Newbie Questions about Units in Physics Equations

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    Physics Units
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Discussion Overview

The discussion revolves around the units used in the equation E=mc^2, specifically how to interpret the variables in terms of their computational values and the implications for mass when energy is given. Participants explore the relationships between energy, mass, and the speed of light within different unit systems.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the units of c in the equation E=mc^2, asking whether it is in meters-per-second or another unit.
  • Another participant explains that in a coherent system of units, E is energy, m is mass, and c is the speed of light, detailing how these relate in SI and cgs units.
  • It is noted that if E is 500 joules, participants can solve for m using the value of c as 3,000,000 meters per second, leading to a small value for m.
  • A humorous exchange occurs regarding the speed of light, with one participant joking about its perceived slowness.
  • Participants acknowledge a mistake in the numerical value of c, with one admitting to forgetting two zeros.

Areas of Agreement / Disagreement

There is no consensus on the specific units used in the equation, as participants discuss different unit systems and their implications. The discussion remains unresolved regarding the interpretation of units and their application.

Contextual Notes

Participants reference different unit systems (SI and cgs) and the implications of these choices on the interpretation of the equation. There are unresolved aspects regarding the exact numerical values and their implications for mass calculations.

Who May Find This Useful

This discussion may be useful for students or individuals new to physics who are trying to understand 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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