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Homework Help: Rotating ruler

  1. Apr 15, 2010 #1
    1. The problem statement, all variables and given/known data

    We have a rule of mass m and length l lying on a table without friction.

    We give a boost I to the rule at a distance d from the center of mass so that it now has a rotation and a translation.

    1) The rule rotates about which point?

    2) Determine the velocity, vm, of the center of mass.

    3) Determine the angular velocity, w, of the ruler.

    4)What should be the distance d so that the instantaneous displacement ds of the oposite side of the ruler be a minimum?

    2. Relevant equations

    I= Variation momentum

    3. The attempt at a solution

    I think the answer to 1 is that it rotates about it center of mass but I don't have a clue about the other ones
  2. jcsd
  3. Apr 15, 2010 #2


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    You are correct on 1.

    Another word for "boost I" is "impulse I". Does this mean something to you?
  4. Apr 16, 2010 #3
    Yes impulse is the change in momentum but how do I know the direction of the resulting velocity?
  5. Apr 16, 2010 #4


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    Put a ruler on a table and give it a sharp flick with your finger. That's the impulse. In what direction is the impulse? In what direction does the ruler move?
  6. Apr 16, 2010 #5

    Doc Al

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    I suspect they are asking about the instantaneous axis of rotation, which won't be the center of mass. (Since it's also translating.)
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