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Unformatted text preview: 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 seesaw 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 seesaw. When Bob and Anna sit on the seesaw, it is completely stationary. The seesaw itself has a mass of 80 kg . a) What is the net torque on the seesaw? What is the net force on the seesaw? Very briefly and clearly explain your reasoning. The seesaw is not rotating nor is its center of mass moving. This means that neither the linear nor the angular momentum of the seesaw is changing . Therefore the net torque and the net force on the seesaw must both be zero. b) Find Bobs 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 seesaw!). 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 nonzero torques are due to Anna, Bob and the Earth. If we add all of the torques, we must get zero, and this will allow us to find Bob's mass. zero, and this will allow us to find Bob's mass....
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This note was uploaded on 10/24/2011 for the course PHYSICS 7B taught by Professor Johnconway during the Spring '07 term at UC Davis.
 Spring '07
 JOHNCONWAY
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