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Homework Help: Work & kinetic energy problem

  1. Sep 30, 2006 #1
    The human brain consumes about 21.9 W of power under normal conditions, though more power may be required during exams. How long can one Snickers bar (280 cal per bar) power the normally functioning brain?[/B] (Note: The nutritional calorie, 1 Cal, is equivalent to 1000 calories (1000 cal) as defined in physics. In addition, the conversion factor between calories and joules is as follows: 1 Cal = 1000 cal = 1 kcal = 4186 J.)

    I keep getting the answer 53.52 seconds after I do the conversion and use the formula P = W/t

    However, 53.52 seconds isnt the correct answer (LON CAPA did not accept it), anyone else have any ideas on this one?
  2. jcsd
  3. Sep 30, 2006 #2
    You did the problem almost correctly. Read carefully that a food calorie is actually 1000 calories.

    Also, think about the 53.52 seconds for a brain to run on a snickers bar; you'd need to eat about 60 of them just to make it through an exam at that rate :) The actual answer makes more sense.
  4. Sep 30, 2006 #3

    Doc Al

    User Avatar

    Staff: Mentor

    You are off by an order of magnitude. How many Joules of energy is stored in that Snickers bar?
  5. Sep 30, 2006 #4
    thanks so much, you guys really helped me out! I knew it didn't make sense, but I couldn't figure out what I was doing wrong.
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