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Mass/energy problem

  1. Oct 11, 2013 #1

    462chevelle

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    1. The problem statement, all variables and given/known data
    if you burn 1 gallon of gas and it makes 48kJ. can you calculate the amount of mass lost.


    2. Relevant equations
    law of conservation: Energy cannot be created or destroyed it just changes form.


    3. The attempt at a solution
    the way i look at this problem is that there is no loss of mass. the mass is just converted from a liquid to a gas since you cannot destroy energy. but i feel like i shouldnt be looking at it this way and i should be thinking of a way to model it in an equation. the question header is rest energy. and the only equation the book has under rest energy is e=mc^2. and i dont feel like that would apply here since i know all the variables. i dont need to solve for anything. any ideas?

    thanks,
    Lonnie
     
  2. jcsd
  3. Oct 11, 2013 #2

    phinds

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    If there is no mass lost at all, where did the heat energy come from?
     
  4. Oct 11, 2013 #3

    462chevelle

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    would the mass converted to heat energy be considered loss of mass?
     
  5. Oct 12, 2013 #4

    haruspex

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    If the heat is lost, yes. But if it is burnt within a closed system, so the heat stays there, I do not see why there should be s loss of mass.
     
  6. Oct 12, 2013 #5

    462chevelle

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    hmm. so as vague as the question is I bet there are 2 answers. that there is no loss of mass if its a closed system and the amount of mass it would lose if it loses all the heat from the system.
     
  7. Oct 13, 2013 #6

    462chevelle

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    so.
    initial mass= 2.8kg
    final mass = 48kJ=m(3x10^8)^2
    final mass-initial mass=mass lost
    does this look like the correct way to model this problem?
    I think im still going to answer with 2 answers with a ya but,

    thanks,
    Lonnie
     
    Last edited: Oct 13, 2013
  8. Oct 13, 2013 #7

    haruspex

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    No, you're ignoring the mass of the waste products. If the heat is lost then the mass lost is the mass equivalent of the heat energy.
     
  9. Oct 13, 2013 #8

    462chevelle

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    so I just convert joules to kg and if I have 48000 joules. in kg that is 2.0833 X 10^-5??
     
  10. Oct 14, 2013 #9

    haruspex

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    Yes.
     
  11. Oct 14, 2013 #10

    462chevelle

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    thanks. that makes it a lot more clear. i am clearly overthinking this stuff.
     
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