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Homework Help: Determine the speeds of the 2 masses

  1. Jan 20, 2017 #1
    1. The problem statement, all variables and given/known data
    upload_2017-1-20_15-28-39.png

    2. Relevant equations


    3. The attempt at a solution
    Here is the solution in the answers sheet. Why the initial potential energy of the Pulley (M) = 0?

    upload_2017-1-20_15-29-12.png
     
  2. jcsd
  3. Jan 20, 2017 #2

    mfb

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    2017 Award

    Staff: Mentor

    The initial potential energy is not zero in the calculation.

    It would not matter - potential energies are arbitrary, only differences in potential energy (before/after) matter. The difference is not zero.
     
  4. Jan 20, 2017 #3
    you don't get what I mean.
    He put the potential zero of the pulley =0. Why is that? The pulley is at a height so it accuires potential energy.
     
  5. Jan 20, 2017 #4

    mfb

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    2017 Award

    Staff: Mentor

    Ah, that.
    Same thing: It does not matter, as its height does not change. Any potential energy you assign to it would appear on both sides of the equality, and cancel.
     
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