1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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


    User Avatar
    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!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook