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Trying to help my son

  1. Dec 1, 2004 #1


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    will someone please help me help my son with this physics problem?
    a board is held horizontal by a cable. the cable is vertical (90degrees to the board). the board is 11 meters long. at the left end of the board is a fulcrum at the right end is the cable. 5 meters from the left end is a weight of 100n and 9 meters from the left end is a weight of 150n. what is the force on the cable.
  2. jcsd
  3. Dec 1, 2004 #2
    Whenever you have forces acting on a lever you have torque. In this case the lever is balanced, so the net torque is zero.
    \tau_{down} & = \tau_{up} \\
    (100N)(5m) + (150N)(9m) & = T(11m) \\
    T & = 168N
  4. Dec 2, 2004 #3


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    Thank You For Your Help. I Wasn't Sure Which End Of The Board To Use For Calc The Moment From.
  5. Dec 2, 2004 #4


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    To make sure this is very clear, there is NO definite point that you use to calculate the moment. This is entirely by choice! You can even choose a point on the wall or on the ceiling 3 miles away if you want to (very tall ceiling, that is).

    However, there certainly is a "proper" choice of where to calculate the moment that will make the calculation a lot easier. This is where the "physics" comes in - knowing where to make the most appropriate choice. It is only when one does problems like this, and does them repeatedly can one get a "feel" for where to put this point.

  6. Dec 2, 2004 #5


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    The cable does not provide a torque about the end that the cable is tied to. So, in this case, it would make sense to calculate torques about the other end (or at least, any other point). There's another reason that makes the other end a smart choice : the force exerted by the fulcrum is not known (yet), so calculating moments about a point in line with this force makes the torque due to the fulcrum = 0.

    Without saying this in as many words, this was the idea that ZapperZ was trying to convey (I hope).
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