Would a laser gun recoil? - Polidicks

[QUOTE] Posted by Jesse at 11:16 PM Today I'd like to approach a question near and dear to many a geek heart: do laser guns have recoil? [IMG]http://2.bp.blogspot.com/_SYZpxZOlcb0/S76cWh0vVrI/AAAAAAAAAA8/3AVsx5suF0A/s200/19990814dogfight.jpg[/IMG] Let's motivate our question a little bit. I've wondered about this question since I saw star wars. Though I'm no firearms expert, the recoil in guns must come from conservation of momentum principles. Momentum is conserved in a system. The gun starts with zero momentum. We fire, give the bullet momentum, and so to keep the system at zero momentum, the gun must gain equal and opposite momentum. That is, the gun will move backwards. All of that was for conventional guns. Light carries momentum, so if we fire a pulse of light, we expect our laser gun to recoil. So yes, they do have recoil. Satisfied, dear readers? Neither am I. The question we really mean to ask is, does a laser gun have noticeable recoil? We need to make a few reasonable assumptions. Let's assume that the laser gun fires a pulse with as much energy as a bullet has kinetic energy, KE. The energy, E, of light is related to its momentum, p, by E=pc, where c is the speed of light. This gives a momentum of [IMG]http://i46.tinypic.com/2zntuuo.jpg[/IMG] What is the kinetic energy of a bullet? A little searching reveals that a .22 bullet is ~2.5g and fires with a muzzle velocity of ~330m/s. Kinetic energy is given by KE=1/2mv^2, where m is mass and v velocity. So, the momentum of a laser pulse with equal energy would be [IMG]http://i46.tinypic.com/24zbqz8.png[/IMG] For comparison, the momentum (p=mv) of a .22 bullet is .83 kg*m/s. The momentum of a laser gun is 2 million times less than the momentum of a .22. But is momentum all we should consider? I suspect the 'kick' we feel on the recoil is directly related to the force that the gun exerts on the holder. This means that instead of momentum we need to consider impulse, momentum per time. We estimate the time it takes to fire a .22 is ~.1s, so the force delivered 8.3 N. Let's estimate the time it takes a laser gun to fire. Unfortunately, not having a laser gun to fire (feel free to send me one, dear readers), we're more or less going to have to guess at the firing time. Most movies with laser guns show pulses of light (which, incidentally would move so fast we wouldn't see them) on the order of a meter or two long. Given the speed of light, this would give a firing time of ~30 nanoseconds. This would give a force delivered of 15 N. This is close to what we estimated or a .22. So, if movies are to be believed (and really, why wouldn't we believe them?), it seems like laser guns may well have recoil. [I]Note: It is worth questioning if we need the same energy for a laser as for a bullet. That could certainly change our estimate. Maybe we'll return to this question again.[/I] [/QUOTE] Source: [url]http://thevirtuosi.blogspot.com/2010/04/today-id-like-to-approach-question-near.html[/url]

Fun fact, lasers in star wars are not realistic, because you cannot see lasers in space.

[QUOTE=Str4t0s;22084169]Fun fact, lasers in star wars are not realistic, because you cannot see lasers in space.[/QUOTE] unrealistic things in a space opera franchise? I would have never known.

[QUOTE=Str4t0s;22084169]Fun fact, lasers in star wars are not realistic, because you cannot see lasers in space.[/QUOTE] No shit sherlock. Nor are the sounds the X-Wings or other spacecraft produce in space, or the explosions in space etc.

[QUOTE=Str4t0s;22084169]Fun fact, lasers in star wars are not realistic, because you cannot see lasers in space.[/QUOTE] Actually you can, if you are standing in front. If you can see the laser on it's path then it's a shitty laser because of dispersion. Or because there are particles there reflecting stuff,

Photons Don't Have Mass, So No

Yes, it would have recoil

100%

[QUOTE=Ishmael12;22084225]Photons Don't Have Mass, So No[/QUOTE] Photons have momentum. Energy and mass are interchangeable at the subatomic level.

[QUOTE=petieng;22084304]Photons have momentum. Energy and mass are interchangeable at the subatomic level.[/QUOTE] :ssh:

What a waste of time

It will not.

