1. Physics With Technology
Friction
Physics With Technology

2. Microscopic Jagged Edges
Friction
Friction opposes slipping and is caused by microscopic jagged edges.
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Microscopic Jagged Edges

3. Friction GOOD: BAD: Gives us traction. Prevents slipping.
Wastes energy.
Slows things down.
Burns us (rug or rope burn).

4. Friction Static Friction: Kinetic Friction: How hard is it to:
Start sliding
Good friction.
How hard is it to:
Keep sliding
Bad Friction.

5. BEFORE THE LAB:
Draw a force diagram for a brick that is being pulled forward at a CONSTANT speed:
How does the tension of the string compare to the resistive force of friction?
Tension in string = resistance of friction
∑Fx = 0 n f T W
More Friction = More Tension
Therefore:
T – f = 0
T = f

6. Friction LAB

7. FRICTION LAB CONCLUSIONS:
f = Force of friction μ = Coefficient of friction n = normal force
f = μn

8. Friction Depends on 2 things:
Surface Texture(μ):
How rough, smooth, sticky a surface is.
Called the Coefficient of Friction.
Contact Pressure (n):
Normal force between the surfaces.

9. Coefficient of Friction:
Depends on ___________________ that are involved.
Tells you the ________________ between the normal force and the friction force.
Example:  = 0.32 means friction is ___________ as strong as the contact pressure
Usually ranges from _______ to _______ but can get higher with special materials.
both surfaces
percentage
32 % 1

10. Static vs. Kinetic:
Static friction is _____________ than kinetic friction because
The jagged edges have time to sink deep inside each other.
stronger

11. Why doesn’t surface area matter?
Small surface area results in:
Only a few jagged edges.
Sink DEEP.
Large surface area results in:
A LOT of jagged edges.
Don’t sink far.

12. Bob is pushing a large crate across the floor. The crate weighs 425 N
Bob is pushing a large crate across the floor. The crate weighs 425 N. It takes Bob 125 N to start the crate moving and 85 N to keep the crate moving. Calculate the coefficient of static friction and the coefficient of kinetic friction between the crate and the floor.

13. The coefficient of static friction between tennis shoes and ice is 0
The coefficient of static friction between tennis shoes and ice is Susy weighs 650 N. What is the hardest Susy can push against the ice before her shoes will start to slide and she will fall? (This is why we usually walk slowly on ice. If you push too hard then you will slip.)

14. Ferdinand is sliding down a snowy hill in his sled of science
Ferdinand is sliding down a snowy hill in his sled of science. The coefficient of kinetic friction between his sled and the snow is The normal force between his sled and the hill is 320 N. Calculate the force of friction that is slowing him down.