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Conservative and non-conservative Forces Conservative forces – have the following properties: 1.Work done by a conservative force on a particle moving between two points is independent of the path taken by the particle. 2.The work done by a conservative force on a particle moving through an closed path is zero. Non-conservative forces – Do not satisfy either of the two properties of conservative forces. Examples: Gravity, elastic (spring) force Example: Friction Equilibrium Compressed Equilibrium Spring is compressed Stored energy increases Spring is released Stored energy decreases Friction acts on the book as it is slid across the table. Is the amount of work done by friction the same for both paths? Is the amount of work done by friction on the book zero if it is moved forward along path 1 then backward along path 1? 1 2 No, more work is done over the greater distance. No, twice the work is done.

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At the bowling alley, the ball-feeder mechanism must exert a force to push the bowling balls up a 1.0-m long ramp. The ramp leads the balls to a chute 0.5 m above the base of the ramp. Approximately how much force must be exerted on a 5.0-kg bowling ball? 1. 200 N 2. 50 N 3. 25 N 4. 5.0 N 5. impossible to determine

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Suppose you want to ride your mountain bike up a steep hill. Two paths lead from the base to the top, one twice as long as the other. Compared to the average force you would exert if you took the short path, the average force you exert along the longer a path is 1. four times as small. 2. three times as small. 3. half as small. 4. the same. 5. undetermined—it depends on the time taken.

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A piano mover raises a 100-kg piano at a constant rate using the frictionless pulley system shown here. With how much force is he pulling on the rope? Ignore friction and assume g = 10 m/s 2. 1. 2,000 N 2. 1,500 N 3. 1,000 N 4. 750 N 5. 500 N 6. 200 N 7. 50 N 8. impossible to determine

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A 50-kg person stands on a 25-kg platform. He pulls on the rope that is attached to the platform via the frictionless pulley system shown here. If he pulls the platform up at a steady rate, with how much force is he pulling on the rope? Ignore friction and assume g = 10 m/s 2. 1. 750 N 2. 625 N 3. 500 N 4. 250 N 5. 75 N 6. 50 N 7. 25 N 8. impossible to determine T T T

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Suppose you drop a 1-kg rock from a height of 5 m above the ground. When it hits, how much force does the rock exert on the ground? 1. 0.2 N 2. 5 N 3. 50 N 4. 100 N 5. impossible to determine

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A pendulum, shown in the photograph below, is lifted to the 30 cm level toward the left of the picture and released. When it falls down to its minimum level, just below the suspension point, it encounters a peg at the 35 cm level. What will then happen? In particular, how high will the pendulum rise as it moves to the right? The pendulum bob will: (1) rise to a height less than 30. (2) rise to a height of 30. (3) rise to a height of greater than 30. (4) loop around the lower peg.

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