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Unformatted text preview: PHY 2053 Spring 2011 Recitation # 5 Chapter # 4 Feb 14 – Feb 18, 2011 45. A 20.0kg 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 freebody 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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 Spring '09
 Staff
 Force, Friction, Mass, static frictional force

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