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| Physics Question - ma = mg |
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| bostonman:
--- Quote ---herefore, lifting a 50 kg dumbbell by 1 m is 490 J, or only 0.117 kcal. --- End quote --- Just tell me if I'm going over the deep end or just reverting to "calories" being incorrectly used. So if I eat a Snicker's Bar at whatever.... 100 calories, does this theoretically mean I have to lift that dumbbell 855 times (100 divided by 0.117) to burn off the snack? Or are calories on nutrition labels using the wrong units? |
| TimFox:
The question about the conversion of food energy to mechanical energy through biological processes is far more complicated than the Newtonian physics behind lifting a mass in a gravitational field. Since your body is very inefficient, it takes more food energy to do a given amount of physical work, therefore it takes less physical work to burn off a given amount of food energy. Nicolas Léonard Sadi Carnot in 1824 originated the careful consideration of "heat engines", that convert thermal energy into mechanical energy, just as steam engines (invented in the previous century) were becoming important. (A distinguished family. His father Lazare Nicolas Marguerite, Count Carnot was a French revolutionary and did fundamental work on engineering mechanics, and his nephew Marie François Sadi Carnot was President of France at the end of the 19th century.) Compared to a Diesel engine, the human body is not an energy-efficient engine (but it suffices for our normal purposes). As I stated, what dietitians and candy-bar manufacturers call a "calorie" is technically a "kilocalorie" in physics, and sometimes called a "large calorie" to distinguish it from the "small calorie" as defined properly. |
| CatalinaWOW:
Just to pound the point in calories are not imprecise, but calculations of their consumption in the human body are. There are a great number of processes going on, many variables associated with each process all of which are difficult to measure and many of which are either not simple enough or well enough understood to write an input-otput equation. |
| TimFox:
Of course, nutritional scientists work hard to develop legitimate quantitative models and calculations of the human body's usage of and needs for food energy. It is a far more complex problem than the Newtonian mechanics of apples dropping from trees. Since calories are popular units for chemists, the nutritional guys have tended to use them. Not my field, man. |
| CatalinaWOW:
Not only do they work hard, but they do good work. But one size definitely does not fit all and there is a huge population demanding simple answers. Bostonman is on his way to understanding the complexity. Most aren't interested or don't have the capacity. |
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