1. Dynamics
Friction

2. What is Friction? Consider a ball rolling down a double sided ramp.
The ball should continue to roll until it reaches the same height. In the case below, roll forever.

3. Friction always opposes motion.
Since the ball does not continue to roll forever it must be acted upon by an unbalanced external force.
This force is called friction.
Two types:
Static Friction: Friction that opposes motion from starting.
Kinetic Friction: Friction that opposes motion of all moving objects.
Friction always opposes motion.

4. Facts About Friction Static friction is greater than kinetic friction.
Friction is independent of surface area and depends solely on the mass and the surfaces involved.
Force of friction is given by:
Where: Ff = force of friction (N)
 = friction coefficient
FN = normal force (N)

5. Example #1
A 5kg block of wood is sliding along a tabletop with a force of 20N.
What is the force of friction acting on the block?
What is the acceleration of the block?
What would happen to the force of friction if the mass was doubled?
What would happen to the acceleration?

6. Example #2
A parked car with a mass of 1000kg needs to be moved. Unfortunately, the doors are locked and the car must be pushed.
How much force must be applied to the car before it starts to move (assume a concrete road surface)?
What is the force of friction as it’s moving?
What will be its acceleration?