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Homework Help: How to calculate work done using a graph

  1. Mar 24, 2009 #1
    Can someone please tell me how do i calculate work done in 4 process of engine cycles using a graph?

    i drew the p-v graph of an gasoline engine cycle now i dont know how to calcualte work done. is it by calculating the area inside or how?

    no need of formulas...just using the graph..

  2. jcsd
  3. Mar 24, 2009 #2

    Doc Al

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    Staff: Mentor

    Last edited by a moderator: Apr 24, 2017
  4. Mar 24, 2009 #3
    now graphically means do i need to just count the Squares in the closed area or is there any formula to use to find it?
  5. Mar 24, 2009 #4


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    How are you given the graph? Just a picture or are you given a formula? If all you have is a picture, the "counting squares is all you can do. If you are given a formula you can integrate over the area or perhaps integrate around the boundary and use Green's Theorem.
  6. Mar 24, 2009 #5
    Thanks hallsofIvy

    Just a picture. now becaususe it's a p-v diagram. i got 31 full squares. i didnt count the half squares which are halfed because of the curve. do i need to count them all as well?

    and if it's 31 then what is the unit do i have to use for the answer?
  7. Mar 24, 2009 #6
    You should count them all. You could try scanning the image into Photoshop or similar, calibrating the scale of the squares to pixels and then measuring the area. Or you could use a planimeter - that's typically how you would measure it from an indicator diagram from an engine.

    As for units, you know that you're looking for the area of a pressure-volume plot. Thus the units of your area should be the same as the units of pressure multiplied by the units of volume. In an MLT system (mass, length, time) your units should be equivalent to:


    or in other words, equivalent to a force times a length, which gives you work.
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