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Homework Help: Quick question about a FBDs involving two masses held vertically by strings.

  1. Oct 9, 2011 #1
    I just wanted to double check my logic on something! If two masses are held vertically by two strings like so:

    [ceiling]
    [string #1]
    [mass #1]
    [string #2]
    [mass #2]

    ...then what forces are acting on mass #1?

    My free body diagrams are below, could someone double check if they're correct? (You'll need pretend they are rotated 90degrees clockwise!).

    for mass 1:
    Tension in string 1 <------ [mass#1]------>Tension in string 2 AND mass1*gravity

    for mass 2:
    Tension in string 2 <------ [mass#2]------>(mass1+mass2)*gravity

    for both masses:
    Tension in string 1 <------ [mass#1 and mass#2]------>(mass1+mass2)*gravity


    Do these make sense?
    Feedback is appreciated (:
     
  2. jcsd
  3. Oct 9, 2011 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Good!

    The weight of mass 1 acts only on mass 1, so it doesn't belong here.

    Good!
     
  4. Oct 9, 2011 #3
    Ah, thank you!

    So, just conceptually, the weight of mass 1 acts transmits as a force to mass 2 only through the the string between them? And this ends up translating into the "force of tension in string 2"? (mass 1 just pulls up on mass 2)
     
  5. Oct 9, 2011 #4

    Doc Al

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    Staff: Mentor

    I think you have things a bit backwards.

    The weight of mass 1 has no effect (even indirectly) on mass 2. You could take string 2 and attach it directly to the ceiling (without mass 1 or string 1) and the the tension will be the same.

    Now the weight of mass 2 is transmitted to mass 1 via the tension in string 2, which in turn adds to the tension in string 1. But yes, the only way that the weight of mass 2 can affect mass 1 is via the string between them.
     
  6. Oct 9, 2011 #5
    Thank you Doc Al! I'm glad I asked because I see the flaw in my logic now -- there is no way for mass 1 to act on mass 2.

    Thanks again!
     
  7. Oct 9, 2011 #6
    By the way, is there any way for me to mark this thread as "solved"?
     
  8. Oct 9, 2011 #7

    Doc Al

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    Staff: Mentor

    You are most welcome.
    Unfortunately not. (But anyone reading this will know that it's 'solved'.)
     
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