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Homework Help: Energy needed for car to travel-thermodynamics

  1. Apr 12, 2010 #1
    1. The problem statement, all variables and given/known data
    Gasoline yields 4.8x10^7 J/kg when burned. How much energy does a car use travelling a distance of 150 km if it gets 25 miles per (British) gallon? Assume that the density of gasoline is 68% that of water.

    2. Relevant equations

    3. The attempt at a solution
    I dont really know where to begin with this question. All i know is that 25mpg is equal to 40.225km per gallon.
    Any suggestions?
  2. jcsd
  3. Apr 12, 2010 #2
    The crux of this question really is in unit conversion. It assumes that you know that a litre of water has a mass of 1 kg so that you can work out the mass of 25 gallons of gasoline.

    Does this help?
  4. Apr 12, 2010 #3
    So i do 40.225 x 68% = 27.35 kg to get the mass of 25 gallons of gasoline?

    and what next? I dont know a formula to figure out the energy given the information.
  5. Apr 14, 2010 #4
    ok does this look right to anyone:
    25gallons x 1.609 = 40.225km per gallon

    150km/40.225 = 3.729 gallons to travel 150km

    3.729 x 68% = 2.536kg

    2.536 x 4.8x10^7J/kg = 1.22x10^9J ???
  6. Apr 24, 2010 #5
    One of the most helpful tools I have found is the Unit Conversion in Excel (http://www.unitconversionaddin.com [Broken]). This simply works because students (like me) are able to do the unit conversion automatically in excel like a program. I don't have to either search on the internet for the conversion value or have to write out some excel function. The download is like 5 bucks and has saved me hours of stress and time.
    Last edited by a moderator: May 4, 2017
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