Main Ideas in Class Today

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1 Main Ideas in Class Today Kinetic Friction Static Friction Friction w/inclines Solving Problems Extra Practice: 4.39, 4.41, 4.61, 4.63

2 Test 1 grades on ecampus (out of 20, post-curve) Curve has been implemented (+1 out of 20) Distribution shown is post-curve Clicker grades also on ecampus, both by session and a total percentage as of this morning 45 Frequency More

3 My Advice Look at solution/key on course website Compare your test to solution Go to office hours to ask any parts you don t understand Print blank exam from course website and test yourself again Check answers Go to office hours with more questions Keep up with new material Get help during office hours/find a tutor

4 watch?v=rzehncbu1_g Introduction to Friction YouTube is a wonderful resource for physics videos (in addition to other fun stuff)

5 Friction Kinetic friction: force that opposes the motion of an object when sliding against a surface (even air causes air resistance) Friction is the reason you always have to put energy into machines in order for them to continue working. The amount of friction between two objects depends upon how smooth or rough the surfaces are. No surface is perfectly smooth.

6 Kinetic Friction (f k ): friction when moving, slows you down f k = m k n (Greek letter pronounced mu ) f k m k is coefficient of kinetic friction, depends on materials (table 4.2 in book gives examples) equation valid for magnitudes only ( vector formula) since f k ^ n n F pull Draw a free body diagram mg If you are pulling this, do you want much friction?

7 A cable attached to a cart pulls the cart up the ramp. Which direction should friction point? A. B. C. D. Inclines & Friction E. not enough information given to decide Q19

8 A cable attached to a cart lowers the cart down the ramp. Which direction should friction point? A. B. C. D. E. not enough information given to decide Q20

9 If the cable moves at a constant speed, in which case (if any) would the tension in the chain be larger? n T f k f k A. When car goes up ramp w B. When car goes down ramp C. Both cases would have the same tension D. Not enough information given to determine Q21

10 Stopping distance After brief shove, what is the acceleration of the object? If it starts at a velocity of 10 m/s, how far will it go before it stops? n f k coefficient of kinetic friction is 0.4 mg +x

11 Static Friction Force that keeps object from sliding against a surface when it is at rest f stat m s n m s is coefficient of static friction, also depends on surface equation valid for magnitudes only ( vector formula) Static friction force increases with applied force until object starts moving: F stat,max = m s n F fr =µ s n Once it starts moving, kinetic friction takes over. Why it is easier to move something after it gets started moving.

12 Rolling Along or Not (A little tricky, but useful to know) What kind of friction allows you to roll things? For example, your car tires on the road. A)Static friction B)Kinetic friction C)Both D)Neither

13 A 20 kg sled slides down a 30 hill. A) If there is a coefficient of friction of µ k =0.1, what is the acceleration of the sled? B) If the length of the hill is 20 m, how long does it take the sled to reach the bottom of the hill if it starts from rest? Inclines & Friction 20 m 30 o

14 Hockey puck A hockey puck is struck by a hockey stick and given an initial speed of 20.0 m/s on a frozen pond. The puck remains on the ice and slides 120 m, slowing down steadily (AKA constant acceleration) until it comes to rest. Determine the coefficient of kinetic friction between the puck and the ice. n

15 A. B. C. D. Q22

16 Q23 Forces on Cars (a) (b) A car accelerates down a straight highway. Which of the free-body diagrams shown best represents the forces on the car? (c) (d) (e) None of the Above Friction prevents the wheels from just spinning in place. This is why a car on ice sometimes can t move.

17 A. B. C. D. E. Q24

18 Clicker Answers Chapter/Section: Clicker #=Answer Ch.4: 19=B, 20=A, 21=A, 22=C, 23=D, 24=C

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