Example 7.13 from College Physics, 7.8 Work, Energy, and Power in Humans

Calculating Weight Loss from Exercising

If a person who normally requires an average of 12,000 kJ (3000 kcal) of food energy per day consumes 13,000 kJ per day, he will steadily gain weight. How much bicycling per day is required to work off this extra 1000 kJ?

Work it out on paper first. Then open the solution one step at a time, and stop as soon as you can finish on your own.

Solution
Table 7.5 states that 400 W are used when cycling at a moderate speed. The time required to work off 1000 kJ at this rate is then

$Time = \frac{energy}{(\frac{energy}{time})} = \frac{1000 kJ}{400 W} = 2500 s = 42 min.$
7.75
Discussion
If this person uses more energy than he or she consumes, the person’s body will obtain the needed energy by metabolizing body fat. If the person uses 13,000 kJ but consumes only 12,000 kJ, then the amount of fat loss will be
$Fat loss = (1000 kJ) (\frac{1.0 g fat}{39 kJ}) = 26 g,$
7.76

assuming the energy content of fat to be 39 kJ/g.

0 of 2 steps open

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Solution

Discussion