1. Aim: How can we define friction? Do Now: A 20 kg object is being pulled across a table at a constant velocity with a force of 58.8 N. Draw a free-body diagram labeling all the forces

2. F = 58.8 N FgFg FNFN F What is the weight? = 196 N F g = mg F g = (20 kg)(9.8 m/s 2 ) F g = 196 N

3. What is the normal force? F N = F g (at rest) F N = 196 N F = 58.8 N FgFg FNFN F = 196 N What is the force of friction? F F = F (constant velocity) F F = 58.8 N = 58.8 N

4. Friction Friction occurs at the boundary of two surfaces No surface is perfectly smooth

5. Friction The force that opposes motion measured in newtons (N) Always opposite direction of motion “Static Friction” is the force that opposes the “start of motion” “Kinetic Friction” is the force of friction between objects in contact that are in motion

6. What Friction Depends On There are two factors that affect friction: The types of surfaces The normal force What can Mr. Foederer do to not slip as much in the winter?

7. Coefficient of Friction Represented as µ F F = µF N µ s = coefficient of static friction (objects at rest) µ k = coefficient of kinetic friction (objects in motion) In the reference table µ is a unitless number between 0 (no friction) and 1 (100% friction) Snow and ice have a low µ Rubber has a high µ

8. Low coefficient of friction Helpful Skiing Hurtful Driving in the snow High coefficient of friction Helpful Applying the brakes to stop a car Hurtful Trying to push an object that has too much friction http://www.youtube.com/watch?v=hOt-D_ee-JE Which was harder to do? – start the object moving or keep the object moving?

9. Which µ is larger? µ s µ s > µ k Ex: Turning a revolving door Harder to start it moving then to keep it moving µ is not dependent on the surface area – only the texture 1 2 3 1, 2, and 3 are all the same item. Which one has a higher µ? They all have the same µ

10. What is the coefficient of friction? F F = µ k F N (µ k because the object is moving) 58.8 N = µ k (196 N) 0.3 = µ k What are the object and table made out of? wood and wood

11. A piece of copper whose mass is 10 kg is placed on a steel beam as a force is applied to it causing it to move with a constant velocity. F FgFg FNFN F Draw a free-body diagram labeling all the forces

12. What is the weight? F g = mg F g = (10 kg)(9.8 m/s 2 ) F g = 98 N What is the normal force? F N = F g F N = 98 N What is the net force? 0 N (velocity is constant) F FgFg FNFN F = 98 N

13. A force of 30 N is applied and the copper does not move. What is the force of friction? F F = 30 N F FgFg FNFN F = 98 N = 30 N

14. What is the maximum force that can be applied before the object begins to move? Solve for the maximum static force of friction F F = µ s F N F F = (0.53)(98N) F F = 51.94 N