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Torque is same all along lever?

  1. Oct 15, 2014 #1
    Here's my background so you know where I'm having trouble. I've attached a picture.

    I have to find the force produced by the muscle to keep the arm and ball at its current position in the picture. Now, I originally assumed Torque was Force in rotation, and intuitively thought you would need to use more force if you're closer to the pivot (shorter lever arm) than compared to the end (longer lever arm). I was right about force needed being greater as the lever arm got shorter, but I'm having touble understanding why the Torque along the lever must be the same. So, now I know Torque and Force are not the same.
    Mathematically, I know there's only one right answer, but how about conceptually? What is torque if it's not force? What does it represent? Why isn't the torque increasing as the lever arm length decreases?


    Attached Files:

    Last edited: Oct 15, 2014
  2. jcsd
  3. Oct 15, 2014 #2
    Torque is the perpendicular force times the distance to the pivot point. It is completely different from force. it is F*d.
    Given that, you can see that as d decreases, so does the torque.

    In this case, in order to hold the weight steady, the two torques must be equal. Because d for the muscle is much less than the other, it requires a much larger force.

    does that help?
    Last edited: Oct 15, 2014
  4. Oct 15, 2014 #3
    sorry, it still hasn't clicked. I mean, mathematically I agree with what you've said, but I still can't understand what Torque represents. What is it that makes it the same everywhere along the lever?
  5. Oct 15, 2014 #4
    it is not the same everywhere along the lever. torque is only defined about a pivot point. For the arm to hold the weight steady, the two torques about that point must be equal.

    I am at a loss for offering an easy conceptual understanding. I just think of it as F*d.
  6. Oct 15, 2014 #5
    wait I think I've got it now. I was originally thinking of the upward force that the forearm would provide to support the ball, but now I'm thinking of the ball dragging down the forearm. And to balance that we need an upward force and the torques have to equal. Thanks a bunch!
  7. Oct 15, 2014 #6
    you're welcome :)
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