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Homework Help: A block is released from rest, determine its velocity

  1. Dec 18, 2017 #1
    1. The problem statement, all variables and given/known data
    14.10 Solve Prob. 12.47 by the work-energy method

    12.47 When the 1.8-kg block is in the position shown, the attached spring is
    undeformed. If the block is released from rest in this position, determine its velocity
    when it hits the floor

    Fig P12_47.jpg

    2. Relevant equations


    3. The attempt at a solution
    Here is my solution to both 12.47 and 14.10.
    I get in 14.10 the correct answer but I am confused with 12.47. Is it correct?
    Pytels_Dynamics109.jpg


    Pytels_Dynamics108.jpg
     
  2. jcsd
  3. Dec 18, 2017 #2

    TSny

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    When you wrote
    upload_2017-12-18_13-7-12.png
    you assumed ##\int a dx = a \Delta x##. Is this true if the acceleration is not constant?

    What is the main topic of chapter 12?
     
  4. Dec 18, 2017 #3
    Pytels_Dynamics111.jpg

    Hi! After reading a sample problem of the book I came up with the above solution.
    I have a question though: If I integrate both sides of the equation just above equation (3)
    shouldn't I be getting on both sides a constant C that cancels out?
     
  5. Dec 18, 2017 #4

    TSny

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    Each side of the equation would have its own constant of integration. You can combine the two constants into one constant. Then, this constant is determined by the initial conditions as you have done.

    Your work here is essentially the derivation of the work-energy principle for this particular problem.
     
  6. Dec 25, 2017 #5
    Ok thanks for that.
    I need to revise my calculus.

    Alexandros
     
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