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FBD Description

  1. Apr 7, 2014 #1
    1. The problem statement, all variables and given/known data
    2z6bl8y.png

    Describe a real situation that would give rise to the FBD of the block. N is normal, T is tension, W weight, f static friction

    2. Relevant equations
    Fnet = ma



    3. The attempt at a solution
    A block glued to a wall, while someone pulling a string down on it.
     
  2. jcsd
  3. Apr 7, 2014 #2

    SammyS

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    The glue's effect probably won't count as friction.

    Where would the normal force come from ?
     
  4. Apr 7, 2014 #3
    The normal force would come from the surface, say a wall
     
  5. Apr 7, 2014 #4

    SammyS

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    Under what condition(s) would the wall be exerting a horizontal force on the block ?
     
  6. Apr 7, 2014 #5
    If you push with a horizontal force, canceling the normal force. Problem is I don't think you can add your own force.
     
  7. Apr 7, 2014 #6

    SammyS

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    How about if the wall pushes with a constant force ? (In what direction would that have to be?)

    What situation would require such a force? -- At least the magnitude of the force might be constant.
     
  8. Apr 7, 2014 #7
    Block at rest on a rough wall. Man pulls the string attached to block to balance with static friction?
     
  9. Apr 7, 2014 #8

    SammyS

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    You need a normal force for there to be friction.

    The normal force is generally a "reaction" force, so if the wall exerts a force of N (to the left) on the block, the block is exerting a force of magnitude N on the wall (to the right.)

    It looks like there is only one horizontal force exerted on the block, namely N, which must be unbalanced if the magnitude of N is not zero. What would Newton say about that?
     
  10. Apr 8, 2014 #9
    Will accelerate in normal force's direction
     
  11. Apr 8, 2014 #10
    Person just lets go off block originally held by hand
     
  12. Apr 8, 2014 #11

    SammyS

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    Then there is no longer any normal force -- once the person lets go.
     
  13. Apr 8, 2014 #12

    SammyS

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    Yes.
     
  14. Apr 8, 2014 #13
    So what is an example of a real situation?
     
  15. Apr 8, 2014 #14

    SammyS

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    It seems that you are to come up with the answer.
     
  16. Apr 8, 2014 #15
    Two blocks on table. Second block accelerating to left, exerting a normal force on first block. Person pulls down string on block.
     
  17. Apr 8, 2014 #16
    Anyone else willing to offer more tips/guidelines to solving this problem?
     
  18. Apr 8, 2014 #17

    PhanthomJay

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    Think with imagination about a trip to Palisades Park.
     
  19. Apr 9, 2014 #18
    Man still not getting it... can someone please offer specific advice? I understand the principles. It must be accelerating to the left, but I can't think of a real life example.
     
  20. Apr 9, 2014 #19

    SammyS

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    Wow!

    Jay gave an excellent suggestion -- to think about (rides) at an amusement park.

    Initially, you might not have the tension involved.
     
  21. Apr 9, 2014 #20
    How about a larger block (m3) moving to the left which exerts a normal force on m2. m2 is connected to m1 via a string. m2 and m1 have the right values so they do not move vertically; instead they move at the same acceleration as m3.
     
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