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## Unit of measurment of C in E=MC^2 to use?

The c in this equation stand for the velocity of light right? Well depending of what unit of velocity I use, wouldn't that change the answer I get? What unit should i use?

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• #### Lawren Jones20+

• +2
Oct 5 2012: You should use whatever units you want E to be, since E will be in whatever units you've started with for M and C. For instance, if you want E to be expressed in kg·m/s^2, then your velocity of light should begin in m/s^2, not km/hr^2 and certainly not miles/s^2. Remember that the units you begin with will carry across the entire equation.
• #### John Smith30+

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Oct 5 2012: This.
• #### edward long100+

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Oct 5 2012: Just to be clear, pedantic even, Energy is always expressed in joules. It would never be correct to express Energy in Kg X m/s^2.
• #### John Smith30+

• +1
Oct 5 2012: It's the same thing, it really doesn't matter if you choose to express it in joules or kgm/s^2.
• #### Lawren Jones20+

• +2
Oct 5 2012: Energy is always expressed in joules, except when its expressed in electronvolts (eV), ergs, foot-pounds, Btus, kilowatt-hour (kWh), calories, etc.
• #### edward long100+

• 0
Oct 5 2012: I do not recall seeing the energy quantity for Einstein's equation, which we are discussing, expressed in any of the units you mention. The only appropriate unit in the SI or MKS system is the joule. Context is important. Foot-pounds are for use in the foot-pound-second system. The electron volt is a unit of electrical energy. Ergs are for use in the CGS system. The BTU is a measure of heat. The kilowatt-hour is a unit of electrical energy. The calorie is a measure of heat.
• #### John Smith30+

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Oct 5 2012: eV is often used alongside official SI units.
• #### Lawren Jones20+

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Oct 5 2012: I fear that you are being pedantic now, Edward. There's noting mystical about the equation E=mc^2, famous though it may be. It's simply a way to calculated energy, in whatever units you wish, as long as you use consistent units across the entire equation. By the way, heat IS energy.

http://en.wikipedia.org/wiki/Units_of_energy
• #### edward long100+

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Oct 5 2012: Perhaps our shoving contest is due to my lingering confusion regarding the posted question (as I originally commented). Exactly what is the question Mr. Sankar is asking? I think he is asking if he should use joules or ft/lbs or some other unit. My answer is joules.
• #### edward long100+

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Oct 5 2012: It is not clear what you mean by the phrase "unit of velocity". As you know VELOCITY is a measure of both speed and direction expressed in units of distance and time such as Meters Per Second (m/s), or Miles Per Hour (mph). The (c) in Einstein's equation represents the SPEED of light in a vacuum. There are several ways of expressing that speed, for example: c = 2.99792458 X 10^8 meters/sec. = 2.99792458 X 10^5 km/sec. = 1 Planck Unit = 1.08 X 10^6 km/hr = 1.86282 X 10^5 mi/s = 6.706166 X 10^8 mi/hr = 173 AU/day. Take your pick. The SPEED of light in a vacuum does not change depending on which unit-of-measure is used.
• #### John Smith30+

• 0
Oct 5 2012: Some languages don't have different words for "speed" and "velocity", the difference only matters when working with vectors, both have unit m/s.
• #### Entropy Driven20+

• +1
Oct 5 2012: Exactly. Thus any units will work. The final results will be equivalent, only expressed in different units. That's it. Basic math, basic physics.