# Work & kinetic energy problem

1. Sep 30, 2006

### UCrazyBeautifulU

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. Sep 30, 2006

### drpizza

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.

3. Sep 30, 2006

### Staff: Mentor

You are off by an order of magnitude. How many Joules of energy is stored in that Snickers bar?

4. Sep 30, 2006

### UCrazyBeautifulU

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.