- #1

artie

**Speed of Light. What is c? Why use the letter "c'?**

In the equation E=mc2

**E**is energy

**is mass**

m

m

What is

**c**? I know that

**c**is the speed of light, but why use the letter

**c**? What does

**c**stand for?

- Thread starter artie
- Start date

- #1

artie

In the equation E=mc2

m

What is

- #2

nicksauce

Science Advisor

Homework Helper

- 1,272

- 5

- #3

artie

That's a little strange in this case, because

- #4

Doc Al

Mentor

- 45,032

- 1,330

I believe that

Last edited by a moderator:

- #5

- 68

- 0

- #6

- 68

- 0

^^^^^^^^^^^^I believe thatwas chosen forcceleritas, Latin for "speed". (Butfor "constant" works too.) Read: http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/c.html" [Broken]c

or that haha

Last edited by a moderator:

- #7

Mapes

Science Advisor

Homework Helper

Gold Member

- 2,593

- 20

- #8

artie

Excellent!I believe thatwas chosen forcceleritas, Latin for "speed". (Butfor "constant" works too.) Read: http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/c.html" [Broken]c

Since Einstein was German, I had tried to find a German word for "light" that begins with

Thanks

Last edited by a moderator:

- #9

malawi_glenn

Science Advisor

Homework Helper

- 4,786

- 22

that is just a joke.^{2}and crossed it out, then wrote E=mb^{2}and crossed that out...

- #10

- 2,226

- 9

wow, i thought i saw that one too, but it had different powers for^{2}and crossed it out, then wrote E=mb^{2}and crossed that out...

i guess it's a way to do physics; guess (and see if experiment supports your guess). about

- #11

malawi_glenn

Science Advisor

Homework Helper

- 4,786

- 22

Hmm we have derived E = mc^2 at school, so I dont think Einstein just "guessed" that solution.. ;-)

- #12

- 22

- 0

and as i think c^2 is the greatest constant in the physics

i dont know exactly how Eisntein found it...

- #13

malawi_glenn

Science Advisor

Homework Helper

- 4,786

- 22

Just google "derivation of E=mc^2" or search in textbooks about special relativity

- #14

- 22

- 0

i said i dont know how Einstein found this equation....

Just google "derivation of E=mc^2" or search in textbooks about special relativity

when E=mc^2 apllicated on the particles...it was right

so the scientists has no reason to change c^2 to c^5345345345

- #15

- 22

- 0

my own explanation why E=mc^2

Just google "derivation of E=mc^2" or search in textbooks about special relativity

E means Energy, measured by Joule

we all now that F=ma and E=Fr =mar(r is the distance)

so Joule=Newton*Meter=Kg*Meter*sec^-2

for E=mc^2 --> Joule=Kg*(Meter*sec^-2)^2

anybody agree with me??

- #16

malawi_glenn

Science Advisor

Homework Helper

- 4,786

- 22

That is just an argument from units, why couldn't it be: E = 8*mc^2 ?my own explanation why E=mc^2

E means Energy, measured by Joule

we all now that F=ma and E=Fr =mar(r is the distance)

so Joule=Newton*Meter=Kg*Meter*sec^-2

for E=mc^2 --> Joule=Kg*(Meter*sec^-2)^2

anybody agree with me??

You must do the full derivation.

And WHY is c^2 the biggest constant in physics?

i) There are formulas which have c^6 .. aren't that a bigger constant?

ii) c is the constant, c^2 = c*c, i.e the constant c is multiplied with another constant c ...

iii) c is not a real constant, it also have a unit: Lenght/time, so c^2 has units lenght^2/time^2

iv) How can we compare e.g G with c? They have different nummerical values, but they have also different units. Also, you can easy come up with a unit system where G has a bigger nummerical value than c. It is like comparing colour with sound.

/Glenn - 1 term from Masters degree in physics.

- #17

HallsofIvy

Science Advisor

Homework Helper

- 41,833

- 961

That was not original with Einstein. Maxwell had already used "c" for the speed of light in his derivation of the wave equation from his equations for the Electric and Magnetic fields and I suspect it had been used that way before. My understanding is that it is from "celerity" which is Latin for "speed".

- #18

- 14

- 0

yes that's rightI'm still unclear as to what the question is! You say you know that "E" is "energy" and "m" is "mass" and understand that "c" represents the speed of light. Are you really only askingwhythe letter "c" is used for "speed of light?

That was not original with Einstein. Maxwell had already used "c" for the speed of light in his derivation of the wave equation from his equations for the Electric and Magnetic fields and I suspect it had been used that way before. My understanding is that it is from "celeritas" which is Latin for "speed".

Last edited by a moderator:

- #19

- 22

- 0

i said first....im not sure of it...but maybe c^2 is theThat is just an argument from units, why couldn't it be: E = 8*mc^2 ?

You must do the full derivation.

And WHY is c^2 the biggest constant in physics?

i) There are formulas which have c^6 .. aren't that a bigger constant?

ii) c is the constant, c^2 = c*c, i.e the constant c is multiplied with another constant c ...

iii) c is not a real constant, it also have a unit: Lenght/time, so c^2 has units lenght^2/time^2

iv) How can we compare e.g G with c? They have different nummerical values, but they have also different units. Also, you can easy come up with a unit system where G has a bigger nummerical value than c. It is like comparing colour with sound.

