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Hemisphere sliding down problem

  1. Jul 14, 2010 #1
    There is a small object, which mass is m on the top of hemisphere, with the mass of M.

    the size of the object is neglectable. Also, the radius of hemisphere is R.

    There is no friction between the hemisphere and the object, and the hemisphere and the surface. What if we slightly hit the object the object will be fall from the hemisphere.

    At what point the mass will completely off from the hemisphere?

    I attatched the the picture.

    I first tried to use energy conservation law to find the velocity of the object.
    [tex]\frac{1}{2} mv^2 + mgR(cos \theta) + \frac{1}{2}MV^2 = mgR [/tex]

    Also by conservation of momentum on x axis, we can deduct

    [tex]mv_x = MV [/tex]

    But I just stucked at here. Plz someone help me

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    Last edited: Jul 14, 2010
  2. jcsd
  3. Jul 14, 2010 #2


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    Let's say that θ in the first equation represents the angle at which the mass flies off. That's one unknown. You also have the final velocities, V and vx that are unknown. So you have three unknowns and only two equations. You need one more equation. What is the condition that must be satisfied if the mass is to fly off? That's your third equation.
  4. Jul 14, 2010 #3
    I found few more equations for that problem.

    First, because the object moves on the hemisphere before it falls off It must satisfy following condition

    [tex]\frac{v_y}{v_x + V} = tan \theta [/tex]

    Also, I am not quite sure about the equation but since the object is having on circular motion (at the frame of hemisphere),
    [tex]\frac{m((v_x + V)^2 + v_y^2)}{R} = mgcos\theta - N[/tex]

    and at the time the object falls from the hemisphere, N will be 0

    Is this right??
  5. Jul 14, 2010 #4


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    In these two equations, you need the horizontal velocity component relative to the hemisphere. I would write that as vx - V to keep V an algebraic quantity. Then when you substitute for V using the momentum conservation equation, you would get vx+(m/M)vx in the expression. This voids confusion.

    So you have four equations (sorry I said three earlier) and four unknowns. You can find an expression for θ in terms of the given quantities.
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