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Homework Help: Moments: rotational and translational velocity

  1. Jan 12, 2006 #1
    Hello everyone,

    I'm working on a basic spaceflight simulator during the break, and I've been unable to come up with some info I need to do the physics.

    Suppose you have a simple object... say, a beam of dimensions w, h, and d. Then suppose (for example) you apply a constant force at the point (w/4, h, d/2) on the board. How would you know how what the rotational and translational velocities would be after a given time interval? According to the NASA site I was reading, the resulting rotational and translational velocities vary, and no equation was given for calculating either of them.

    Some pointers in the right direction would be greatly appreciated!

  2. jcsd
  3. Jan 12, 2006 #2
    Incidentally, I don't think this should have been moved. This is NOT a homework question, and if it's so introductory, I don't know why I've not been able to get a straight answer yet (and I'm not referring to this board).
  4. Jan 13, 2006 #3


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    You would have to resolve the force into a translational force and a rotational force. Then use F=ma and equations of motion to calculate the translational velocity and rotational dynamics and moments of enertia to calculate the angular velocity.
  5. Jan 13, 2006 #4
    I guess that's what I'm stuck on: how do I figure out how much is translational and how much is rotational? I know how to sum moments & all that, but according to this website:


    ... pinned and unpinned objects behave differently when moments are applied. But it doesn't say exactly "how".

    "If the object is confined (or pinned) at some location called a pivot, the object rotates about the pivot, but does not translate. The force is transmitted through the pivot and the details of the rotation depend on the distance from the applied force to the pivot. If the object is unconfined and the force is applied at some distance from the center of gravity, the object both translates and rotates about the center of gravity."
  6. Jan 13, 2006 #5


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    As I understand it, the pivot would exert an normal reaction force equal in magnitude but opposite in direction to the other forces (including the weight of the beam), this prevents any translational movement. I don't exactly know how you decide how much energy is imparted into each form. I imagine the relationship will have the variables of force and distance from centre of gravity, but I am not aware of any such formula. Sorry
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