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ijurcough ijurcough
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11 years ago
The formula of momentum is (mass*velocity) right? So a photon, being massless would have zero momentum too. But i read that it hass momentum, but no mass.

i need a fast answer please!
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#1
11 years ago
a photon has "apparent" mass.  we know it has mass because it has been proved that photons have energy and momentum, so they must have mass, however small.

if an object sits outside in the sun all day long, its mass will increase simply because photons have been striking it and sticking all day.  you may not be able to see that increase using a scale, but it did increase a very small amount.

since a photon has energy, then it must have mass.  you remember E = m*c^2 right?  well flip that around.  to find the mass of a photon, use its energy and speed.

m = E / c^2

see more below.
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#2
11 years ago
Photons travel near the speed of light.  Because of this, relativistic effects much be taken into account.  Momentum is actually equal to E/c.  The energy of a photon is hv where h is planck's constant and v is its speed.  This can be rewritten as E = hc/(lamda) where lamda is the wavelength of the photon.  Now plug this into the momentum equation and you get p = h/(lamda).
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#3
11 years ago
Lamda = h/p
therefore p = h/lamda
now lamda = c/n (n= frequency) and (c = velocity of light   as photons travel at the speed of light)
therefore p = hn/c.
All conditions satisfied without the use of mass
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#4
11 years ago
energy of a photon is E = hv v - frequency.
and momentum of a photon is p =hv / c, c=3*10^8m/s.
from einsteins relativistic energy relation                E = sqrt((cp)^2 + m(rest)c^4).
moving mass of a photon is

E=mc^2 = hv.
therfore mass is hv/c^2 which is not zero.
therfore momentum (p) is hv/c^2 * c = hv/c.
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ijustdontknowijustdontknow
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11 years ago
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