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Unformatted text preview: huynh (hph268) HW #4 clark (56485) 1 This print-out should have 19 questions. Multiple-choice 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