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Homework Help: A few conceptual problems with moments

  1. Feb 2, 2012 #1
    1. The problem statement, all variables and given/known data
    My questions are as follows:
    (1) If I apply a force (B) on a beam with a center of gravity at the middle(A), it will JUST cause a moment, correct?
    (2) If I remove this fixed point(A), and no gravity or any other forces are present, will it still JUST cause a moment or will it cause a shift?
    (3)Can a force be applied on this beam to cause a moment & a movement?

    2. Relevant equations


    3. The attempt at a solution

    I think,
    (1) It will just cause a moment, not a downwards motion
    (2) It will either just cause an acceleration or just a moment
    (3) If I move the force along an angle, it will cause both
  2. jcsd
  3. Feb 3, 2012 #2


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    Homework Helper

    It depends on how you define 'moment'. From the way your questions are worded, it looks like you are defining 'moment' to be a rotational motion around the point A. (And that point A is moving with the beam, so point A is always at the centre of mass).

    The questions are a bit vague. I'd say you've got question 1) correct, because if it is fixed at A, then the only motion must be rotation around A.

    Question 2) could mean a lot of things. The initial change in motion is clear, but then after that, it depends what direction the force is in. Is it always downwards, or does its direction change so that it is perpendicular to the axis of the beam?

    For question 3), you are right that both rotational and translational motion can be caused. But I'm not sure what you mean by 'move the force along an angle, it will cause both'. And why do you think it is necessary to alter the force to cause both types of motion?
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