Calories Burned/Physics Question

Discussion in 'The Bike Cafe' started by vesterholt, Oct 17, 2014.

  1. vesterholt

    vesterholt New Member

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    I wanted to see if there were any other science/engineering types here who could help me out with this question.

    When I bike to work, I ride over the Manhattan bridge, which is approximately a 100 ft climb. For simplicity let's assume that my bike and I weigh 200 lbs.

    From basic physics we know that potential energy due to gravity is M x G x H, where M is mass, G is the gravitational acceleration (32.2 ft/sec/sec) and H is height.

    Based on this my increase in potential energy should be 200 x 32.2 x 100 = 644,000 ft lbs of energy. From wikipedia, 1 ft lb = .000324 food calories, so this equates to 209 calories. This would assume 100% efficiency, which we know is not possible (most estimates of human body efficiency are between 20-30%, which would mean burning around 800 calories). Yet, this seems like an unreasonably high number of calories to burn over a climb which takes me around 5 minutes riding slowly.

    Does anyone know where the error is in this?
     
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  2. jpr95

    jpr95 Active Member

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    Pounds is a unit of force, not mass, so it already "contains" the gravitational constant.

    Convert to metric first, say 100 kg, 9.8 m/s^2, 30 m, which would be 29,400 J, which is about 7 food calories, 25-ish if you consider the human inefficiency.
     
  3. jpr95

    jpr95 Active Member

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    Addendum. Multiply 200 lbs by 100 feet gets you 20,000 foot-pounds (remember to track your units!), which is about 27,000 J. That's a little less than what I had above, but I used 100 kg above, which is 220 lbs. In the ballpark, though.
     
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