1. Unit 2 Forces
Friction

2. Agree or Disagree?
Friction is due to the electrostatic fundamental force.
Heavier objects experience more friction than lighter ones.
Boxes A and B below are identical. Box A experiences more friction than B. B A

3. Try this! Can you pull the books apart?
Link to video

4. Friction Is the force that resists changes in an object’s motion.
Is due to electrostatic forces between the surfaces where two objects are in contact.
Depends on:
The normal force acting on an object;
Which surfaces are in contact. Ff FN Fa FN
Normal force – A reaction force that acts perpendicular to a surface. Fg Fg

5. Static or Starting Friction
Prevents stationary objects from moving Need to apply a force equal to μsFn to cause an object to move. I.e., If Fa doesn’t equal to or exceed μsFN, an object will remain maintain its state of motion.
Ffs ≤ μsFN
coefficient of friction (“mu”)

6. Ffk = μkFN Kinetic Friction
When an object overcomes static friction and begins to move, kinetic friction takes over.
Kinetic friction acts against an object’s motion.
Ffk = μkFN

7. What factors affect friction?
Which surface is easier to slide on?
B: Asphalt
A: Ice
Rank these objects based on how easy they would be to slide:
B: Wooden crate that someone is pushing down on
C: Wooden crate
A: Metal railway shipping container

8. What factors affect friction?
The magnitude of Ff depends on the materials in contact, and the normal force acting.
Ffs ≤ μsFN FA Ff FN Fg
Ffk = μkFN
Note: On level surfaces, and when no vertical forces act, the normal force is equal to the gravitational force.

9. Coefficient of Friction
In general, μs ≠ μk.
In fact, μs is usually greater than μk
It’s harder to start moving an object than it is to keep it moving.

10. Coefficient of Friction

11. Examples
A 1.5 kg book is at rest on a table. It takes 7.35 N to get the book moving, and 3.2 N to keep it moving at constant velocity.
Calculate μs between the book and the table.
Calculate Ffk and μk.
Try Pg. 171 #1, 3

12. Examples A 1200 kg car travels at constant velocity.
What normal force does the road apply on the car?
To avoid an accident, the driver slams on her brakes and the car begins to skid. The car experiences a frictional force of 7500 N. What is the coefficient of friction between the tires and the road?

13. Warm Up
Sara places her shoe on a ramp inclined at 20°. The shoe does not slide. What forces act on the shoe?
On a ramp inclined at 30°, Sara finds that her shoe slides down the ramp at constant velocity. What forces act on the shoe now?

14. Homework
Pg. 172 #2, 3, 4, 5, 6, 7, 10