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B Why can we choose any arbitrary point to calculate the moment?

  1. Mar 26, 2017 #1

    xyz

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    Can someone explain, what means that we can choose any arbitrary point to calculate moment and result will always be identical?

    If I choose point 1 ...20kg x 1m = 20kg x 3m.......seesaw right side will fall down
    If I choose point 2....20kg x 2m = 20kg x 2m.......seesaw stay horizontal

    So this two moments are not the same...Fulcrum point in nature is always just one point,how then we can choose any point we wont??
    safgdfg.png
     
  2. jcsd
  3. Mar 26, 2017 #2

    A.T.

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    There are 3 forces acting on the seesaw, not just 2.
     
  4. Mar 26, 2017 #3

    xyz

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    I know,there is reaction force on fulcrum which is equal to total weight (40kg)..

    But what does mean that we can choose any point for calculate moment?
     
  5. Mar 26, 2017 #4

    A.T.

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    So why do you ignore it in your net moment calculations?
     
  6. Mar 26, 2017 #5
    Not clear what you mean by "result." The net moment (torque) will be different depending on the choice of point. The more general statement is that net moment about any point is proportional to the rate of change of angular momentum about that point.

    Here is an earlier thread with a similar discussion: https://www.physicsforums.com/threads/rotational-equilibrium-torque-about-any-axis.440631/
     
  7. Mar 27, 2017 #6

    A.T.

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    Unless it's zero, like in this case.
     
  8. Mar 27, 2017 #7
    Your comment led me to review some ancient history. If the net moment is zero about one point, it is not necessarily zero about all other points. That is only true if the net force is zero.

    Here's a discussion of this: http://physics.stackexchange.com/qu...red-from-one-point-in-space-it-equals-zero-me

    As shown there, in general the moment transfer from point A to point B is given by Mb = Ma + rabxF where rab is the position vector from a to b and F is the net force. So if Ma is zero and the net force is zero, then Mb is also zero.
     
  9. Mar 27, 2017 #8

    A.T.

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    Good point, but that's also the case here.
     
  10. Mar 27, 2017 #9

    xyz

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    Thanks pixel!
    Now is clear to me...This is the key;

    The sum of the torques on any point is equal, only when the sum of the forces is zero
     
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