2. Work 07/28/09 Physics 103, Spring 2009, U. Wisconsin Requires non-zero external force (F) and displacement ( x) in the direction ( ) of the force Units are Newton-meter defined as a joule also 1 calorie (energy raises a gm of water 1 degree C) 1 cal = 4.186 J + work is done on the object by the external force, if there is a displacement in the same direction - work is done on an object by an external force, if there is a displacement in the opposite direction. W = F x cos
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4. Question 1 07/28/09 Physics 103, Spring 2009, U. Wisconsin A woman holds up a bowling ball in a fixed position. The work she does on the ball 1. Depends on the weight of the ball. 2. Cannot be calculated without more information. 3. Is equal to zero. Although the woman is exerting force against gravity to hold the bowling ball up, she has not shifted its position. Therefore, the work done by her on the ball is zero.
5. Question 2 07/28/09 Physics 103, Spring 2009, U. Wisconsin A man pushes a very heavy load across a horizontal floor. The work done by gravity on the load 1. Depends on the weight of the load. 2. Depends on the coefficient of kinetic friction between the load and the floor. 3. Is equal to zero. The load is moving horizontally, where as gravitational force is vertical.
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11. Converting Form of Energy 07/28/09 Physics 103, Spring 2009, U. Wisconsin
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13. Question 3 07/28/09 Physics 103, Spring 2009, U. Wisconsin When you do positive work on an object, its kinetic energy 1. increases. 2. decreases. 3. remains the same. 4. need more information about the way the work was done Work-energy theorem:
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15. Question 4 07/28/09 Physics 103, Spring 2009, U. Wisconsin 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 exerted along the short path, F av , the average force you exert along the longer path is 1. undetermined, because it depends on the time taken 2. F av / 2 3. F av 4. 2 F av Gravitational potential energy gained is the same for both cases It is equal to average force exerted times distance Since distance traveled is twice, the F av is one-half correct
16. Question 5 07/28/09 Physics 103, Spring 2009, U. Wisconsin Two marbles, one twice as heavy as the other, are dropped to the ground from the roof of a building. Just before hitting the ground, the heavier marble has 1. as much kinetic energy as the lighter one 2. twice as much kinetic energy as the lighter one 3. half as much kinetic energy as the lighter one 4. no kinetic energy Final velocity of the two marbles is the same Kinetic energy is proportional to mass correct
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18. Question 6 07/28/09 Physics 103, Spring 2009, U. Wisconsin The potential energy of a spring is 1. proportional to the amount the spring is stretched. 2. proportional to the square of the amount the spring is stretched. 3. proportional to the amount the spring is compressed. Stretching
The force that accompanies the “capability” takes various forms (labeling the energy). Energy (capability) can increase or decrease. That is, energy can be decreased by exerting a force that does work, and can be increased if an external force does work.
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