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Free body diagram logic

  1. Nov 29, 2015 #1
    1. The problem statement, all variables and given/known data
    How come for the following image when I assume block B slips up, block A does not move. The free body diagram at block B does not have a normal from the ground. This is a static friction problem.
    c8159d667350baaca3628c32559c52fb.png



    2. Relevant equations
    No relevant equations all theory


    3. The attempt at a solution
    This should be the Free body diagram but why does B have no normal from the ground when this assumption is made?
    9125af555fd21b6e87a783fb1a808f7c.png
     
  2. jcsd
  3. Nov 29, 2015 #2

    SteamKing

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    It's not clear what the original problem statement was, since you didn't provide it.

    When blocks A and C are forced together, block B is going to slide upwards on the sloped face of block C. Once Block B is just a minute distance off the ground, there is no more normal force on the block from that direction.
     
  4. Nov 29, 2015 #3
    I would assume if block B is slipping up, it is no longer in contact with the ground and therefore there cannot be a normal force.
     
  5. Nov 29, 2015 #4
    The original problem statement was:
    The three stone blocks have weights of 60lb 150lb and 40 lb respectively Determine the smallest horizontal force P that must be applied to block C in order to move this block. The coefficient of static friction between the blocks is 0.3 and between the floor is 0.5
     
  6. Nov 29, 2015 #5
    Based on the question, the only way to move C, is to lift B. Therefore, no normal.
     
  7. Nov 29, 2015 #6
    Excellent, thank you that is what I thought. I did the question originally assuming the normal and the system was statically indeterminate I was unsure how to solve until I was told this FBD did not contain a normal but I was unsure as to why
     
  8. Nov 30, 2015 #7

    haruspex

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    Is that from checking with the actual numbers? With some combinations of weights etc., the three blocks would slide horizontally, no lifting.
     
  9. Nov 30, 2015 #8
    No, I didn't use numbers. I was trying to visualize the block moving without lifting....I couldn't see it.
     
  10. Nov 30, 2015 #9

    haruspex

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    Despite the way the blocks are drawn, block A is much lighter than block B. If it's light enough it will slide instead of B rising. It's easy to visualise - the three blocks slide as if one unit.
    Note that the clause "block A does not move" in the OP is not part of the problem specification; it is an observed consequence of the assumption that B slides up.
     
  11. Nov 30, 2015 #10
    I guess I just latched onto the phrase Block A does not move. I am not sure if that was a part of the problem originally but that was the assumption I was basing my thoughts on. If it can move, I agree, there is a possibility that the whole unit could move together. In that case, yes I can visualize Block B travelling horizontally. :)
     
  12. Nov 30, 2015 #11

    haruspex

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    The actual problem statement appears to be in post #4.
     
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