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Unformatted text preview: Perez (nap563) HW04 TSOI (58160) 1 This print-out should have 20 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 (part 1 of 2) 10.0 points As shown in the figure, a block is pushed up against the wall. Let the mass of the block be m = 2 . 5 kg, the coefficient of kinetic friction between the block and the wall be = 0 . 61, and = 47 . Suppose F = 61 N. The acceleration of gravity is 9 . 8 m / s 2 . 2 . 5 kg F 4 7 k =0 . 61 Find the force of friction. Correct answer: 25 . 3771 N. Explanation: Recall that f = N . From summationdisplay F = 0 in the horizontal direction, one can see that N = F cos . Hence the force of friction is f = N = F cos = 25 . 3771 N . 002 (part 2 of 2) 10.0 points The force, F , which keeps the block mov- ing upwards with a constant velocity satisfies which equations? 1. F cos = mg + F sin 2. F cos = mg F cos 3. F cos = mg + F cos 4. F sin = mg 5. F cos = mg 6. F sin = mg F sin 7. F sin = mg + F sin 8. F sin = mg + F cos correct 9. F cos = mg F sin 10. F sin = mg F cos Explanation: m F v mg f For constant velocity, acceleration is zero. Hence summationdisplay F y = 0 = F sin mg F cos . The first term is the applied upward force, the second term is the weight of the block, and the third term is the frictional force. 003 10.0 points A 3.00 kg block is pushed along the ceiling with a constant applied force of 84.0 N that acts at an angle of 64.0 with the horizontal. The block accelerates to the right at 6.80 m/s 2 . The acceleration of gravity is 9 . 81 m / s 2 . 3 kg 8 4 N 6 4 6 . 8 m / s 2 Perez (nap563) HW04 TSOI (58160) 2 What is the coefficient of kinetic friction between the block and the ceiling? Correct answer: 0 . 356493. Explanation: Basic Concepts: F applied,x = F applied cos F applied,y = F applied sin F y,net = F applied,y mg F n = 0 F x,net = ma x = F applied,x F k F k = k F n m F a Given: m = 3 . 00 kg = 64 . F applied = 84 N a x = 6 . 80 m g = 9 . 81 m / s 2 Solution: F applied,x = (84 N) cos64 = 36 . 8232 N F applied,y = (84 N) sin64 = 75 . 4987 N The normal force is F n = F applied,y mg = 75 . 4987 N (3 kg)(9 . 81 m / s 2 ) = 46 . 0687 N From the horizontal motion, ma x = F applied,x k F n k = F applied,x ma x F n = 36 . 8232 N (3 kg)(6 . 8 m / s 2 ) 46 . 0687 N = 0 . 356493 004 (part 1 of 3) 10.0 points The suspended 2 . 6 kg mass on the right is moving up, the 1 . 9 kg mass slides down the ramp, and the suspended 8 . 8 kg mass on the left is moving down. The coefficient of friction between the block and the ramp is 0 . 14 ....
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- Spring '08