Newbie Questions about Units in Physics Equations

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SUMMARY

The discussion centers on the units used in the famous equation E=mc², specifically addressing the units of energy (E), mass (m), and the speed of light (c). In the International System of Units (SI), E is measured in joules, m in kilograms, and c in meters per second. The conversation also touches on the coherent system of units, explaining that the equation remains valid across different unit systems, such as cgs, where E is in ergs, m in grams, and c in centimeters per second. A practical example is provided, calculating mass when E is 500 joules and c is 3,000,000 meters per second.

PREREQUISITES
  • Understanding of basic physics concepts, particularly energy, mass, and speed of light.
  • Familiarity with the International System of Units (SI) and coherent systems of units.
  • Basic algebra skills for solving equations.
  • Knowledge of unit conversions between SI and cgs systems.
NEXT STEPS
  • Research the concept of coherent systems of units in physics.
  • Learn about unit conversions between SI and cgs systems.
  • Explore the implications of E=mc² in various physical contexts.
  • Study the derivation of energy units from force and distance in physics.
USEFUL FOR

Students of physics, educators teaching physics concepts, and anyone interested in understanding the relationship between energy, mass, and the speed of light in various unit systems.

Incognito310
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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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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.
 
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.
 
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.
 
Forgot two 0's there jbriggs...
 
Matterwave said:
Forgot two 0's there jbriggs...

So I did. *blush*
 

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