1. Exploring Frictional Forces Friction Friction and Newton’s Laws Static and Kinetic Friction Coefficient of Friction Practice

2. Friction The analysis of friction is very important to practical situations since friction is present everywhere. Friction is a good and bad thing. –Good when our tires meet the pavement –Bad when reducing efficiency and/or producing heat

3. Friction Friction always opposes motion (or F net ) –Kinetic friction –Static friction –Air resistance

4. Friction and Newton’s Laws Friction is simply a force included and accounted for in a FBD. Applies to situations where: –F net = 0 –F net = ma

5. Two types – Static and Kinetic Static friction – stationary scenarios Kinetic friction – moving scenarios The magnitudes of friction depend on –the surfaces in contact with each other (ie. shoe and floor) –the normal force

6. Static Friction The magnitude of the force needed to start a stationary object moving is the maximum static friction, F S max

7. Kinetic friction Kinetic friction acts on any object that is in motion The magnitude of the force needed to keep the object moving at a constant velocity is the kinetic friction, F K.

8. Coefficients of Friction The coefficient of friction is a ratio of the magnitude of the force of friction between two surfaces to the normal force between those surfaces.

9. Coefficients of Friction Determining μ S and μ K for given substances is done through experimentation. i.e.

10. Practice 1.A crate of fish of mass 18.0 kg rests on the floor of a parked delivery truck. The coefficients of friction between the crate and the floor are μ S = 0.450 and μ K = 0.410. The local value of gravitational acceleration is 9.80 m/s 2. What are the force of friction and the acceleration if: a)a horizontal force of 75.0 N [E] is applied to the crate? b)if a horizontal force of 95.0 N [E] is applied?

11. Practice 2.A musician applies a horizontal force of 17 N [W] to an instrument case of mass 5.1 kg. The case slides across a table with an acceleration of 0.39 m/s 2 [W]. What is the coefficient of kinetic friction between the case and the table?

12. Practice Solutions 1. a)No movement because the F app < F s b)a = 1.26 m/s 2 2. μ K = 0.30