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 "kilowatt-hours" of energy your dwelling consumed over the
past month. A kilowatt is 1000 watts--the power that you would use burning
ten 100-watt lightbulbs. One kilowatt-hour is the ENERGY used to keep those
ten lightbulbs burning for one hour. Again, energy (kilowatt-hours) =
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 friction-an idealized situation that cannot occur in the real world.
Still, with a well-made 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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