I think the answer is Time
A theory is a set of sentences which is closed to make logical implications.
Human can produce a maximum mechanical power of 1/3 hp (horse-power).
Horse power is a unit of power, it equals 746 watt (watt is also a power unit, known to be equal to a joule per second)
So:
1 ph = 746 watt = 746 J/s
1/3 ph = 248.6 J/s
we're looking for the calories, so
1 joule = 0.239006 calories
So human produce a maximum mechanical power of:
248.6 J/s = 82.88 calories per second.
We're looking for the number of calories consumed in three hours, so:
82.88 cal/s = 82.88 cal/s * 3600 s * 3 = 895200 calories = 895.2 kilocalories (food calories).
The number of food calories is 895.2 food calories
