
Exercise: Work, Energy and Power 
Basic Physics In physics, work can be defined thus: work (energy) = Power X Time Look at your next bill from the power company for example. You will be billed for the number of "kilowatthours" of energy your dwelling consumed over the past month. A kilowatt is 1000 wattsthe power that you would use burning ten 100watt lightbulbs. One kilowatthour is the ENERGY used to keep those ten lightbulbs burning for one hour. Again, energy (kilowatthours) = power(kilowatts) X time (hours). Since exercise is work that uses energy (calories), we can restate the above equation this way: calories burned = Power X Duration This equation can be summed up as follows: "the harder you push and the longer you go, the more calories you burn." Calculating Number of Calories Burned By Classic Physical Principles Another basic physics equation states that the work done upon an object is equal to the force applied to the object multiplied by the distance that the object is moved or: work = Force X Distance Thus, if a man weighing 200 pounds is lifted 100 feet into the air, the work used to do the lifting can be easily calulated if we assume that the lifting is done very slowly and that the machine doing the lifting is perfectly efficient and has no frictionan idealized situation that cannot occur in the real world. Still, with a wellmade machine the real amount of energy used would be only slightly greater than the equation would predict. Where Basic Physics Fails Now, suppose that instead of using a machine to lift the man 100 feet in the air, he decides to do the work himself by walking up a 100 foot tall hill. Will the man, like the machine, use just a little bit more energy than the equation suggests? No. Not even close. He will use far more energy to walk or climb himself up the hill than the physics equation would predict. Why? Because most of the work he performs climbing the hill will end up as heat, not as altitude. The heat will come from the inneficiency of his muscles, from his clothing causing friction, from the friction of his feet upon the ground, from slipping a little here and there, from needing to use his diaphragm muscle to pump more air into his lungs etc. In fact, the man will need to burn far more energy (calories) to get to the top of the hill than the physics equation predicts. Exactly how much more energy is used depends upon other variables that are much harder to quantify than weight, gravity and distance. Summary: Basic physical principles help to illustrate the relationship between power and duration on the one hand and energy used on the other, but the complexity of human movement and physiology make it impossible to use such a simple equation to calculate the actual number of calories burned from exercise. 
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