Work and Energy (exam 3)

# Work and Energy (exam 3) - Work and Energy Concept...

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Work and Energy Concept Questions CTEnergy-1. Albert Einstein lowers a book of mass m downward a distance h at constant speed v. The work done by … the force of gravity is. . A) + B) - C) 0 the force of Albert's hand is. . A) + B) - C) 0 the net force on the book is . . A) + B) - C) 0 Answers: The work done by gravity is + since displacement and force of gravity are in the same direction. The work done by Al's hand is (–), since displacement and the force of Al's hand are in the opposite direction. The net force is zero, so the work done by the net force is zero. CTEnergy-2. A rock of mass m is twirled on a string in a horizontal plane. The work done by the tension in the string on the rock is. . A) + B) - C) 0 Answer: The work done by the tension force is zero, since in any tiny interval of time, the force of the tension in the string is perpendicular to the direction of the displacement.

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CTEnergy-3. A 1 kg mass is moved part way around a square loop as shown. The square is 1 meter on a side and the final position of the mass is 0.5 m below its original position. Assume that g = 10 m/s 2 . What is the work done by the force of gravity during this journey? mg h = (1 kg) (10 m/s 2 ) ( h) A) 10 J B) 5 J C) 0 J D) -10 J E) -5 J Answer: Total work done is +5 J. The work done during the horizontal segments is zero (since force is perpendicular to displacement there). The work during the first upward vertical segment is negative and this negative work is cancelled by the positive work done during the first half of the downward vertical segment. The total work is that done during the second half of the downward vertical segment where h = 0.5 m. If instead of moving part way around a square, the mass where taken on a long and tortuous journey to the Moon, Tibet, and Lithuania and then returned to the same finish point as before, would the work done by gravity be the . . A) same, or B) different. Answer: the same! Any journey can be thought of a series of small vertical or horizontal displacements. During any horizontal segment, the work done by gravity is zero. All upward vertical segments are cancelled by corresponding downward vertical segments, EXCEPT for the last 0.5 m between the start and the finish. CTEnergy-4. Two marbles, one twice as heavy as the other, are dropped to the ground from the roof of a building. (Assume no air resistance.) Just before hitting the ground, the heavier marble has. .

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A) as much kinetic energy as the lighter one. B) twice as much kinetic energy as the lighter one.
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## This note was uploaded on 05/25/2008 for the course PHYS 2010 taught by Professor Dubson during the Fall '06 term at Colorado.

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Work and Energy (exam 3) - Work and Energy Concept...

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