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Homework Help: Joules and mass

  1. Apr 5, 2010 #1
    1. The problem statement, all variables and given/known data
    Consider two friends who go to Burger King every day for lunch. One of them orders a double whopper sandwich large fries and a large coke (total cal = 1600) while the other orders a whopper junior small fries and a small coke every day (cal = 800).
    if these two friends are very similar otherwise and they have the same metabolic rate, determine the weight difference between these two friends in a year.


    2. Relevant equations
    I know of know no other equation that relates energy to mass besides this one.
    Does it apply here?
    E=mc^2
     
  2. jcsd
  3. Apr 5, 2010 #2
    haha ive never heard of such a question. one calorie is 4.184 joules according to google. thats oddly close to the specific heat capacity of water, off topic sorry.

    and yeah thats what I would do. what are you studying right now in physics, relativity? energy?
     
  4. Apr 5, 2010 #3
    however, that cannot be right. c^2 is like 9e18...which would mean you could eat like thousands of trillions of calories more and not even gain a kilogram.
     
  5. Apr 5, 2010 #4
    sorry for the multiple posts, I just found this little statistic here on yahoo answers

    7709 cal = 1 Kg
     
  6. Apr 5, 2010 #5
    One calorie here is actually one Kcal and thus 4.184 kilo joules. The "kilo" is omitted by the food industry and dieticians.

    Hint: What is the energy content of one kg of fat?
     
  7. Apr 5, 2010 #6
    Yes, that looks plausible, A rule of thumb is that 4 days of fasting will lead to about 1 kg of weight loss. We use about 2000 kcal of energy per day, but if you don't eat, then it will be a bit less.
     
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