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Homework Help: Determining Scale Readings on an Atwood Machine

  1. Oct 22, 2012 #1
    1. The problem statement, all variables and given/known data

    Two masses, measuring 800g and 400g respectively, are attached to spring scales and an Atwood Machine. What will the reading of each scale be?

    View attachment 52223

    2. Relevant equations

    Fg = mg

    3. The attempt at a solution

    I think that the reading on each spring scale will equal the force of gravity acting on each attached mass.

    For Scale 1:

    Reading = Fg = mg
    = (0.8kg)(9.8N/kg) = 7.84 N

    For Scale 2:

    Reading = Fg = mg
    = (0.4kg)(9.8N/kg) = 3.92 N

    Is this correct?
  2. jcsd
  3. Oct 23, 2012 #2


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    Science Advisor
    Homework Helper
    Gold Member

    Can't open the attachment, but assuming a massless frictionless ideal pulley (yes??), the tension in the rope as measured by the scale must be the same on both sides of the pulley. You will need to draw a free body diagram for each mass to identify the forces acting on each, then use Newton's 2nd law on each to get 2 equations to solve for the tension force (scale reading). Note that the system is accelerating.
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