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Newbie Questions about Units in Physics Equations

  1. Jul 19, 2014 #1
    In an equation like E=mc^2, what are the units?

    Like, I know c is a constant, but in the equation is it meters-per-second, kilometers per hour, or what? In this, and other equations how do you know what is meant by the letters in terms of their computational value?

    If E is 500 joules, what is m?
  2. jcsd
  3. Jul 19, 2014 #2
    You know that is work A[J]=F[N]*s[m]
    => J=kg* (m/s)^2
    From here you can see that if the c is in m/s them mass must be in kg.
  4. Jul 19, 2014 #3


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    In any "coherent" system of units, (i.e. in a system where the unit of energy is one unit of mass times one unit of distance squared divided by one unit of time squared) the E will be the standard unit of energy, the m will be the standard unit of mass and the c will be the speed of light measured in the standard units of distance per standard units of time.

    In SI, E would be in Joules, m would be in kilograms and c would be in meters per second.
    In cgs, E would be in ergs, m would be in grams and c would be in centimeters per second.

    You could choose to express E in foot pounds, m in slugs and c in feet per second and the equation would hold.

    c is 3,000,000 meters per second. Solve for m in kg. The answer will be fairly small.
  5. Jul 19, 2014 #4


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    It is? Light's gotten pretty slow since lunchtime.
  6. Jul 19, 2014 #5


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    Forgot two 0's there jbriggs...
  7. Jul 20, 2014 #6


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    So I did. *blush*
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