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# Sci-fi Laser : Ironman

#### Lazerbeak

##### New member
Beam-powered propulsion - Wikipedia, the free encyclopedia

The rule of thumb that is usually quoted is that it takes a megawatt of power beamed to a vehicle per kg of payload while it is being accelerated to permit it to reach low earth orbit.
...although some obtain propulsion directly from light pressure acting on a light sail structure, and at low altitude heating air gives extra thrust
Interesting question :thinking:

~ LB

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#### EpicHam

##### New member
By the way, photons don't have mass. They have energy. While there is a mass-energy equivalence, it's irrelevant in the case of a photon, since they never slow down (which is the only thing that contributes to mass-energy equivalence in matter). Light exerts pressure though because it has momentum (it has energy, velocity, etc.). Weird to think that something without mass can have momentum, but it can!
well where does that radiation pressure come from ??

#### Wolfman29

##### New member
Radiation pressure comes from transfer of momentum! That's what happens when a photon strikes a mirror and bounces off of it. It decreases in wavelength slightly, and thus loses momentum (VERY SLIGHTLY).

#### EpicHam

##### New member
Radiation pressure comes from transfer of momentum! That's what happens when a photon strikes a mirror and bounces off of it. It decreases in wavelength slightly, and thus loses momentum (VERY SLIGHTLY).
so... why does something without mass have momentum?

#### Wolfman29

##### New member
We all agree photons carry energy. That's a thing.

The energy is calculated via (I think?) the de Broglie relation: E = hv, where h is Planck's constant and v is the frequency.

Then, we apply the relativistic definition of energy: E^2 = p^2 c^2 + m^2 c^4. Setting m = 0 (because photons have no momentum), we have:

E = pc

Now, we know E = hv, so we instead have
hv=pc.

Solving for momentum, we have
p = hv/c.

Turns out, this expression is experimentally verified.

EDIT: I suppose you were asking for why, not what. Gimme a minute.

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#### EpicHam

##### New member
We all agree photons carry energy. That's a thing.

The energy is calculated via (I think?) the de Broglie relation: E = hv, where h is Planck's constant and v is the wavelength.

Then, we apply the relativistic definition of energy: E^2 = p^2 c^2 + m^2 c^4. Setting m = 0 (because photons have no momentum), we have:

E = pc

Now, we know E = hv, so we instead have
hv=pc.

Solving for momentum, we have
p = hv/c.

Turns out, this expression is experimentally verified.
ohhhh!
I remember that back from high school :yh:
So thats why !
But there is a problem tho.
Its not an elastic collision , so how do we know how much of the momentum is transferred to Ironman's arm and how much just blings off?

#### Wolfman29

##### New member
We figure it out by the reflectivity of the material

#### EpicHam

##### New member
We figure it out by the reflectivity of the material
now we got the maths done , so how much would the recoil be?

Newton's third law still applies to photons right?

#### Wolfman29

##### New member
It should. I haven't looked deep enough photonic propulsion though to know how to do the calculations exactly =p