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Homework Help: Impulse and Momentum of a puck

  1. Nov 2, 2007 #1
    A 100g puck is revolving in a 20-cm-radius circle on a frictionless table. The string passes through a hole in the center of the table and is tied to two 200g weights.

    Suppose a flame burns through the string and causes the lower weight to fall off while the puck is revolving. What will be the puck's speed and the radius of its trajectory after the weight drops?

    I was able to find the speed prior to the flame burning through the string. But after that, I have no clue how to solve for the radius and speed.
  2. jcsd
  3. Nov 2, 2007 #2


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    It is a trick question.
  4. Nov 2, 2007 #3
    Why is it a trick question?

    I would have thought what would happen is the tangential velocity would be maintained while radius would vary such that centripetal acceleration would equal to the gravitational force acting on the 100g weight left?
  5. Nov 2, 2007 #4

    Shooting Star

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    Hi 2rip,

    Suppose the initial radius is r1 (given). Afterward, let the radius be r2.

    Initially, centripetal force = tension in string = weight of 200g.

    Afterward, centripetal force== tension in string =weight of 100g

    Initial KE of the puck = Final KE + increase of PE of 100g wt to get lifted by (r2-r1).

    From these 3 eqns, you can solve it.
  6. Nov 2, 2007 #5


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    Ah, so you mean that the two 100 gram weights are hanging in tandem below the table and that the string between the two is burned off?

    I agree with Shooting star that energy should be conserved since no external force acts on the system when the string is burned.
    Last edited: Nov 2, 2007
  7. Nov 3, 2007 #6
    Ohh thanks!! That makes so it so much clearer. I didn't use energy to solve ><
  8. Oct 30, 2009 #7
    How about this same question but in a chapter concerning momentum only, energy has not been covered yet.
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