I am trying to help my daughter do her physics homework. Her assignment includes calculating Calories worked off while exercising.
The formula she is using is
Work (Joules) = Force (Newtons) * Distance (Meters).
I calculated that my 65 kg daughter 4.448 is 618 Newtons. And her walking up 3 floors of stairs was 9.144 meters. Multiply those and I get 5,653 joules. Divide that by 4.18 and I show that 1,352 calories were burned per trip. Equating to 1.3 Calories (Kcal) per trip.
She did 15 trips in 30 min, I get about 20 Calories burned for the whole activity. (Note, this is just "up the stairs" time. Down the stairs time is not represented in these numbers.)
Here is the part I am not understanding. If I go to any diet and exercise site I get told of a Metabolic Equivalent number. (It is 7.5 for climbing stairs.) It says to mulitply this by her weight to get Calories burned per hour. So, 7.5 * 65 = 487.5. Divide by 2 because it was only a half hour = 243.75 Calories burned.
So 20 Calories by the physics way, and 243.75 Calories via the "exercise calculation" way.
Why does the Newtonian Physics result differ so greatly from the "exercise calculation"?
I assume that the "exercise calculation" is likely including the inefficiencies of the human body in it, but it still seems like a significant difference.