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Homework Help: Which forces am I missing in the FBDs?

  1. Aug 11, 2018 #1
    1. The problem statement, all variables and given/known data
    Two blocks with pulleys attached lie on a frictionless surface and are connected by a massless rope strung over the pulleys as shown at right. A force of magnitude F is applied to the second block. Draw a FBD for a system consisting of Block 1 and its pulley, and also a system consisting of Block 2 and its pulley. (See the screenshot attached.)

    2. Relevant equations
    F=ma?
    I don't think there are any specific equations needed...
    3. The attempt at a solution

    Obviously, there are normal forces by the table on both systems as well as weight, and from each pulley we have two tension forces emanating. The system with block 2 also has an additional force of magnitude F towards the right. However, apparently there are still more forces I am missing, but I can't seem to find them...
     

    Attached Files:

  2. jcsd
  3. Aug 11, 2018 #2

    Orodruin

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    Unfortunately, your description of what you actually did is too vague for me to understand what you did and what you got. Please provide the equations you obtain from your FBDs with appropriate descriptions of what each term represents.
     
  4. Aug 11, 2018 #3

    haruspex

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    Look at the ends of the rope.
     
  5. Aug 11, 2018 #4

    Chandra Prayaga

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    What is your attempt at drawing the FBDs?
     
  6. Aug 12, 2018 at 2:49 AM #5
    @Chandra Prayaga — please see the screenshot attached.

    @Orodruin

    since neither system is accelerating vertically, we have
    N_(table,1) - W_(Earth,1) = 0 &
    N_(table,2) - W_(Earth,2) = 0
    where 1 denotes the system that has box 1 (System 1) and 2 denotes the system that has box 2 (System 2).

    Horizontally, for System 1, making the right positive, the two tension forces make possible the system's acceleration (from what I have currently):
    2T_(rope,1) = m_(system 1)*a_1

    For System 2, horizontally,
    F - 2T_(rope,2) = m_(system 2)*a_2

    @haruspex — I'm sorry, but it's still not obvious to me what I'm missing... Could you elaborate?
     

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  7. Aug 12, 2018 at 3:59 AM #6

    haruspex

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    What is the rope attached to at its ends?
     
  8. Aug 12, 2018 at 4:42 AM #7

    Orodruin

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    As haruspex points out. You are missing some forces. A rope will also exert a force on anything it is attached to. You cannot just ignore those forces.
     
  9. Aug 12, 2018 at 7:27 AM #8
    @haruspex, @Orodruin

    The rope is attached to box 1 on one end and the table on the other end. But I don't think the table is exerting any direct forces on System 1...
    So would I get something like this (attached)?
     

    Attached Files:

  10. Aug 12, 2018 at 7:28 AM #9

    haruspex

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    That looks right.
     
  11. Aug 12, 2018 at 7:42 AM #10
    Alright, thank you for all the help, everyone!
     
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