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- Thread starter jaydnul
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You know what quantities to use when using SI units, right?

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jtbell

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So do centimeters, grams, seconds, and ergs. These are CGS or Gaussian units.

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In real life, you can use any valid units of mass and velocity in [itex]E=mc^2[/itex] and get a meaningful result. Just keep track of what units you used and don't throw them away. You can measure mass in terms of pounds and velocity in terms of feet per second, and you'd say the energy is measured in terms of pounds feet^2/second^2. As it happens, this combination of units does have a name: it's called a foot-pound. (Why is it called a foot-pound? Because imperial units use "pound" for both mass and force. Sometimes the latter is abbreviated lbf. to say pounds of

But really, in order to talk about quantities like energy in terms of nice units (like joules or ergs), you need to have some understanding of what those mean in terms of a small base of units like meters, kilograms, seconds, and so on.

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