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[Check for errors]- FREE BODY DIAGRAM for 2 Cylinders.

  1. Nov 9, 2009 #1
    Ok as you seen I been trying to get familiar with a few different type of exercises, this is a FBD type exercise.

    I attached the picture showing the exercise done, I just want you to check if you notice any errors, thanks you.

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

    Cylinder A- Mass 75kg and Diameter-400 mm, then Cylinder B- Mass 50kg and Diameter 200mm

    Attached Files:

  2. jcsd
  3. Nov 9, 2009 #2
    In FBD you draw the powers that bodies experience. You wrote apart from their weights the forces exerted by the bodies. Also think that the body B has a net force different than zero but it cant move..
  4. Nov 9, 2009 #3
    Thanks for the reply, but

    What do you mean by that?, I really thought I did it right????

    you sure is not right?
    Last edited: Nov 9, 2009
  5. Nov 9, 2009 #4
    Your scetch is like this Cylinder 1.jpg

    The force F1 is extracted by the body A on the body B. So it should be shown on B's FBD.

    The force F2 is extracted by the body A on the wall. So it should be shown on Wall's FBD

    Cylinder 2.jpg

    The forces -F1 and -F2 in my scetch are those that should be in your A's FBD.
    The point of FBDs is to make easier calculate the accelerations on the different axes of the bodies in our system. We cant calculate their acceleration if we dont draw the forces they experience from other bodies.

    Attached Files:

  6. Nov 9, 2009 #5
    I forgot to add the reaction force from the ground.. Sorry
  7. Nov 9, 2009 #6
    Thanks for the help, you went beyond the call of duty with that one, appreciate it.

    so you're saying the Force from the right and the force from the B cylinders were missing?

    everything else is all good?
  8. Nov 9, 2009 #7
    Wait with what you said there and even posted a pic, I can assume that the Cylinder B

    also needs a Force from the right like this <------

    and not like I have it which is like ------->

    correct? or I am I looking too deep into it??
  9. Nov 9, 2009 #8
    1) yes the Cylinder B needs a force from the right.
    2) to understand the whole thing better, think why you drew your forces like that. Think why you drew F3 pointing to the right.
    3) When you give it a though you ll understand that F3 is the Force that Cylinder B exerts on the Wall.. What you need is the Force that the wall exerts on the Cylinder. Think about it... Think about the Forces that act ON each of the cylinders and try to make sence of it..
  10. Nov 9, 2009 #9
    What do you mean?, the reaction from the group UP^?
  11. Nov 9, 2009 #10
    1- Well thanks to your drawing I figured that's what I missed there

    2- I drew them like I did because I am going by the reaction of the Cylider, you're showing me that it needs to be the other areas reaction with the Cylinder

    3. Yeah part 2 pretty much explains that I didn't follow the correct process
  12. Nov 9, 2009 #11
    Here is the edited picture, with your advise taken into account, I am not sure if I am missing any other reactions,

    You did say something about missing the reaction form the ground???, perhaps my edited still pic is still wrong but check out and provide further guidance please.

    Attached Files:

  13. Nov 9, 2009 #12
    Look.. Thats how the FBDs should look like. Think about it and if you dont understand something tell me. The forces shown in FBDs are the forces that have the potential to accelerate the body

    Attached Files:

  14. Nov 9, 2009 #13
    Thanks for clearing that up, I do have a question,

    Can you explain how in Cylinder A got reaction up and down in the same route???

    I never seen that in class, then again we never did this type of exercise in class just

  15. Nov 9, 2009 #14
    The force that points down is not a reaction. Its the weight of the cylinder ( the force that earth exerts on the cylinder ). Also its clear that the cylinder does not move downwards so there is a force that act upwards. That force is the 3rd newton's pair of the net force on the y -axis (but finding the force is another story).
  16. Nov 9, 2009 #15
    Oh no I got that much, I was just wondering how it was presented because I never seen anything like that before, but like I said before we never did these types in class all we
    done is beams with Pins and Weights on them.

    Anyways, thanks for clearing that up and helping me understand while also going beyond the call of duty, that's very much appreciated, I been on these forums for past 48 hours seeking guidance and I learned alot in that time frame.
    Last edited: Nov 9, 2009
  17. Nov 9, 2009 #16
    No prob man. cya around : )
  18. Nov 9, 2009 #17
    I noticed you used Mg for the Weight, you did that as an example or I am missing something here?,

    I thought for a F.B.D. you show what is given to you?, no conversions or anything like that
    needs to be done.
  19. Nov 9, 2009 #18
    W=mg its a formulae.. You dont need to worry abou this. It's more convenient to write mg rather than W while it means the exact same thing. No1 would never argue about that.
    You always show the weight except if the problem states that it s negligible ( which is very very rare ).
  20. Nov 9, 2009 #19
    Ah ok, I was just making sure, I mean the problem here has specific Kg masses for

    each of the Cylinders, so I was just making sure wondering why you didn't use

    the given Kg mass, clearly I need to use them though.
  21. Nov 9, 2009 #20
    Its better not to put numbers in FBDs. They are to help you. You can then write the equations and substitute the numbers.
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