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What is the stored energy of a wind-up watch?

  1. Oct 2, 2012 #1
    1. The problem statement, all variables and given/known data
    A person does .02 Joules of work winding a watch. Assuming NO loss of energy to friction, how much energy is stored in the watch's main spring?


    2. Relevant equations
    PE=1/2kx^2 ?? I have no idea..


    3. The attempt at a solution
    I've gotten nowhere trying to do this problem without other information, such as mass or length.

    Thank you for looking and helping!
     
  2. jcsd
  3. Oct 2, 2012 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF.

    Hint -- what are the units of energy?
     
  4. Oct 2, 2012 #3

    HallsofIvy

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    Staff Emeritus
    Science Advisor

    Also crucial- conservation of energy since there is no friction. Don't use a formula- think.
     
  5. Oct 2, 2012 #4
    The unit of energy is just Joules.. Right? Well in this case.

    So would it just be a potential energy of .02J since the total work done is calculated by the Newtons of force and meters which they don't give in the problem?
     
  6. Oct 2, 2012 #5

    berkeman

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    Staff: Mentor

    Correct. As Halls alludes to, since there is no energy lost to friction, all of the work has to go into storing potential energy in the spring. Good job!
     
  7. Oct 2, 2012 #6
    Thank you both!
     
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