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Quiz+8+Solution+Section+C

# Quiz+8+Solution+Section+C - 7B.S11 Quiz 8C Last 6 digits of...

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7B.S11 - Quiz 8C DL SECTION _ Grade: 05/25/2011 No books or notes. Calculators OK. Show all your work below. Answers alone will receive no credit! This quiz has two sides! Anna and Bob each sit on opposite ends of a see-saw which is 10 m long, as shown in the picture. Anna, who has a mass of 50 kg , is 7 m from the pivot and Bob is 3 m from the pivot. Note that the pivot is not in the middle of the see-saw. When Bob and Anna sit on the see-saw, it is completely stationary. The see-saw itself has a mass of 80 kg . a) What is the net torque on the see-saw? What is the net force on the see-saw? Very briefly and clearly explain your reasoning. The see-saw is not rotating nor is its center of mass moving. This means that neither the linear nor the angular momentum of the see-saw is changing . Therefore the net torque and the net force on the see-saw must both be zero. b) Find Bob’s mass . Ignore the thickness of the seesaw. (Hint: An extended force diagram may be useful, and don't forget the force of Earth on the see-saw!). Show your work. There are four forces in the problem, but the pivot force does not exert a torque since it has no moment arm. The three non-zero torques are due to Anna, Bob and the Earth. If we add all of the torques, we must get

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Quiz+8+Solution+Section+C - 7B.S11 Quiz 8C Last 6 digits of...

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