1. T HE E FFECTS OF F RICTION

2. F RICTION Friction resists motion and acts in a direction opposite to the direction of motion. It occurs because of the electrical forces between the surfaces where two objects are in contact.

3. S TATIC F RICTION AND K INETIC F RICTION Static friction is the force that tends to prevent a stationary object from starting to move. The maximum static friction is called the starting friction. Kinetic friction is the force that acts against an object’s motion in a direction opposite to the direction of motion.

4. Sliding friction affects a toboggan; rolling friction affects a bicycle; and fluid friction affects a boat moving through water and an airplane flying through air. The Effect of friction is heat and loss of efficiency Unwanted friction can be reduced by changing sliding to rolling, by polishing by using bearings, and by using lubrication.

5. C OEFFICIENT OF F RICTION By experiment, the coefficient of friction can tell a physicist how much friction will be present between an object and a surface. Coefficients are ratios of the frictional force to the normal force.

6. C OEFFICIENT OF S TATIC F RICTION Every two contacting objects have unique properties and therefore a different coefficient of static friction:

7. C OEFFICIENT OF K INETIC F RICTION Every two contacting objects have unique properties and therefore a different coefficient of kinetic friction:

8. M EASURING COEFFICIENT OF FRICTION = tan θ =

9. C OMMON C OEFFICIENTS Materials in Contact Kinetic Coefficient Static Coefficient Wood on wood0.20.25-5 Steel on wood0.250.45 Steel on ice0.0100.10 Rubber on dry asphalt 1.01.1 Rubber on wet asphalt 0.951.0 More on page 101 in your textbook.

10. E XAMPLE # 1 It takes 50N to pull a 6.0 kg object along a desk at constant speed. What is the coefficient of friction?

11. E XAMPLE #2 The coefficient of sliding friction between two materials is 0.35. A 5.0 kg object made of one material is being pulled along a table made of another material. What is the force of friction?

12. E XAMPLE # 3 A packing crate is placed on a plane inclined at an angle of 35° from horizontal. If the coefficient of static friction between the crate and the plane is 0.65, will the crate slide down the plane?

13. E XAMPLE # 4 A piece of wood is sliding on a steel table saw (  k =0.25) is pushed with a force of 50N. If the wood is moving at constant velocity, what is the mass of the wood?

14. E XAMPLE # 5 A 800-N student is Rollerblading at a velocity of 6m/s [W]when the student trips and slides horizontally along the trail, coming up to stop in 1.5 seconds. a. Determine the Student’s average acceleration. b. What is the kinetic friction as the student slides? c. Determine the coefficient of kinetic friction between the student and the trail.