2. Announcements CAPA Set #6 due Friday at 10 pm This week in Section  Lab 2: Acceleration due to gravity Make sure to do the pre-lab and print materials before lab * Slight mismatch of pre-lab file and web input. Just one simple question added on web input. Follow web input. Read Chapter 5 Sections 5.1-5.5 (Circular Motion) Advanced reminder  Exam #2 on Tuesday, October 11

3. Friction Why have you been ignoring me all this time? Force resisting the movement of two objects in physical contact past each other. Force of Friction depends on: 1. Characteristics of materials 2. Vertical force pushing the materials together

4. Coefficient of Friction (  ) When we try to push this green block sideways… The Force of Friction acts in the opposite direction to resist this pushing force. Maximum Force of Friction = coefficient of friction (  ) x Force pushing surfaces together (Normal Force) F f (max) =  N

5. Static versus Kinetic Friction When an object is not moving (static), molecules can form more bonds and materials can deform into each other. Thus, the coefficient of friction can be larger (call it coefficient of static friction  s ). Once the object is moving, friction is lower (call it the coefficient of kinetic friction  k ). Also, the force of friction never creates an acceleration, it always acts to oppose motion caused by some other effect.

6. Demonstration with Newton Meter and Blocks Idealization – Not an Exact Law of Physics

7. Clicker Question Room Frequency BA You are pushing horizontally with a force of 5000 Newtons on a car that has a weight of 10,000 Newtons. The car is not moving. What can you say for certain about the coefficient of friction? A)  s = 0 B)  s = 0.1 C)  s = 0.5 D)  k = 0.5 E) None of the above F net = 0 = F push – F friction 0=F push –  s x Normal  s x Mg = F push  s = F push /Mg = 5000/10k  s = 0.5 But actually,  s could be even larger since we do not know if we have reached the maximum.

8. Surfacesµ (static)µ (kinetic) Steel on steel0.740.57 Glass on glass0.940.40 Metal on Metal (lubricated)0.150.06 Ice on ice0.100.03 Teflon on Teflon0.04 Tire on concrete1.000.80 Tire on wet road0.600.40 Tire on snow0.300.20

9. A block of mass m is being pushed across a rough horizontal table. A constant velocity v is maintained with an external force F ext. What is μ K ? - -

10. M m An object with mass M is resting on a rough table whose coefficients of static and kinetic friction are μ S and μ K, respectively. Which of the following is a necessary condition to start the object in motion? A) M > m B) M < m C) mg > μ K Mg D) Mg > μ S mg E) mg > μ s Mg mg f max = -μ S N = -μ S Mg T=mg (little block) T>  s N (large block) For motion to begin: mg > μ S Mg Clicker Question Room Frequency BA T T Mg N=Mg

11. A block of mass m is pulled across a rough (μ K ) flat table with a constant force F ext at an angle θ. - - Clicker Question Room Frequency BA What is the correct expression for the magnitude of the Normal force to use for calculating the friction force? A)N=mg B)N=mg sin  C)N=mg sin  F ext sin  D)N=mg  –F ext cos  E)None of the above F net,y =ma y =0 0=N-mg+F ext sin  N=mg-F ext sin 

12. A block of mass m slides down a rough (μ K ) incline tilted at an angle θ from the horizontal. N F g = mg θ θ mg sinθ mg cosθ +x +y f = μ K N

13. Circular Motion