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Free body diagram for a bag of coins suspended by an elastic band

  1. Feb 19, 2009 #1
    1. The problem statement, all variables and given/known data
    Draw a free body diagram showing all the forces on the elastic band, and a free body diagram showing all the forces on the bag of coins

    2. Relevant equations


    3. The attempt at a solution

    I drew the forces on the elastic band as T = mg (m = mass of coins since the elastic band's mass isn't stated) upwards

    and mg downwards

    The forces I drew on the bag were F = kx upwards (the force of the elastic's stretch pulling proportionally upwards on the bag) and F = mg downwards (m = mass of coins)

    If it were a rope attaching the bag to the ceiling I'd be ok but I'm a bit thrown off since it's an elastic band. Please let me know if this seems logical.

    Thanks in advance for your help!
  2. jcsd
  3. Feb 19, 2009 #2


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    Homework Helper

    Welcome to PF.

    That sounds good.
  4. Feb 19, 2009 #3
    Wow, what a fast response! Thanks for the reassurance :)
  5. Feb 19, 2009 #4
    Hi, I just had one thought. This was from an experiment where coins were added one by one to a bag (attached to the elastic) and then the elastic stretched with each coin added (and I have to draw a free body diagram of the forces acting on the elastic and those acting on the bag).

    Since the elastic band did move when the coins were added, am I wrong to say that these forces are in equilibrium? Maybe I should have instead said that the upward tension force on the elastic is equal to the spring force (F = kx)? Thanks again!
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