Power: The Energy Flow Rate (Part II)

Upon further investigation I realize now that I made a rather large mistake in one of my previous blogs entitled, Power Output: The Energy Flow Rate, (03/08/2010), or so I think. First off, the title was not wrong, but inaccurate. I revised the title now to say simply ‘Power’ because the rate at which energy flows can refer to either a power input or output.

Secondly, my calculation of the amount of Watts that my body was operating at was, let’s say, skewed. I originally stated that for a 30-minute workout, I burned around 400 calories. I then converted these 400 calories to Joules and divided by the number of seconds in 30 minutes. This gave me 0.928 Watts. At first I thought this seemed low, but was at the time completely convinced that the calculations were done correctly.

It turns out that the calculations themselves were performed correctly, but instead it was in my conversions where there was a discrepancy. A unit for energy is the ‘thermodynamic’ calorie, but workout equipment measures food calories, or large calories, which equates to approximately 4.1868 kilojoules per food calorie. I assumed ‘thermodynamic’ calories in my original calculations.

So in actuality I burned 400 food calories x 4186.8 Joules of energy in my workout, which equals 1,674,720 Joules total, assuming the workout bike measures food calories. Dividing this by the number of seconds in 30 minutes, 1800, gives me 930 Watts. This is a huge difference from my original result, but I believe more accurate. Again, if anyone has any input into this subject feel free to email at: studentjournalist1@gmail.com, because at this point I’m not sure if workout equipment manufacturers know what the heck they are talking about.