# HW9.sol - homework 09 RAHMAN TARIQUE Due 3:00 am Question 1...

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homework 09 – RAHMAN, TARIQUE – Due: Feb 25 2008, 3:00 am 1 Question 1, chap 6, sect 1. part 1 of 1 10 points In order to slide a heavy desk across the ±oor at constant speed in a straight line, you have to exert a horizontal force of 500 New- tons. Compare the 500-Newton horizontal push- ing force F to the frictional force f between the desk and the ground. 1. F > f ; the frictional force is always less than any other force acting on the object. 2. It depends on which direction the desk is moving. 3. F = f from Newton’s ²rst law. correct 4. F = f from Newton’s third law. 5. F < f ; the frictional force is greater than the pushing force since it is such a heavy desk. Explanation: Since the desk moves at constant speed in a straight line, Newton’s ²rst law tells us the two forces must balance. Question 2, chap 6, sect 1. part 1 of 1 10 points When an object is moving in the air, the air drag force is in the opposite direction to the velocity. Consider a light foam ball that is thrown up into the air. When is the net force on the ball greatest? 1. When the ball moves down 2. When the ball is at the top of its trajec- tory 3. When the ball moves up correct 4. The net force is constant. Explanation: Let f be the air drag and F the force of gravity. F always acts down and f opposes the direction of motion. When the ball moves up, both the air drag and gravity act downward, so the net force is F + f downward. When the ball moves down, the air drag pushes up and gravity pulls down, so the net force is F f downward. When it is at the top of its trajectory, the air drag is zero, so the net force is F downward. F f < F < F + f Question 3, chap 6, sect 1. part 1 of 1 10 points The force F exerted on the block pushes the block against the ceiling and at the same time accelerates the block to the right. m F θ μ a Find the kinetic friction between the block and the ceiling. 1. f k = μ ( F sin θ mg ) correct 2. f k = μ ( F sin θ + mg ) 3. f k = μ F sin θ 4. f k = μ mg Explanation: The net normal force which pushes against the ceiling is F sin θ mg . So the kinetic friction is f k = μ ( F sin θ mg ). Question 4, chap 6, sect 1. part 1 of 3 10 points Consider a force pulling 3 blocks along a rough horizontal surface, where the masses are a multiple of a given mass m , as shown in the ²gure below. The coe³cient of the kinetic

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homework 09 – RAHMAN, TARIQUE – Due: Feb 25 2008, 3:00 am 2 friction is μ . The blocks are pulled by a force of 90 μ mg .
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HW9.sol - homework 09 RAHMAN TARIQUE Due 3:00 am Question 1...

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