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Homework Help: Work, box, spring (dynamics)

  1. Feb 5, 2012 #1
    1. The problem statement, all variables and given/known data

    See attachments.

    2. Relevant equations

    3. The attempt at a solution
    In Example 2.11, why is the work for friction -6(10+Δ)? Why is it negative 6?

    In Example 2.12, again why is the friction -6, and then where did (Δ-d) come from? I thought Δ is the distance the spring is initially compressed, and d is the distance of its maximum extension? So why minus? And also in this example why is there a negative sign in front of the work for weight? Totally confused about orientations in this problem.

    Attached Files:

  2. jcsd
  3. Feb 6, 2012 #2

    rude man

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    Homework Helper
    Gold Member

    Too hard to read your attachments.
  4. Feb 6, 2012 #3
    you sure do live up to your moniker. mods please ban this guy
  5. Feb 8, 2012 #4
    I can see where the way the solution is laid out that it might be confusing.

    I always look at it from the first law of thermodynamics. With this problem, consider energy at the start and then at the end of the process. Put the terms on opposite sides of the equals sign.

    Remember that if the system is doing work (such as overcoming friction) it will subtract from the FINAL energy of the system so it should be a negative sign if on that side of the equation.

    If work is done on the system, it will add to the final energy of the system so it will add to the FINAL energy in the system. Therefore if you wish to place it on the INITIAL energy side of the equation, it must be negative.

    If you think about it in these terms, you'll keep your signs correct.
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