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Use Newtonian Model to determine what the scale reads

  1. Aug 30, 2012 #1
    1. The problem statement, all variables and given/known data

    forcesystem-1.jpg

    The strings attach to a scale which reads the force. What is the force that the scale will read?


    2. Relevant equations

    F=ma according to Newton's Laws.



    3. The attempt at a solution

    Thinking logically, I simply calculated the force of the left side:

    F=ma => (2kg)(9.8m/ssq) = 19.6N

    Then calculated the force of the right side:

    F=ma => (1kg)(9.8m/ssq) = 9.8N


    Then simply subtracted the two, leaving the final answer to be 9.8N. My teacher told me that this is not the right answer. I'm confused as to how else I could have done this problem. Thanks for the help!
     
  2. jcsd
  3. Aug 30, 2012 #2

    Doc Al

    User Avatar

    Staff: Mentor

    What you found here is not 'ma', but mg. That's the weight of the left mass. That's just one of the forces acting on that mass. What's the other?

    Same issue here.

    What you need to do is identify all the forces acting on each mass and then apply ƩF = ma to each mass separately. Then you can solve for the acceleration of the masses and the tension in the cable (which will equal the scale reading).
     
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