A 51 kg woman cheats on her diet and eats a 440 Calorie (440 kcal) jelly donut f

Question

A 51 kg woman cheats on her diet and eats a 440 Calorie (440 kcal) jelly donut for breakfast.
(a) How many joules of energy are the equivalent of one jelly doughnut?
=?J

(b) How many stairs must the woman climb to perform an amount of mechanical work equivalent to the food energy in one jelly doughnut? Assume the height of a single stair is 15 cm.
=?stairs

(c) If the human body is only 25% efficient in converting chemical energy to mechanical energy, how many stairs must the woman climb to work off her breakfast?
=?stairs

Explanation / Answer

a) 1cal = 4.18J 440kcal = 440 *4.18 = 1.839MJ. b) let number of steps = n. work done = n*m*g*h where m = mass of woman, g = accn due to gravity, h = height of one step. n*m*g*h = 1.839*10^6 n = 19,227 c) mechanical work out = 0.25*chemical energy consumed n is proportional to the chemical energy of the doughnut, since it is calculated on that basis. Let n2 be the number of steps representing 'mechanical work out' n2 = 0.25*n = 4,806.75

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