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Newton law's - Finding acceleration

  1. Apr 28, 2013 #1
    1. The problem statement, all variables and given/known data
    (see attachment)


    2. Relevant equations



    3. The attempt at a solution
    I assumed that the acceleration of 5M is towards the right and is ##a##. Moving to the frame of 5M, the other two blocks experience a pseudo force.
    The forces acting on M are the tension in the string, pseudo force, its weight and the normal reaction from 5M.
    ##T+Ma=Ma'##
    Similarly for 2M,
    ##2Mg-T=2Ma'##
    But I still need one more equation to reach the answer.

    Any help is appreciated. Thanks!
     

    Attached Files:

  2. jcsd
  3. Apr 28, 2013 #2

    ehild

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    The equation for the CM?

    ehild
     
  4. Apr 28, 2013 #3
    [tex]a_{cm}=\frac{Ma'+2Ma'}{8M}=\frac{3a'}{8}[/tex]
    Is this correct? And how does this help me? :confused:
     
  5. Apr 28, 2013 #4

    ehild

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    Does the CM accelerate in horizontal direction in the inertial frame of reference?
    By the way , I would use inertial frame for all parts.

    ehild
     
  6. Apr 28, 2013 #5
    Whoops, missed that.
    [tex]a_{cm,x}=\frac{-a'}{8}[/tex]
    [tex]a_{cm,y}=\frac{-a'}{4}[/tex]
    I have assumed that the direction to right is positive x-axis and the direction vertically upwards is positive y-axis.

    How will you do this using the inertial frame? I assume I will have to consider the motion of the 2M block in horizontal and vertical directions.
     
  7. Apr 28, 2013 #6

    ehild

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    The cm accelerates in vertical direction, but is there any external horizontal force?

    ehild
     
  8. Apr 28, 2013 #7
    No. Do you mean I should have ##a_{cm,x}=0##? I think my expression for ##a_{cm,x}## is incorrect.
     
  9. Apr 28, 2013 #8

    ehild

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    I mean the CM of the whole system in the inertial frame of reference.

    ehild
     
  10. Apr 28, 2013 #9
    Sorry, I still don't get it. Can you elaborate a bit more? :)
     
  11. Apr 28, 2013 #10

    haruspex

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    If you're not comfortable thinking about the horizontal accn of CM, you can get two more equations by introducing one more unknown, the normal reaction between 5M and 2M.
     
  12. Apr 28, 2013 #11

    ehild

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    You need the acceleration of the big block with respect to the ground. Of course, the acceleration is zero with respect to itself, so you can not avoid using the inertial frame of reference.

    ehild
     
  13. Apr 28, 2013 #12
    I have to do this in the frame fixed to 5M or inertial frame?

    I will make equations again for the inertial frame of reference.
    For M, the forces acting on it are tension, weight & normal reaction from 5M.
    ##T=Ma## (I feel this is not correct)

    For 2M, it performs two motions, one in horizontal and the other in vertical direction.
    For horizontal direction,
    ##N=2Ma'## (where N is the normal reaction on 2M by 5M)
    For vertical direction,
    ##2Mg-T=2Ma##

    For 5M, it perform horizontal motion, the forces acting on 5M in the horizontal direction are the tension and the normal reaction due to 2M but how would should be the value of tension? Should it be T?
     
  14. Apr 28, 2013 #13

    ehild

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    The tension is the same along a single rope if the pulley has zero mass.

    ehild
     
  15. Apr 28, 2013 #14
    I should have used the proper words. I meant the direction of force of tension on 5M.
     
  16. Apr 28, 2013 #15

    ehild

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    Actually there are three tensional forces on the big block...
     
  17. Apr 28, 2013 #16
    But I need to consider only one, the other two are in vertical direction. Right?
     
  18. Apr 28, 2013 #17

    ehild

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    I meant three horizontal forces of tension.
     
  19. Apr 28, 2013 #18
    I still don't see how there are three horizontal forces of tension. :confused:
     
  20. Apr 28, 2013 #19

    ehild

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    Two at the left pulley and one at the right one.
     
  21. Apr 29, 2013 #20
    Its all Newton's THIRD Law.

    At the right pulley, there is a string passing over it. That pulley, due to strands of the string exerts a force on the big block rightwards due to newton's third law. (Why ?) Can you find that force ?

    Also, at the left pulley, there are two forces acting on the big block and they are horizontal. (Find their directions as a hint.)

    Also, on observing carefully the right pulley, there is one strand of the string pushing the big block downwards. (That's Newton's third law. Do not consider that as tension.)
     
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