Newbie Questions about Units in Physics Equations

In summary, the units in an equation like E=mc^2 can vary depending on the system of units used. 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. The value of m can be calculated by solving the equation for m in terms of the given values for E and c.
  • #1
Incognito310
21
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?
 
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  • #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.
 
  • #3
Incognito310 said:
In an equation like E=mc^2, what are the units?
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.

If E is 500 joules, what is m?

c is 3,000,000 meters per second. Solve for m in kg. The answer will be fairly small.
 
  • #4
jbriggs444 said:
c is 3,000,000 meters per second. Solve for m in kg. The answer will be fairly small.

It is? Light's gotten pretty slow since lunchtime.
 
  • #5
Forgot two 0's there jbriggs...
 
  • #6
Matterwave said:
Forgot two 0's there jbriggs...

So I did. *blush*
 

Related to Newbie Questions about Units in Physics Equations

1. What are the units used in physics equations?

The units used in physics equations are derived from the seven base units in the International System of Units (SI), which are meter (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electric current, kelvin (K) for temperature, mole (mol) for amount of substance, and candela (cd) for luminous intensity.

2. Can I use different units in physics equations?

No, it is important to use the correct units in physics equations as they represent physical quantities and their relationships. Using different units can result in incorrect calculations and conclusions.

3. How do I convert between units in physics equations?

To convert between units in physics equations, you can use conversion factors or unit conversion formulas. It is important to pay attention to the units and their relationships in order to convert correctly.

4. What happens if I forget to include units in a physics equation?

Forgetting to include units in a physics equation can result in incorrect calculations and incorrect interpretations of the results. Units provide important context and meaning to the numerical values in an equation.

5. Are there different systems of units used in physics equations?

Yes, there are different systems of units used in physics equations, such as the SI system and the CGS system. It is important to be familiar with the specific system of units being used in order to correctly interpret and use equations.

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