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Mass/energy problem

  • #1
462chevelle
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Homework Statement


if you burn 1 gallon of gas and it makes 48kJ. can you calculate the amount of mass lost.


Homework Equations


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


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
 

Answers and Replies

  • #2
phinds
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If there is no mass lost at all, where did the heat energy come from?
 
  • #3
462chevelle
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would the mass converted to heat energy be considered loss of mass?
 
  • #4
haruspex
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would the mass converted to heat energy be considered loss of mass?
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.
 
  • #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.
 
  • #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:
  • #7
haruspex
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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 steal going to answer with 2 answers with a ya but,

thanks,
Lonnie
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.
 
  • #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??
 
  • #9
haruspex
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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??
Yes.
 
  • #10
462chevelle
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thanks. that makes it a lot more clear. i am clearly overthinking this stuff.
 

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