1. Friction is a type of force between two touching surfaces.

2. 5.2 Friction
I. What is the friction force? A. Acts in a direction opposite to the motion of an object. B. Friction is all around us. C. Two types: kinetic and static

3. 5.2 Friction
II. How do static and kinetic friction differ? A. Kinetic friction 1. Exerted on one surface by another when the two surfaces rub against each other because one or both surfaces are moving.

4. 5.2 Friction
2. Measure the force to move an object at constant speed using a spring scale.
3. Add more weight to the blocks increases the normal force which increases the kinetic friction.

5. 5.2 Friction Static Friction
1. Force exerted on one surface by another when there is NO motion between the two surfaces.

6. 5.2 Friction
If the object doesn’t move, the force of static friction is increasing.
When you push hard enough on the object, the object will move.
There is a limit to how large the static friction will be.
When your force on the object is larger than the maximum static friction, the object will move and kinetic friction will take over.

7. 5.2 Friction Friction depends upon:
1. Materials that the surfaces are made of
2. Normal force between the surfaces
Friction does not depend upon:
1. Surface area between the surfaces
2. Speed that the object moves

8. 5.2 Friction
E. Coefficient of kinetic friction 1. The linear relationship between the kinetic kinetic friction and the normal force.

9. 5.2 Friction
2. Values for coefficient of friction on common objects (Table 2 p. 132). 3. Measured on dry surfaces. 4. Impurities can affect the measurements.

10. 5.2 Friction Ff, kinetic – measured in N (newtons)
5. Formula for Kinetic Friction Force
Ff, kinetic =  FN
The kinetic friction force equals the product of the coefficient of kinetic friction and the normal force.
Ff, kinetic – measured in N (newtons)
 - (mu) – NO UNITS
FN – measure in N (can determine from the weight of the object)

11. 5.2 Friction Coefficient of Static Friction
1. Acts in response to a force trying to cause a stationary object to start moving.
2. The maximum static force will balance the force of person pushing an object the instant before it begins moving.

12. 5.2 Friction Ff, kinetic   FN
3. Formula for Static Friction Force
Ff, kinetic   FN
The static friction force is less than or equal to the product of the coefficient of static friction and the normal force.
Fs, kinetic – measured in N (newtons)
 - (mu) – NO UNITS
FN – measure in N (can determine from the weight of the object)

13. 5.2 Friction Review of Formulas: 1. Fnet = ma (Newton’s 1st Law)
2. FN = Fg = mg (weight of the object)
3. Ff, kinetic =  FN
4. Fnet = Fapplied –Ffriction
5. ma = Fapplied –  FN
6. ma = Fapplied –  mg

14. 5.2 Friction Practice Problems
18. Gwen exerts a 36-N horizontal force as she pulls a 52-N sled across a cement sidewalk at constant speed. What is the coefficient of kinetic friction between the sidewalk and the metal sled runners? Ignore air resistance.

15. 5.2 Friction Practice Problems
19. Mr. Ames is dragging a box full of books from his office to his car. The box and books together have a combined weight of 134 N. If the coefficient of static friction between the pavement and the box is 0.55, how hard must Mr. Ames push horizontally on the box in order to start it moving?

16. 5.2 Friction Practice Problems
20. Thomas sits on a small rug on a polished wooden floor. The coefficient of kinetic friction between the rug and the slippery wooden floor is only If Thomas weighs 650 N, what horizontal force is needed to pull the rug and Thomas across the floor at a constant speed?

17. 5.2 Friction Practice Problems
21. You need to move a 105-kg sofa to a different location in the room. It takes a 403-N force to start the sofa moving. What is the coefficient of static friction between the sofa and the carpet?

18. 5.2 Friction Practice Problems
22. A 1.4-kg block slides freely across a rough surface such that the block slows down with an acceleration of −1.25 m/s2. What is the coefficient of kinetic friction between the block and the surface?

19. 5.2 Friction Practice Problems
23. You want to move a 41-kg bookcase to a different place in the living room. If you push with a force of 65 N and the bookcase accelerates at 0.12 m/s2, what is the coefficient of kinetic friction between the bookcase and the carpet?

20. 5.2 Friction Practice Problems
25. Ke Min is driving at 23 m/s. He sees a tree branch lying across the road. He slams on the brakes when the branch is 60.0 m in front of him. If the coefficient of kinetic friction between the car’s locked tires and the road is 0.41, will the car stop before hitting the branch? The car has a mass of 1200 kg.