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Homework Help: Block slides from rest from the top of a fixed frictionless sphere

  1. Nov 12, 2008 #1
    can anyone help me to solve this question? please and thank you :D

    A small block slides from rest from the top of a fixed frictionless sphere of
    radius R.

    Determine the vertical distance traveled by the block, x , where it loses
    contact with the surface of the sphere and its speed at this point.
  2. jcsd
  3. Nov 13, 2008 #2
    let m be the mass of the block.
    let y be the angular displacement of the block between its starting point and the point in which it loses contact with the sphere.

    we can see that cos(y) = (R-x)/R, x = R[1 - cos(y)]

    since there is conservation of mechanical energy, loss of gravitational potential energy will result in a gain in kinetic energy of the block, mgx = (1/2)mv2
    hence, v2 = 2gx = 2gR[1 - cos(y)]

    next, as the block moves down the sphere, a portion of its weight must act radially into the sphere so as to provide the required centripetal force for the motion of the block.
    hence, required centripetal force = mg*cos(y) = mv2/R
    substituting the value of v2 found previously into the equation, mg*cos(y) = 2mgR[1 - cos(y)]/ R
    hence, rearranging the terms, we get cos(y) = 2/3

    since x = R[1 - cos(y)], x = R(1 - 2/3) = R/3

    since v2 = 2gx = 2gR[1 - cos(y)], v2 = 2gR/3, v = (2gr/3)1/2

    is it correct?

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  4. Nov 13, 2008 #3
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