Solution #2 fall 2007

# Solution #2 fall 2007 - Physics 2211 Fall 2007 Quiz#2...

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Physics 2211 Quiz #2 Solutions Fall 2007 g Earth = 9 . 8m / s 2 Unless otherwise directed, all springs, cords, and pulleys are ideal, and drag should be neglected. I . (16 points) A block of mass m is being slid to the right along a horizontal ceiling by an applied force of magnitude A that makes an angle θ with the vertical, as illustrated. The coeﬃcient of static friction between the block and the ceiling is μ s , while the coeﬃcient of kinetic friction is μ k . What is the acceleration magnitude the block in terms of any or all of m , A , μ s , μ k , and physical or mathematical constants? ( On Earth. ) . . . . . . . . . . . . . . . . . . . . . . . As announced during the quiz, the acceleration may depend on θ . Use Newton’s Second Law. Sketch a Free Body Diagram. X F x = A x - f k = ma x A sin θ - μ k n = ma x X F y = A y - n - w = ma y = 0 n = A cos θ - mg Substituting this expression for the normal force, n , into the x -equation above, A sin θ - μ k ( A cos θ - mg ) = ma x a x = A sin θ - μ k ( A cos θ - mg ) m Quiz #2 Solutions Page 1 of 5

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II . (16 points) Zach, of mass m , is riding upward in an elevator when the ﬂoor partially breaks free and hangs at an angle θ below the horizontal! Fortunately, the safety system of the elevator detects the problem and brings the elevator to a stop with acceleration magnitude a . Also fortunately, the coeﬃcient of static friction, μ s , is just suﬃcient to prevent Zach from sliding out of the gap as it slows. What is Zach’s apparent weight as the elevator slows, in terms of any or all of m , θ , μ s , a , and physical or mathematical constants? (
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Solution #2 fall 2007 - Physics 2211 Fall 2007 Quiz#2...

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