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ssope
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Does a laser have a recoil since photons have momentum. Would a moving laser be slowed down (even a fraction) if was locked in the on position and then thrown at the ground
Matterwave said:The laser does indeed have recoil; however, the momentum of the particles p=h/lambda isn't very much (h is a very small number).
The laser would be slowed a very small fraction. (very small!)
Matterwave said:I mean, we can work out some math.
p=h/lambda=E/c
This laser I have at home has <30 miliwatts of power. So, each second, my laser's momentum flow is only about 1*10^-10 kgm/s. Which isn't enough to make much of a difference. To get me, a 60kg person, moving at 10 m/s, I would have to be in perfectly frictionless space and be pushed by that laser for 190,000 years.
Now, there are much larger lasers in the world. I am only talking about those hand held lasers.
Matterwave said:By my calculations, it would take a 14.7 trillion watt laser to accelerate a 5 ton payload at 1g...
At 1g it would take about half a year to go .5c (I didn't want to mess with relativity, so I keep gamma small.)
hamster143 said:Thrust to power ratio is inversely proportional to exhaust speed. Any rocket scientist would tell you that. Lasers have huge exhaust speed (300,000 m/s), therefore very low thrust per unit of energy. Turboprops have modest exhaust speeds, therefore they are quite useful for propulsion. The real reason to go for lasers appears when you're already moving extremely fast to begin with, say, if you want to accelerate beyond 0.5c ... then lasers, and, more generally, photon engines are the way to go.
ssope said:Is the recoil of a laser precisely Planck's constant divided by wavelength?
LostConjugate said:The recoil is the force on the laser device. Wouldn't a laser at 1 Watt experience a force of 1 Newton per second?
Frame Dragger said:a 50 Watt laser pointer
LostConjugate said:The recoil is the force on the laser device. Wouldn't a laser at 1 Watt experience a force of 1 Newton per second?
LostConjugate said:The recoil is the force on the laser device. Wouldn't a laser at 1 Watt experience a force of 1 Newton per second?
SpectraCat said:WOW ... where can I buy one of those?
sylas said:Force is in Newtons, not Newtons per second. The force from a 1 Watt laser is 1/c Newtons.
Matterwave said:Which turns out to be ~(1/3*10^-8 Newtons). A 1 watt laser is also quite strong...most conventional lasers are in the milliwatts range.
Matterwave said:Yes, I'm talking about those laser pointers. Sorry if "conventional" wasn't the correct word to use. I have a 30mW laser, and it will point (at night of course) pretty much across town...
In order to go from Watts to N/s one would have to divide by meters...there is to reason to divide the power by 1 meter tho...
LostConjugate said:Ok I guess the correct term is 1 Watt is 1 Newton Meters per second.
So if you pointed a 1-Watt laser at a mirror the mirror should experience a force of 1 Newton Meter per second, so by conservation of energy the kick in the laser would be the same.
Or how about this. 1 Watt laser would be 1 Joule per second kick
SpectraCat said:LostConjugate said:The recoil is the force on the laser device. Wouldn't a laser at 1 Watt experience a force of 1 Newton per second?
Ummm ... not sure how you got there.
1) *IF* the total energy in the beam were carried as kinetic energy by massive bodies, then Newton's third law would apply and you could analyze the problem in such terms. But as has already been posted, the momentum carried by a photon is given by p=h/lambda, while the energy is given by E=pc = h x frequency. So the beam carries a lot of energy per unit momentum.
2) How did you arrive at those units? Power is energy/time, energy (work) is force x distance ... how does the distance factor into your estimate?
Matterwave said:Sylas said:Force is in Newtons, not Newtons per second. The force from a 1 Watt laser is 1/c Newtons.
Which turns out to be ~(1/3*10^-8 Newtons). A 1 watt laser is also quite strong...most conventional lasers are in the milliwatts range.
LostConjugate said:It does not make sense to have the 1/c term in there, c is imbedded in the laser being 1 Watt. If you shine 1 Watt of light off a reflective disc in space it should gain 1 joule per second of kinetic energy, which means you would get a 1 joule kick per second from shining the laser.
