1. Aim: How do we explain the force of friction?

2. Visualizing Friction

3. Friction
When an object is in contact with a surface, the surface exerts a contact force on the object. The contact force that acts parallel to the surface, opposing the motion of an object is called friction.

4. Static Friction vs Kinetic Friction
Static friction- the force that must be overcome to set an object in motion. Static friction acts on objects at rest Kinetic Friction- the force of friction that acts on an object that is moving. Which is greater, the force of static friction or kinetic friction? STATIC FRICTION

5. Equations for Friction
Ff static =< μs FN Ff kinetic = μk FN Ff = force of friction FN= normal force μs = coefficient of static friction μk = coefficient of kinetic friction

6. Thought Question
In which scenario, is it easier to move the sled? It is easier to move the sled in scenario B because the normal force is smaller and so the friction force is smaller.

7. Sample Friction Problem
The coefficients of friction for an old physics book upon a shelf are:  μs = 0.50 and μk = 0.40.  Suppose the book has weight 14.5 N and a horizontal force is applied to it.  (a) Determine the maximum force that can be applied without moving the book.  (b) Determine the force required to keep the book moving at a constant speed across the shelf.  (c) If the magnitude of the applied force is increased gradually, find the initial acceleration of the book just as it starts to move.
a. 7.3 N b. 5.8 N c m/s2

9. Sample Friction Problems
2. A car with speed vo applies the brakes and slows to a stop. Derive and simplify an equation for the stopping distance d in terms of vo and μ. (Use Newton’s equations of motion)

10. Friction Problem 2

11. Sample Friction Problem
3. A traveler pulls a suitcase of mass 8.00 kg across a level surface by pulling on the handle 20.0 N at an angle of 50.0° relative to horizontal.  Friction against the suitcase can be modeled by μk =   (a) Determine the acceleration of the suitcase.  (b) What amount of force applied at the same angle would be needed to keep the suitcase moving at constant velocity? 
a m/s2, forward b N

12. Friction Problem 3

13. Sample Friction Problems
4. A horse of mass 509 kg pulls a sleigh of mass 255 kg and both horse and sleigh accelerate at m/s2.  The coefficient of friction for the sleigh is 0.15 as it moves over the snow.  (a) Find the force that the horse must exert on the sleigh.  (b) Determine the amount of horizontal force that the horse’s feet must exert.
a. 458 N, forward b. 713 N, backward