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Unformatted text preview: huynh (hph268) HW #4 clark (56485) 1 This printout should have 19 questions. Multiplechoice questions may continue on the next column or page find all choices before answering. 001 10.0 points Two identical objects are moving directly to ward one another at the same speed v . vectorv vectorv m m What is the total kinetic energy of the sys tem of the two objects? 1. Zero since ( vectorv vectorv ) 2 = 0 2. None of these 3. 4 mv 2 since (2 v ) 2 = 4 v 2 4. Still 1 2 mv 2 from the definition of kinetic energy 5. mv 2 since energy is a scalar correct Explanation: Since kinetic energy is an additive scalar, the total kinetic energy of the system of two objects is 1 2 mv 2 + 1 2 mv 2 = mv 2 . 002 10.0 points An object with mass M is attached to the end of a string and is being lowered vertically at a constant acceleration of g 6 . If it has been lowered a distance from rest, how much work has been done by the tension in the string? 1. M g 6 2. M g 6 3. M g 3 4. 5 M g 6 correct 5. M g 3 6. M g 7. M g 8. 5 M g 6 Explanation: Using Newtons Second Law, F net = T + M g j M a = T j M g T = j parenleftbigg M g M g 6 parenrightbigg = j parenleftbigg 5 M g 6 parenrightbigg , so the work done when x = j is W = F x = parenleftbigg 5 M g 6 parenrightbigg ( ) = 5 M g 6 . 003 10.0 points A force F is exerted by a broom handle on the head of the broom, which has a mass m . The handle is at an angle to the horizontal, as shown below. F What is the work done by the force on the head of the broom as it moves a distance d across a horizontal floor? 1. W = F m tan 2. W = F d sin 3. W = F m cos 4. W = F md sin 5. W = F d cos correct huynh (hph268) HW #4 clark (56485) 2 Explanation: By definition, the work done by a force is given by the formula W = vector F vector d = F d cos , where vector d is the displacement vector, and is the angle between vector F and vector d . Simply apply the definition, the work is W = F d cos . 004 10.0 points A block of mass m is pushed a horizontal dis tance D from position A to position B, along a horizontal plane with friction coefficient . Then m is pushed from B to A. If the force pushing m from A to B is vector P , and the force pushing m from B to A is vector P , what is the total work done by friction? 1. 2 ( mg P ) D 2. 3. 2 mg D correct 4. 2 ( P mg ) D 5. +2 mg D Explanation: From A to B, the work done by friction is mg D . From B to A, the work done by frition is mg D too since the work done by kinetic friction is always negative. So the total work done by friction is equal to the sum of these two works, and we get...
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This note was uploaded on 09/28/2011 for the course PH 303K taught by Professor Zao during the Spring '11 term at University of Texas at Austin.
 Spring '11
 Zao
 Physics

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