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Bicycling & Fermi thinking

Estimate the force on a bicycle by a rider traveling at 9 m/s.


(There is the drag equation which will help us estimate the answer but here, we want to be able to guess the answer without the equation)


Solution


We can say that, intuitively, the power generated to ride the bicycle will be equal to the force we exert and the speed with which we exert this force. So, how much power does the human body generate?


From our chips packets, we guess that the human body needs 2000 kcal per day.


Or, 2000 kcal per day = 2 000 000 Calories (calories are different from Calories!)

Or, converting Calorie into Joules = 2 000 000 Calorie x 4.187 = 8 374 000 Joules/day (1 Joule = 4.187 calories)

Or, 8.37 * 10^6 Joules/day


Converting joules to power,

If 1 W = 1 Joule/sec, then

8.37 * 10^6 Joules/day / 86 400 sec = 96.9 Joule/sec or 96.9 W/hour

Or, about 100 W/hour


But, cycling is an intensive activity. So, its reasonable to assume that we would expend about 200 W/hour for this activity.


Or, about 200/9 = 22 Newtons (Newton is how we measure force)


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