# Newbie Questions about Units in Physics Equations

1. Jul 19, 2014

### Incognito310

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. Jul 19, 2014

### Anonymous119

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.

3. Jul 19, 2014

### jbriggs444

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.

4. Jul 19, 2014

### SteamKing

Staff Emeritus
It is? Light's gotten pretty slow since lunchtime.

5. Jul 19, 2014

### Matterwave

Forgot two 0's there jbriggs...

6. Jul 20, 2014

### jbriggs444

So I did. *blush*