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Newton's Second Law -- Accelerations in a pulley system

  1. Feb 28, 2017 #1
    1. The problem statement, all variables and given/known data
    A 20-kg block with a pulley attached slides along a frictionless ledge. It is connected by a massless string to a 5.0-kg block via the arrangement shown in Figure. (a) draw complete free body diagrams for the masses and Find (b) the acceleration of each block and (c) the tension in he connecting string.

    2. Relevant equations
    net force= ma
    Fweight=ga
    Fnormal=Fweight

    3. The attempt at a solution
    aTension=aTension
    Fweight-Ftenison=5a
    Ftension=20a
    Fweight-20a=5a
    ga=25a
     
  2. jcsd
  3. Feb 28, 2017 #2

    gneill

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    Staff: Mentor

    Hi Jonathan, Welcome to Physics Forums.

    The problem mentions a figure that is absent from your post. Without it it would be difficult to solve the problem or evaluate your solution efforts. Can you post the image? Did you have a particular question about the work you've shown?
     
  4. Feb 28, 2017 #3
     
  5. Feb 28, 2017 #4

    gneill

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    Staff: Mentor

    How does the pulley arrangement affect the relative rates of motion of the two blocks (do they both experience the same acceleration)? How does the pulley arrangement affect the net force delivered through string tension to each block?

    In other words, does mechanical advantage play a role in this problem? If so, does your current work reflect this?
     
  6. Feb 28, 2017 #5
    They both experience the same acceleration. The tension pulling both blocks will be the same because the string is massless
     
  7. Feb 28, 2017 #6
    I don't believe mechanical advantage would play a role in this problem.
     
  8. Feb 28, 2017 #7

    gneill

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    Staff: Mentor

    Better check again! :wink:

    Make yourself a sketch of just the block on the horizontal surface's pulley and its string connections. Suppose the pulley moves distance x to the right. How far does the top string have to move to take up the slack created?

    upload_2017-2-28_16-4-58.png
     
  9. Mar 2, 2017 #8
    No it actually doesn't play a role in this problem. We havent learned that yet.
     
  10. Mar 2, 2017 #9

    gneill

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    Staff: Mentor

    Nevertheless, solving this problem requires that you take into account the relative motions of the two blocks and the sum of tension forces at each block. If the problem requires concepts that you haven't learned yet, you should speak to your instructor.
     
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