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Atwood Machine Problem

  1. Jun 30, 2014 #1
    1. A 77.00 Nt object (denoted as object 1) rests on the ground. A light cord is connected to this object which runs vertically upward over a light frictionless pulley and is attached to another object denoted as object 2.

    a) Calculate the force that the ground exerts on object 1 if object 2 is 30 N.

    b) Calculate the force that the ground exerts on object 1 if object 2 is 60 N.

    c) Calculate the force that the ground exerts on object 1 if object 2 is 90 N.



    http://imgur.com/cVEQW07 http://imgur.com/cVEQW07

    g (m1 - m2) / (m1) + m2 = ay


    I drew FBD's for both of the weights and solved for the acceleration for situation a. I then used the equation w - t = ma and solved for the tension value, I then subtracted the tension value from the original weight of object 1, and I got 33.88 N.

    I feel like this is the wrong way of solving the problem because once you get to situation c, you would have a negative force which can't be possible.
     
  2. jcsd
  3. Jun 30, 2014 #2

    SammyS

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    That seems to be a correct result for part c. -- but ...

    If the ground can't produce a negative (downward) force on m1, then what do you suppose happens to the system?

    attachment.png
     

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  4. Jun 30, 2014 #3
    The system would change where object 1 would be "in the air" correct?
     
  5. Jun 30, 2014 #4

    SammyS

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    Yes, so the ground will exert what force on m1?
     
  6. Jun 30, 2014 #5
    It would have to be 0 Newton's. If I'm correct about that, then I already assumed that, however I rethought my approach on part a and b.

    I redid the FBD's for (a) and (b) and added the two equations for Newton's second law, and the tensions canceled each other out, leaving me with w1 - w2 = Fn. From there I calculated that in situation (a), the force that the ground exerts is 47 Newton's, and in situation (b) it would be 17 Newton's.
     
    Last edited: Jun 30, 2014
  7. Jun 30, 2014 #6

    SammyS

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    That all looks good!
     
  8. Jun 30, 2014 #7
    Thank you so much for the help!
     
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