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Calculating weight using a Seesaw=

  1. Feb 5, 2009 #1
    1. The problem statement, all variables and given/known data
    Describe how one could find the weight of some unmarked object using only a stick of known weight. Assume that the unknown weight can be attached to the stick and that we know about torques and centers of mass.

    2. Relevant equations
    I'm not sure this seems to be my problem.
    Torque = lever arm x force perpendicular to level arm?

    3. The attempt at a solution
    I don't know. I know it has something to do with the distance to the axis and the center of mass, but I don't know.
  2. jcsd
  3. Feb 5, 2009 #2


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    is there a diagram given with distances to pivot point etc?
    are you given the torques for a particular configurations of the system?
  4. Feb 5, 2009 #3
    No, I don't have any measurements. The post is all the information I have. No diagrams...I'm just supposed to know how to to figure it out. I thought it might be the weight of the stick x the distance to the pivot point? I'm just not sure...
  5. Feb 6, 2009 #4


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    ok, it seems that the question is asking you to describe what you may try to achieve your aim. well, since the object can be attached to the stick and the stick's weight is known (as well as all torques). The simplest situation to consider is when you suspend the stick (with the object attached somewhere on it, doesn't matter where, eg. as long as the object distance to the suspended location is known), you arrange it such that the stick-object system balances perfectly (ie. in equilibrium), then net torque about the point of suspension must be zero and you can then work out the appropriate weight for the object for that to happen, given the stick's center of mass location, its weight and other torques (which I believe is supposedly known).

    of course, it can also be done when the system is not in equilibrium, you just need to know the value of the net torque.
  6. Feb 6, 2009 #5
    The question is saying assuming I know about torques. I don't know the torque of the problem I am trying to solve.

    So I would have to measure the distance from the center of mass to the position where the weight is suspended. Then multiply that distance by the weight of stick. That result should be my unknown weight?(of course doing so while the stick is in equilibrium)

    Lets assume the weight of stick is 2N, and to balance the the stick I have to place the unknown weight at 30 cm. My unknown weight would be .3Nm?
  7. Feb 7, 2009 #6


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    where is the location of the centre of mass of the stick relative to the point of suspension?

    result: torque due to centre of mass of stick equals torque from unknown weight 30 cm away
    then you have equilibrium.
  8. Feb 9, 2009 #7
    Thanks for you help!
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