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Unformatted text preview: hiarker (srh959) homework 09 Turner (58220) 1 This print-out should have 13 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 10.0 points A block is pushed upward along a frictionless inclined plane. m F Determine the horizontal force F on the block which causes it to move upward with a constant velocity. 1. F = m g cot 2. F = m g sin 3. F = m g sin 4. F = m g tan correct 5. F = m g cos 6. F = m g cos Explanation: The block is subject to three forces: its weight vector W , the normal force vector N of the surface, and the external force vector F which pushes it to the right. The external force acts in the hor- izontal direction, the weight acts vertically, and the normal force acts in the direction nor- mal (perpendicular) to the surface leftward from vertically up by angle . Let the x axis point horizontally to the left while the y axix points vertically up. In these coordinates, the net force on the block has components F net x = F N sin , F net y = N cos W. Since the block does not accelerate, both com- ponents must vanish, hence N = W cos = mg cos and F = N sin = mg sin cos = mg tan . 002 (part 1 of 2) 10.0 points The 7 . 1 N weight is in equilibrium under the influence of the three forces acting on it. The F force acts from above on the left at an angle of with the horizontal. The 5 . 9 N force acts from above on the right at an angle of 51 with the horizontal. The force 7 . 1 N acts straight down. F 5 . 9 N 7 . 1 N 5 1 What is the magnitude of the force F ? Correct answer: 4 . 4845 N. Explanation: Standard angular measurements are from the positive x-axis in a counter-clockwise di- rection. Let : F 1 = F , F 2 = 5 . 9 N , 2 = 51 , and F 3 = 7 . 1 N . Consider the free body diagram. The green vectors are the components of the slanted forces. F 1 F 2 F 3 2 hiarker (srh959) homework 09 Turner (58220) 2 The weight is is equilibrium, so summationdisplay F x = F 1 x + F 2 x + F 3 x = 0 F 1 x = F 2 cos 2 = (5 . 9 N) cos 51 = 3 . 71299 N and summationdisplay F y = F 1 y + F 2 y + F 3 y = 0 F 1 y = F 2 sin 2 F 3 = (5 . 9 N) sin51 ( 7 . 1 N) = 2 . 51484 N...
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