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PH 211 Winter 2014 Friday February 14

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Lab calendar update > Tuesday 2/18NO LAB Wednesday 2/19MIDTERM 2 Thursday 2/20Lab 5

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Midterm pickup Tuesday 14:00 – 15:30 Compare your exam and solutions. Come to office hours for questions. If asking for re-grading, use form from web- site, and follow procedure.

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Google moderator Which topic would you like to discuss in class on Monday? http://www.google.com/moderator/#16/e=20e 597

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Google moderator Which topic would you like to discuss in class on today? http://www.google.com/moderator/#16/e=20e 889

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On question 6.44, the problem states that a bag is dropped on a conveyor belt moving at a constant velocity. I am confused about how to determine the acceleration and/or force imparted on the bag." "Can you explain how to do problem 6.28 from MasteringPhysics? "A 1400 kg steel beam is supported by two ropes." rope 1 is 20° left of vertical, rope 2 is 30° right of vertical. What is the tension in ropes 1 and 2?"

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A box on a rough surface is pulled by a horizontal rope with tension T. The box is not moving. In this situation: A. f s > T. B. f s = T. C. f s < T. D. f s = s mg. E. f s = 0. Slide 6-75

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A box on a rough surface is pulled by a horizontal rope with tension T. The box is not moving. In this situation: A. f s > T. B. f s = T. C. f s < T. D. f s = s mg. E. f s = 0.

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A box has a weight of 100 N. It sits on the floor. It is then pulled by a 30 N horizontal force. Does the box move? A.Yes B.No C.Not enough information to say. Slide 6-78

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A box has a weight of 100 N. It sits on the floor. It is then pulled by a 30 N horizontal force. Does the box move? A.Yes B.No C.Not enough information to say.

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An object is held in place by friction on an inclined surface. The angle of inclination is increased until the object starts moving. If the surface is kept at this angle, the object 1.slows down. 2.moves at uniform speed. 3.speeds up. 4.none of the above 5.There’s not enough information to answer this

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An object is held in place by friction on an inclined surface. The angle of inclination is increased until the object starts moving. If the surface is kept at this angle, the object A.slows down. B.moves at uniform speed. C.speeds up. D.none of the above E.There’s not enough information to answer this

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Friction depends on normal force Static friction: f s ≤μ s F normal, opposes cause Kinetic friction: f k =μ k F normal, opposes velocity

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But how about this?

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In case a the weight of the hanging block is 10 N, in case b the tension force is 10 N. Compare the accelerations of the two systems. (the big block is the same in both cases) The acceleration is: 1.The same 2.Greater on the left 3.Greater on the right 4.I have no idea

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In case a the weight of the hanging block is 10 N, in case b the tension force is 10 N. Compare the accelerations of the two systems. (the big block is the same in both cases) The acceleration is: 1.The same 2.Greater on the left 3.Greater on the right 4.I have no idea

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A compact car and a large truck collide head on and stick together. Which vehicle undergoes the larger acceleration during the collision? 1.Car 2.Truck 3.Both experience the same acceleration. 4.Can’t tell without knowing the final velocity of the combined mass.

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A compact car and a large truck collide head on and stick together. Which vehicle undergoes the larger acceleration during the collision? A.Car B.Truck C.Both experience the same acceleration. D.Can’t tell without knowing the final velocity of the combined mass.

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Newton 3 a b Act on different objects!

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If F ab and F ba are not the same we get spontaneous motion!

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Action-reaction pairs

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And now …. this…..It is moving!

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T1T1 m1m1 m1gm1g

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And now …. this…..Still moving

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T2T2 m2m2 m2gm2g fkfk FnFn

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And now …. this…..It is moving! z s Δz=Δs→a 1 =a 2 T1T1 T2T2 T 1 = T 2

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T1T1 m1m1 m1gm1g

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T2T2 m2m2 m2gm2g fkfk FnFn

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Demo http://www.physics.umn.edu/outreach/pforce/c ircus/ Hammer and nail

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