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Some Mechanics Questions

  1. Sep 16, 2009 #1
    Hi, I've been learning about moment and there's a lot of things I don't understand

    First, in the formula moment = I * alpha, what does I represent and how do we calculate it? All the formulas that I've seen are defined as M = Fr.

    Also, I'm wondering, if, say, two people are playing tug of war, they are leaning back. Why must a person lean back to exert force? Also, as they are leaning back, how does couples of moments relate to the fact that they don't start spinning?

    I have another problem regarding tension. What is the definition of tension exactly and how do we determine which direction it goes (ex. a rock is hanging on the rope, i know that tension goes up, but why)? Also, when the two people are pulling a rope and both are exerting 100N of force, why is the tension of the rope 100N and not 200n or 0 N

    Thanks for any help that you can provide
    Last edited: Sep 16, 2009
  2. jcsd
  3. Sep 16, 2009 #2


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    I is the http://en.wikipedia.org/wiki/Moment_of_inertia" [Broken], a body's resistance to rotating due to a moment.

    Your other questions will become clear, I think, if you look into how to construct a free body diagram (check any statics book) and how to use static equilibrium equations (e.g., the sum of the forces in the x-direction is zero, etc.).
    Last edited by a moderator: May 4, 2017
  4. Sep 17, 2009 #3

    Andy Resnick

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    Unless I misunderstand your first formula, what you wrote is incorrect- the torque is equal to I*alpha. The moment equation M=Fr is close to a torque=(Fr*sin(angle between the two)), but in your context 'moment' can be considered a more sophisiticated version of torque.

    Have you ever played tug-of-war? Or pulled on anything heavy? You need to lean back for a few reasons: one, to engage your leg muscles and take the strain off your lower back, and two, by getting lower than the other person gives you additional mechanical advantage. You don't spin becasue of the ground.

    The best explanation of tension I ever got was posted here (I forget by whom, sorry)- in order to understand tension in a rope, simply cut the rope. What happens to the ends of the rope? Does it matter where you cut the rope?
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