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Homework Help: Speed of chain?

  1. Mar 12, 2009 #1
    speed of chain??

    1. The problem statement, all variables and given/known data

    A chain of metal links with total mass M = 6 kg is coiled up in a tight ball on a low-friction table. You pull on a link at one end of the chain with a constant force F = 69 N. Eventually the chain straightens out to its full length L = 1.0 m, and you keep pulling until you have pulled your end of the chain a total distance d = 3.6 m.

    Consider the point particle system:
    What is the speed of the chain at this instant?

    2. Relevant equations

    sqrt((2FA)/M) where F is force (69 n)
    A is change in center of mass aka center of mass final - center of mass inital (this is where im having some troubles)
    M is mass (6 kg)

    3. The attempt at a solution

    i not sure what A is for this equation. i tried 3 different ways as followes and i am wrong all 3 times

    i figured that it would be 3.6 becuae it moved this far when i do the formula i get

    then i figured 3.6 + 1.0 (from length of chain) use formula using 4.6
    and get 10.28591

    finally i did half of lenght plus the 3.6 distance to get 4.1
    did formula and got 9.7101

    all these are wrong and im out of ideas if you have one let me know
  2. jcsd
  3. Mar 12, 2009 #2

    Doc Al

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    Staff: Mentor

    Re: speed of chain??

    Draw yourself a before and after diagram. If the chain starts out balled up at x = 0, what are the coordinates of the ends of the chain after pulling it the stated distance?
  4. Mar 12, 2009 #3
    Re: speed of chain??

    I really dont need to know the end points of the chain do i?
    the A in the equation is for the center of mass. you compare its
    final position to its inital.
    so its inital is 0.
    final would be 4.1
    here is how i get that.
    the ball stretched out will start at 0 then go to 1. the center (A) would be at .5. then you move (A) a distance of 3.6 which means the (A) would be at 4.1.
    the left side would be at 3.6 and the right side would be at 4.6
    but when i did the equation with 4.1 it said the answer was wrong.
  5. Mar 12, 2009 #4
    Re: speed of chain??

    Keep in mind that the force is not acting on a constant mass (the entire chain is not moving at the start).
  6. Mar 12, 2009 #5

    Doc Al

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    Staff: Mentor

    Re: speed of chain??

    Understanding where the end points of the chain are will help you determine where the center of mass is.
    This is incorrect. Think this way: You grab one end of the chain (when it's at x=0) and you pull that end a distance of 3.6 m. So where is that end of the chain? Where's the other end? Where's the center of mass?
  7. Mar 12, 2009 #6
    Re: speed of chain??

    thanks guys. i was reading the problem as if the chain straightens the 1 m first then you move it the 3.6 so actually moving it 4.6. just need to learn how to read haha
    thanks again for all the help.
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