law of conservation

Question

What is the efficiency of an out-of-condition professor who does 2.30 ✕ 10⁵ J of useful work while metabolizing 510 kcal of food energy? (Enter your answer as a percentage.

(a) 

How many food calories (in kcal) would a well-conditioned athlete metabolize in doing the same work with an efficiency of 20%?

(b)

Answer 1

a)

Energy output = 2.3E5 J

Energy input = 510 kcal = 2.134E6 J (from the conversion 1 kcal = 4184 J)

Efficiency = energy output / energy input = 2.3E5 / 2.134E6 = 0.107 = 10.7%

b)

The athlete is 20% efficient, so

0.2 = 2.3E5 J / energy input

energy input = 2.3E5 / 0.2 = 1.15E6 J