# Homework Help: Energy from a Fast Food Meal

1. Dec 4, 2009

### KatlynEdwards

1. The problem statement, all variables and given/known data

For how long would a 68 kg person have to swim at a fast crawl to use all the energy available in a typical fast food meal of burger, fries, and a drink? Assume 25% efficiency.

2. Relevant equations

Fast food meal provides 1350 Cal or 5670 kJ
A 68 kg person swimming at a fast crawl uses 800 W of metabolic power.

3. The attempt at a solution

I'm having trouble just setting up the equation.
I know that Change in Thermal Energy + Change in Chemical Energy = 0 because it is an isolated system, but I don't know where to go from here, nor do I know what units I need each part to be in.

2. Dec 4, 2009

### Pengwuino

Well, just think about this question for a second. This person intakes a certain amount of energy by eating the food. 25% of that energy is useable. The person then performs some activity that takes a certain amount of energy per second (or 800 J per second). You should be able to determine how long the person can swim just by giving the question some thought.

3. Dec 4, 2009

### KatlynEdwards

So would it be 5670*.25=800W* time?

Solving for time gives me 1.77 - but what units is this?

4. Dec 4, 2009

### KatlynEdwards

Op never mind, I needed to change kJ into Joules, then it gives me time in seconds. Whoops. :D

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