SpectraCat said:Why on Earth do you think that? What is the kinetic energy of a photon? I already told you that you can not think of photons as massive bodies that transfer kinetic energy to their targets ... do you think that I am wrong? Photons have momentum ... I gave you the formula for that too .. p=E/c ... that is why matterwave correctly said you needed to divide by c.
Also, if the object is reflective, then most of the energy will propagate in a different direction after the reflection ... so all of the energy is not absorbed by the target. How do you account for that?
SpectraCat said:Why on Earth do you think that? What is the kinetic energy of a photon? I already told you that you can not think of photons as massive bodies that transfer kinetic energy to their targets ... do you think that I am wrong? Photons have momentum ... I gave you the formula for that too .. p=E/c ... that is why matterwave correctly said you needed to divide by c.
Also, if the object is reflective, then most of the energy will propagate in a different direction after the reflection ... so all of the energy is not absorbed by the target. How do you account for that?
Frame Dragger said:30mW POINTER?! Heh... You have good toys. So... has it ever had any noticable 'kick' when you use it to perform home LASIK surgery? :rofl:
Just give in and buy a 60 watt laser engraver, and take out the laser assembly...
There was a great comic called Bloom County; in it a young boy (scientist) name Oliver shows up at school for his science project. His professor asks him what he made, and Oliver points to a large object and says, "A thermonuclear bomb". The professor asks, "And where did you get the fissile material?" Oliver replies, "I scraped the glowing stuff off a million Micky Mouse watch hands." -A pause...- the professor turns to the room and claps, "Ok, fire drill everyone!"
:rofl:
Ok, so micky mouse watches don't have tritium, and that's not fissile material, but sometimes these ideas are too fun to leave alone. Laser pointers that can spot weld are clearly the next logical concept.
Matterwave said:Obviously, not all of the energy of the laser will go into kinetic energy of the object. Almost all of the energy is scattered off. What the laser DOES instill into the object is momentum... and the momentum of light is precisely p=E/c.
Matterwave said:lol!
I think some of those old glow in the dark stuff actually had Uranium in it...although, it wouldn't be weapons grade uranium but still...maybe the kid was real smart and found a way to enrich it XD. The fusion-able material needed (Deuterium - .015% abundance, and tritium - unstable) is easier to get than the fissile material...not easy by any means, but if the kid was able to enrich the uranium from glow in the dark watch hands, maybe he had something up his sleeve!
Anyways, @Lost Conjugate, the "kick" should be a force (which is measured in Newtons, not Joules!) and not some energy/time deal you have going on. What you are saying though is that a 1 Watt laser will get a 1kg block to 1 m/s in half a second (K=.5mv^2)...which is obviously not true. If you hold this view, my 30mW (.03W) laser should be able to accelerate a paperclip (1g) to something like 8m/s in 1 second! I've never witnessed my laser pointer moving anything like that haha. Although having such a laser pointer would be the epitome of cool...coolness does not equal fact.
Obviously, not all of the energy of the laser will go into kinetic energy of the object. Almost all of the energy is scattered off. What the laser DOES instill into the object is momentum... and the momentum of light is precisely p=E/c.
LostConjugate said:However if you measured the energy from a light beam over 1 second and it came out to be 1 Watt, all that energy would be measured in force.
Also p =E/c but the light is traveling at c, you not measuring the force of a single photon, your measuring over a crap load of photons traveling at c for 1 second.
Frame Dragger said:Actually I think the joke was in reference to the tritium that is used on high end watches (still used), not to mention ACOG and other high-end gun sights. Of course, mickey mouse watches do NOT use tritium. :rofl: The idea was that he was going to make a real mutliple stage weapon a la Teller-Ulam! Good stuff. The tritium would fuse with deuterium... of course now this role is filled with the single Lithium-Deuteride. *shrug* a mushroom cloud is a mushroom cloud.
As for the laser pointer, it would be a coherent BEAM! So... if you swept it across a room it should act like a giant bat in that scenario. I think LostConjugate might have been thinking of the machines the Ghostbusters use.
LostConjugate said:well I might be wrong, but I that was just my opinion seems like a laser measured at 1Watt would give a return kick of 1Watt unless the energy was somehow dispersed as heat which I don't think it is.