. Friction is a force that always exists between any two surfaces in contact with each other.

3. There are two kinds of friction, based on who the two surfaces are moving relative to each other:
Static friction – exists between two surfaces that are NOT moving relative to each other.
Kinetic friction – exists between two surfaces that ARE moving relative to each other.
In any situation, static friction is greater than kinetic friction!

4. Friction acts in the direction OPPOSITE to the motion of the object, relative to the other surface.

5. Friction is proportional to the normal force (which remember is NOT always equal to the force of gravity).
The proportionality constant is called the “coefficient of friction” and is dependent on two surfaces sliding together.
Frictional Force
Coefficient of Friction =
Normal Force

6. In equations, the coefficient is represented by “µ” (the Greek letter “mu”)
µs – coefficient of static friction
µk – coefficient of kinetic friction
The smaller the number, the less friction.
For any two surfaces, µs > µk
(Values are determined empirically)

7. The Formulas: Static Friction fs ≤ µs FN
Notice the inequality here. The force of static friction can change, but there is a maximum before the object starts to move. fs,max = µs FN
KineticFriction fk = µk FN
The force of static friction is always the same once the object is moving.

8. Sample Problem 1
A sled is resting on a horizontal patch of snow and the coefficient of static friction is µs = The sled and its rider have a total mass of 38.0kg. What is the magnitude of the maximum horizontal force that can be applied to the sled before it just begins to move?
Kinetic Friction
Static Friction

9. Sample Problem 2
A sled is traveling at 4.00 m/s along a horizontal stretch of snow. The coefficient of kinetic friction is µk = How far does the sled go before stopping?