/Glenn - 1 term from Masters degree in physics.

biggest physical constant

and tell me about these formulas which have c^6... i hav no idea abt them...

about c is not a real constant?? i didnt understand

...it is a unit that`s right.... so where`s the problem???

c is specified unit (at least in caccum)

- #20

malawi_glenn

Science Advisor

Homework Helper

- 4,786

- 22

Can you please spell better?i said first....im not sure of it...but maybe c^2 is the

biggest physical constant

and tell me about these formulas which have c^6... i hav no idea abt them...

about c is not a real constant?? i didnt understand

...it is a unit that`s right.... so where`s the problem???

c is specified unit (at least in caccum)

The thing is that we can only compare things that have equal units. For example 3Joules - 1 Joule = 2 Joule. But what is 3Joules - 5m/s ? And what is biggest between 300 000 000m/s and 1.626*10^-34 J*s ?

Using your argument, why is not h^-1 bigger than c^2? If we only look at the figures, using SI-units, h^-1 = 1.51*10^33 ...

It is only meaningful to compare quantities with the same units. c.f. pi vs. natural number e, or 2000m/s with 10m/s.

- #21

- 22

- 0

excuse me.....i learned myself the englishCan you please spell better?

The thing is that we can only compare things that have equal units. For example 3Joules - 1 Joule = 2 Joule. But what is 3Joules - 5m/s ? And what is biggest between 300 000 000m/s and 1.626*10^-34 J*s ?

Using your argument, why is not h^-1 bigger than c^2? If we only look at the figures, using SI-units, h^-1 = 1.51*10^33 ...

It is only meaningful to compare quantities with the same units. c.f. pi vs. natural number e, or 2000m/s with 10m/s.

if u know arabic....maybe we can communicate better :D

coz im from Syria

- #22

malawi_glenn

Science Advisor

Homework Helper

- 4,786

- 22

I hope you understand what I wrote.

- #23

- 14

- 0

W= T2-T1= integral F.dr

W Work

T kenitic energy

F force

dr deplacement element

this formula is generalisation of the case in classical mechanics

the force is rate of change of momentum in time

the component // to r is

F//=G(v)m(dv/dt)

m rest mass

G Gamma factor = 1/sqrt(1-v²/c²)

then F.dr=F//.dr

and dr=vdt

W= integrale m G(v) v.dv

the bound of velocity 0"rest" and v

W= T2-T1=G(v)mc²-mc²

this is for free particule " no interaction "

like we know in this case E=T=G(v)mc²

rest energy =mc²

in presence of interaction the effect must be take into account in enrgy's formula

- #24

- 14

- 0

I think that i understand your viewpointCan you please spell better?

The thing is that we can only compare things that have equal units. For example 3Joules - 1 Joule = 2 Joule. But what is 3Joules - 5m/s ? And what is biggest between 300 000 000m/s and 1.626*10^-34 J*s ?

Using your argument, why is not h^-1 bigger than c^2? If we only look at the figures, using SI-units, h^-1 = 1.51*10^33 ...

It is only meaningful to compare quantities with the same units. c.f. pi vs. natural number e, or 2000m/s with 10m/s.

in classiccal mechanics the kenitic energy is T=(1/2)mv²

but if we take the expression of energy in relativity for free particule

E=mc²/sqrt[1-(v²/c²)]

and make a taylor development for v very smaller than c we find

E=mc²+ (1/2)mv²+... =T

u may ask about the origin of mc²

this is not a physical problem because the principle of consevation energy is not broken if we add a constant , because the interising is the amount of change make by the interaction of physical systems in this case representing by the work W= T2-T1 =(1/2)mv2²-(1/2)mv1²

Last edited:

- #25

- 2,226

- 9

let's say it's non-relativistic to start with:

work done: [tex] T = \int_0^{x_0} F \ dx [/tex]

the force

in the classical case. we can pull the mass

so this work integral comes out as:

[tex] T = \int_0^{x_0} F \ dx = \int_0^{x_0} \frac{d(m_0 v)}{dt} \ dx [/tex]

with two substitutions of variable (that we learn in calculus):

[tex] T = \int_0^{m_0 v_0} v d(m_0 v) = m_0 \int_0^{v_0} v dv [/tex]

where

[tex] T = m_0 \int_0^{v_0} v dv = \frac{1}{2} m_0 v_0^2 [/tex]

the familiar kinetic energy formula for classic mechanics. we had to do

hey guys, i just realized i gotta get going before i finish this. i'll come back to it tonight. you can kinda see how it will go. in the relativistic case, the mass is not constant, the integral will come out different and the kinetic energy will come out to be

[tex] T = m c^2 - m_0 c^2 [/tex]

where [tex] m = m_0 \frac{1}{\sqrt{1 - \frac{v_0^2}{c^2}}} [/tex]

the relativistic mass. the interpretation of the kinetic energy in the above equation is

[tex] T = m c^2 - m_0 c^2 = E - E_0 [/tex]

where

i'll get back to this later.

- Last Post

- Replies
- 15

- Views
- 8K

- Replies
- 4

- Views
- 1K