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Unformatted text preview: Physics 53 Summer 2008 1 Exam II Solutions Part A: Multiple choice questions. Check the best answer. Each question carries a value of 4 points. 1. A system is subject to only two external forces, of equal magnitude. If the forces are in opposite directions but not along the same line, there is at least one point about which total angular momentum is conserved. [Total torque is ( r 2 ! r 1 ) " F , independent of the reference point and never zero.] If the forces are in opposite directions and along the same line, total angular momentum is conserved about every point. [Now r 2 ! r 1 is parallel to F .] If the forces are in the same direction but not along the same line, there is no point about which total angular momentum is conserved. [About the point halfway between them the total torque is zero.] If the forces are in the same direction and along the same line, there is no point about which total angular momentum is conserved. [About any point on that line the total torque is zero.] 2. You are at one end of a platform on wheels that can roll freely on a horizontal floor. You and the platform are at rest. The platform has length ! , and you and the platform have the same mass m . You walk to the other end of the platform and stop. Which of the following is NOT true? The CM of the system of you plus platform does not move. Total horizontal momentum of the system is conserved. The platform moves across the floor distance ! /2 during your walk. You do not move at all relative to the floor. [You move distance ! /2 the other way.] 3. The door shown has two hinges supporting it. Over time: The screws holding the bottom hinge to the door and the wall tend to work loose, because the door pulls on the hinge. The screws holding the top hinge to the door and the wall tend to work loose, because the door pulls on the hinge. Both sets of screws tend to pull loose, because the door pulls on both hinges. There is no force pulling either set of screws loose. [Consider torques about the bottom hinge.] Physics 53 Summer 2008 2 4. A satellite is in an elliptical orbit around the earth. Points A and B are at opposite ends of the major axis of the ellipse, and the distance between them is 2 a . The earth is at the center of the ellipse. This orbit has less energy than a circular orbit of radius a....
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