In short no. Unlike a bullet, a 'Lazer' would not make any recoil and as said previously has no mass. Seems like a stupid question to ask...

[QUOTE=Retarded Turtle;22084925]In short no. Unlike a bullet, a 'Lazer' would not make any recoil and as said previously has no mass. Seems like a stupid question to ask...[/QUOTE] [QUOTE=petieng;22084304]Photons have momentum. Energy and mass are interchangeable at the subatomic level.[/QUOTE] :eng101:

[QUOTE=petieng;22084304]Photons have momentum. Energy and mass are interchangeable at the subatomic level.[/QUOTE] They aren't "interchangeable" in the sense you can use one instead of another, they're interchangeable in that they can be converted into each other.

[QUOTE=petieng;22084304]Photons have momentum. Energy and mass are interchangeable at the subatomic level.[/QUOTE] Very little momentum. That's why using photons as a propulsion system is ridiculous, it requires 300 terawatts of power for a single newton of thrust. Unless you exchange the momentum with a solar sail, but it still requires a lot of power.

[QUOTE=metallics;22084994]They aren't "interchangeable" in the sense you can use one instead of another, they're interchangeable in that they can be converted into each other.[/QUOTE] Yes but this is the reason photons have momentum and so a laser gun would have recoil. If you're doing a calculation on particle collisions, you can treat energy and mass as the same thing.

[QUOTE=Ishmael12;22084225]Photons Don't Have Mass, So No[/QUOTE] [url]http://en.wikipedia.org/wiki/Radiation_pressure[/url] Also, capital letters only go at the start of sentences, headlines, or proper nouns. :downs:

[quote=snoops;22084247]yes, it would have recoil[/quote] no, it wouldn't

Even someone who only paid attention to Star Wars as a little kid should know that the "lasers" in Star Wars actually fired bolts of pressurized plasma.

[QUOTE=Mr Donovan;22087276]Even someone who only paid attention to Star Wars as a little kid should know that the "lasers" in Star Wars actually fired bolts of pressurized plasma.[/QUOTE] God, I mean, common knowledge.

[QUOTE=Mr Donovan;22087276]Even someone who only paid attention to Star Wars as a little kid should know that the "lasers" in Star Wars actually fired bolts of pressurized plasma.[/QUOTE] Well duh, anyone with half a brain also noticed that the floating mountains in Avatar were room-temperature superconductors or that the Sun in Sunshine was dying due to a Q-ball infection. I mean seriously, how can someone not realize?

Everything that uses momentum and moves has an equal and opposite reaction. It might be less noticeable, but there will still be recoil.

[QUOTE=Str4t0s;22084169]Fun fact, lasers in star wars are not realistic, because you cannot see lasers in space.[/QUOTE] If you weren't so uneducated, you would know that weapons in Star Wars were BLASTERS and not Lasers. Projectiles made of plasma are visible.

Either way, a "laser" gun is still accelerating a mass of some sort, and due to the conservation of momentum some of the force has to be put back on the gun. It's no different than firing a bullet.

[QUOTE=OrionChronicles;22087104]no, it wouldn't[/QUOTE] But...it would. [QUOTE=Eudoxia;22087866]... or that the Sun in Sunshine was dying due to a Q-ball infection. I mean seriously, how can someone not realize?[/QUOTE] It's a shame they left that bit of science out of the film (probably to make it more accessible), but it just meant people slated it for being unrealistic.

Do flashlights have recoil? Do the awesome little laser pointers that can light stuff on fire have recoil? No.

[QUOTE=starpluck;22084216]No shit sherlock. Nor are the sounds the X-Wings or other spacecraft produce in space, or the explosions in space etc.[/QUOTE] Actually, as the novelization of A New Hope (I believe) pointed out, most every starfighters is equipped with a virtual sound system, in which the ship's various sensors work out a sort of sound space outside of the starfighter which is then fed back to the pilot to give them a better sense of position in the battle. From that, one could assume that the sounds we hear in the movies in outer space are merely the synthesized sounds of battle from inside of the fighter's cockpit.

Mirrors are the lasers worst enemy.

[QUOTE=Chris122990;22089551]Do flashlights have recoil? Do the awesome little laser pointers that can light stuff on fire have recoil? No.[/QUOTE] They do, it's just immeasurably small.