# Thread: Kinetic energy of a wave question

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## Kinetic energy of a wave question

E = hv and also 0.5mv**2

E = hv and also 0.5mc**2

Does light have mass? If not what is it?

2. light has no mass, but it has momentum

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Originally Posted by tusenfem
light has no mass, but it has momentum
P = mv

P = 0 x v

= 0

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No...

momentum p = E/c = hf/c = h/lambda, where h is Planck's constant, f is the frequency, and c is, well, c! (speed of light). From: E=hf (Max Planck's black-body radiation equation).

5. Originally Posted by mugaliens
No...

momentum p = E/c = hf/c = h/lambda, where h is Planck's constant, f is the frequency, and c is, well, c! (speed of light). From: E=hf (Max Planck's black-body radiation equation).
My bold, which comes from the full equation for energy:

E2 = p2c2 + m02c4, which can be derived somehow. Look it up in a basic physics book to get to know the somehow.

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Originally Posted by tusenfem
My bold, which comes from the full equation for energy.
I was disagreeing with you, tusenfem. I was disagreeing with the Newtonian equation for momentum being applied to relativistic velocities.

Oops! I meant, I "wasn't" disagreeing..." Sorry!
Last edited by mugaliens; 2009-Oct-07 at 09:34 AM.

7. Originally Posted by mugaliens
I was disagreeing with you, tusenfem. I was disagreeing with the Newtonian equation for momentum being applied to relativistic velocities.
Well, yeah, relativistic is not Newtonian.
Newtonian momentum: m v
Relativistic momentum: m v = γ m0 v

Relativistic total energy: E2 = p2c2 + m02c4
As a photon has no m0 this reduces to: E2 = p2c2
As the energy of a photon is: E = h ν (which is btw not "Max Planck's black-body radiation equation")
One can get the momentum of a photon by: p = E/c = h ν / c

Methinks you were disagreeing with rodin.

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Originally Posted by tusenfem
Methinks you were disagreeing with rodin.
Yes.

9. Originally Posted by tusenfem
Relativistic momentum: m v = γ m0 v
And note that there's a good reason why you can't just use the relativistic formula p = γ m0 v for a photon. It's true that m0 is 0, which would lead you to think that the momentum must be zero, but gamma is infinite, which means the value of this expression is indeterminate.

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Originally Posted by Grey
...which means the value of this expression is indeterminate.
Exactly. Fortunately, Planck's equation is determinate.

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