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Recit%205%20ch.%204[1]

Recit%205%20ch.%204[1] - PHY 2053 Spring 2011 Recitation 5...

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Unformatted text preview: PHY 2053 Spring 2011 Recitation # 5 Chapter # 4 Feb 14 – Feb 18, 2011 45. A 20.0-kg sled is being pulled across a horizontal surface at a constant velocity. The pulling force has a magnitude of 80.0 N and is directed at an angle of 30.0 o above the horizontal. Determine the coefficient of kinetic friction. 45. REASONING AND SOLUTION Four forces act on the sled. They are the pulling force P , the force of kinetic friction k f , the weight m g of the sled, and the normal force N F exerted on the sled by the surface on which it slides. The following figures show free-body diagrams for the sled. In the diagram on the right, the forces have been resolved into their x and y components. f k x y m g F N P cos θ P sin θ θ f k x y m g F N P Since the sled is pulled at constant velocity, its acceleration is zero, and Newton's second law in the direction of motion is (with right chosen as the positive direction) k cos x x F P f ma θ =- = = ∑ From Equation 4.8, we know that k k N f F μ = , so that the above expression becomes k N cos P F θ μ- = (1) In the vertical direction, N sin y y F P F mg ma θ = +- = = ∑ (2) Solving Equation (2) for the normal force, and substituting into Equation (1), we obtain...
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Recit%205%20ch.%204[1] - PHY 2053 Spring 2011 Recitation 5...

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