1. Force of Friction

2. What is friction?
The force that is opposite of the direction of motion.

3. What does the amount of friction depend on?
Friction depends on the surfaces and the force pressing the surfaces together. 
For example: An icy surface has less friction than a dry surface.
This car slid off the
road because of an icy
surface!

4. Friction also depends on the force pressing those surfaces together.
For example: If your book was beginning to slide off of your desk, what do you do?
You slam your hand down on it to keep it from sliding!
(The force (or your hand) pressing on that surface is friction!)

5. In other words…. f = µ N (It’s a fun equation )
The types of surfaces and weight (or forces that are pressing on a surface) are factors that affect friction.
So how do we find that amount of friction?
Through the coefficient of friction (µ):
f = µ N (It’s a fun equation )
f = friction µ = coefficient of friction
N = normal force (support force)

6. 1. The surfaces relate to the: µ - coefficient of friction.
2. pressure of surfaces relate to the: wt - weight or N - normal force (also the support force)
The higher the coefficient of friction, the more friction present!

7. Let’s try a problem:
A desk is exerting a 5N normal force (support force) on a block that is sliding on a table. The block is being slowed by a .75N frictional force. What is the coefficient of friction between the block and the desk?
Give yourself a couple of minutes to do the problem.

8. Were you correct? Here’s the answer:
f = µN
f = .75N Step 1: .75 = µ5
N = 5N
µ = ? Step 2: .75 = µ5
Step 3: µ = .15
Step 4: That’s it! There are no units for a coefficient of friction!

9. The coefficient of friction and the floor is. 2
The coefficient of friction and the floor is .2. I push a 50kg box at a constant speed across the floor. Draw a free body diagram