The momentum added to an object by light energy (E) shining on it (eg solar sails) is E/C. Why?
My physics is only up to A-Level, so I'm probably wrongly mixing Newtonian physics with photons.
Can someone explain a simple derivation?
For kinetic energy:
E = 1/2 x m v^2
So I wondered why mv [momentum] does not equal 2E/c (kinetic energy equation) when it's applied to photons.
Answers:
p = mv = (E/c^2) * c = E/c
Sorry matey, no idea, but it looks really intelligent! Keep up the good work.
You're right about being wrong, photon don't follow Newton's laws.
A photon sail isn't like a traditional sail, where the air particles bounce off the sail and directly transmit their kinetic energy.
Photons are absorbed into atoms, and are consumed by exciting electrons. I don't fully understand the process myself, but having excited electrons makes you move. I think they transmit some other sort of energy when they relax to ground state.
The equations you have quoted are classical equations which do not account for relativistic (fast moving) effects. They are a simplification of the relativistic equations. At slow (everyday) speeds, the relativistic equations reduce to the formulas above. At fast moving speeds, like a photon at the speed of light, the relativistic effect becomes the dominant part of the equation.
See the wikipedia entry for relativistic mass for further understanding.
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Can someone explain a simple derivation?
For kinetic energy:
E = 1/2 x m v^2
So I wondered why mv [momentum] does not equal 2E/c (kinetic energy equation) when it's applied to photons.
Answers:
p = mv = (E/c^2) * c = E/c
Sorry matey, no idea, but it looks really intelligent! Keep up the good work.
You're right about being wrong, photon don't follow Newton's laws.
A photon sail isn't like a traditional sail, where the air particles bounce off the sail and directly transmit their kinetic energy.
Photons are absorbed into atoms, and are consumed by exciting electrons. I don't fully understand the process myself, but having excited electrons makes you move. I think they transmit some other sort of energy when they relax to ground state.
The equations you have quoted are classical equations which do not account for relativistic (fast moving) effects. They are a simplification of the relativistic equations. At slow (everyday) speeds, the relativistic equations reduce to the formulas above. At fast moving speeds, like a photon at the speed of light, the relativistic effect becomes the dominant part of the equation.
See the wikipedia entry for relativistic mass for further understanding.
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