Question
Find a snack or drink item in the classroom, or at your next meal. Find the total Calories (kilocalories) in the item, and calculate how long it would take exercising at 150 W (moderately, climbing stairs) at 20% efficiency to burn off this energy.
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Solution video
OpenStax College Physics for AP® Courses, Chapter 15, Problem 4 (Test Prep for AP® Courses)
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This is College Physics Answers with Shaun Dychko. I found a chocolate bar and it has 500 Calories in it so we multiply that by 4186 joules for every Calorie and that is 2.093 times 10 to the 6 joules; this is Calorie with a capital 'C' by the way which is also a kilocalorie with lower case 'c'... it's very confusing all these different units here but... anyhow... this is the number of joules we have to burn. Efficiency is the power output divided by the power input and I am going to assume that this figure given to us in the problem is the output power and so we'll solve for P in by multiplying both sides by P in divided by Eff and we get the input power is the power output divided by efficiency so it's a 150 watts divided by 20 percent, written as a decimal 0.20 and that is 750 watts of power consumed in the body to do this exercise... power output. So this energy that needs to be burned equals power multiplied by time and we can solve for time by dividing both sides by P and the time then is the energy divided by the power and the power we'll use here is the power input because we are consuming this candy bar and so that's 2.093 times 10 to the 6 joules divided by 750 watts which is this many seconds multiplied by 1 min for every 60 seconds and that works out to 46.5 minutes of exercise to burn the candy bar.