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Question: 

A 65-kg person decides to lose weight by sleeping one hour less per day, using the time for light activity. How much weight (or mass) can this person expect to lose in 1 year, assuming no change in food intake? Assume that 1 kg of fat stores about 40,000 kJ of energy.

Source: Giancoli, Douglas C., Physics: Principles with Applications, 7th Edition, 2014.

Quick Answer: 

$5 \textrm{ kg}$

Giancoli 7th Edition, Chapter 15, Problem 16

(1:27)

Chapter 15, Problem 16 is solved.

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Transcript for this Giancoli solution

This is Giancoli Answers with Mister Dychko. We're going to substitute the energy burned while sleeping for one hour, so that's 70 joules per second times 2600 seconds and we're going to replace it instead with some light activity, 230 joules per second times 3600 seconds. This is 230 watts times an hour basically but converted into seconds. So, the difference then is burning an additional 576 kilojoules per day. So yeah, this happened. This is the only difference for the entire day otherwise the food intake is the same and their other activities are the same. Take that energy difference per year and we go 576 kilojoules per day of extra energy burned times 365 days and you get 210000 kilojoules per year. Take that 210240 kilojoules per year multiplied by one kilogram of fat for every 40000 kilojoules and we get about five kilograms burned per year as a result of sleeping one hour less and doing light activity instead of sleeping.