# Calories Burned/Physics Question

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

1. ### vesterholt New Member

Joined:
Jun 27, 2014
Messages:
7
0
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?

Tags:

2. ### jpr95 Active Member

Joined:
Oct 11, 2010
Messages:
870
30
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.

Joined:
Oct 11, 2010
Messages:
870