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Homework Help: Static equilibrium marbles problem

  1. Apr 2, 2009 #1
    Hey all, new member here
    1. The problem statement, all variables and given/known data
    Two uniform 75g marbles 2cm in diameter are stacked on top of each other in a container that is 3cm wide
    the top marble touches right side of the container, the bottom touches the left side.
    Point A is the bottom marble left contact point with the container, point B is the bottom marble touch the bottom of the container, point c is the top marble's contact point with the container

    Attempt at a figure:
    l top marble, point Cl
    lmarble contact pointl
    lpoint A, bottom marble l
    l_____point B________l
    What is the force the container exerts at point A,B, and C?
    What force does each marble exert on each other?

    2. Relevant equations
    Basically since its static equilibrium, , all force and torques add up to zero. Also the equation for Torque, T=sin(phi)*lever arm

    3. The attempt at a solution

    force at B is easy to get since thats just the normal force equal to the combined weight of the two marbles.
    At A and C, its obvious that they're equal. I can also get the torque due to the top marble on the bottome marble: weight*(.01m)*(sin(30)) after that I don't know what else to do.
  2. jcsd
  3. Apr 2, 2009 #2


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    Science Advisor
    Homework Helper

    Welcome to PF!

    Hey, krko ! Welcome to PF! :smile:
    Is this frictionless?

    You're taking moments about the point of contact between the marbles … why? :confused:

    If you take moments (torques) about a point, you must do it for all the forces … that's why we usually choose a point about which some of the forces have zero moment.

    Try again, taking moments about a different point. :smile:
  4. Apr 2, 2009 #3
    Okay, i uploaded an image of the problem. The torque i'm talking about is the torque caused by the top marble about point the center of the bottom marble. Although looking back i think I found my mistake, that I neglected the effect of the force of the container on point C on the bottom marble.

    Attached Files:

    • img.bmp
      File size:
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