What would happen if you had a solar sail with a lightsource attached?

[ussfletcher]

So, say you put a solar sail in space and attached a very low mass, high intensity light source to it. Would the system propel itself?

On earth we can easily see that having a fan blow on a sail of a ship would have no effect. Photons, however, are massless but they can impart a force, so would the same rules apply?

For clarity: The light source's energy is infinite through some fantastical invention. The question specifically is about the force imparted on both the 'flash light' and the sail. I think we can all agree that photons would exert force on the sail, the question is: would a counter force be applied to the 'flashlight' as well?

 

[NikolaeVarius]

Uh you're kidding right?

 

[M0oG0oGaiPan]

wouldn't the light require a source of energy?

 

[Sukhoi]

Um it's the same concept as any engine/motor. Put energy in, get force out.

 

[ussfletcher]

I don't think this really has anything to do with the energy, consider the lightsource to have infinite energy. This is more about the force imparted on the 'flashlight' than anything else.

 

[Inferno0032]

Location: Michigan State University

I see now.

JKz ;-)

 

[zinfamous]

wouldn't the light require a source of energy?

duh, the solar sail, of course!

/facepalm

 

[SphinxnihpS]

I can make a fish float in water.

 

[ussfletcher]

Edited OP for clarity.

 

[Meghan54]

Guess it'd work, provided your flashlight was as large as our Sun.

For clarity: The light source's energy is infinite through some fantastical invention.

But if you had an infinite power source, why would you even mess around with a solar sail?

 

[ussfletcher]

Guess it'd work, provided your flashlight was as large as our Sun.




But if you had an infinite power source, why would you even mess around with a solar sail ?

 

[rivan]

Even with your magical flashlight, it wouldn't work.

 

[DrPizza]

OP, that's an absolutely brilliant idea! Another equally brilliant idea - lets build sailboats. But, in case of days without any wind, we'll put big fans on them.

/sarcasm.

If you have an infinitely powered flashlight, just aim the flashlight in the opposite direction that you want to go. No sail required. (Duh!)

 

[gophins72]

So, say you put a solar sail in space and attached a very low mass, high intensity light source to it. Would the system propel itself?

On earth we can easily see that having a fan blow on a sail of a ship would have no effect. Photons, however, are massless but they can impart a force, so would the same rules apply?

For clarity: The light source's energy is infinite through some fantastical invention. The question specifically is about the force imparted on both the 'flash light' and the sail. I think we can all agree that photons would exert force on the sail, the question is: would a counter force be applied to the 'flashlight' as well?

assuming that the flash light was not so fantastical that it can avoid newton's third law of physics then yes, there would be an equal but opposite force that exerts from the flash light. And unless the sail itself was so efficient that it doesn't lose any energy, then the system would propel itself, but in the opposite direction that the flash light is shining. The sail would act as a brake or dampening force.

 

[SunnyD]

I've got this perpetual motion device I'd like to sell you...

 

[gorcorps]

If we're assuming as something as absurd as an infinite energy source for your light, then you can say whatever you damned well please. In this newly constructed world of yours it's entirely possible that your energy sail will spontaneously transform into a unicorn which will propel itself by shooting rainbows out its ass.

 

[ussfletcher]

assuming that the flash light was not so fantastical that it can avoid newton's third law of physics then yes, there would be an equal but opposite force that exerts from the flash light. And unless the sail itself was so efficient that it doesn't lose any energy, then the system would propel itself, but in the opposite direction that the flash light is shining. The sail would act as a brake or dampening force.

So, if an object (such as a matter-antimatter collision) is naturally emitting photons, the photons impart a force opposite the direction of their motion to the emitter?

 

[Ken g6]

For clarity: The light source's energy is infinite through some fantastical invention. The question specifically is about the force imparted on both the 'flash light' and the sail. I think we can all agree that photons would exert force on the sail, the question is: would a counter force be applied to the 'flashlight' as well?

Absolutely. Photons are said to be "massless" only because they have no intrinsic rest mass. Because a photon carries energy, and energy is mass (e=mc^2), photons in practice "have" (carry) mass.

Now, practically, whether your light sail would move the system depends on the reflectivity and configuration of the sail. Any practical light sail would act just like the reflector inside the flashlight, effectively giving you a bigger flashlight, with two exceptions:

1. If the sail is absolutely black, absorbing all light, it will absorb the flashlight's light and re-emit it in the infrared. This light will be emitted in all directions equally, producing no net force, except that some will re-impact the flashlight, so that could produce a tiny reverse thrust.

2. If the sail reflects all light back into the flashlight...the flashlight will probably heat up until it explodes. If it doesn't, the sail must be absorbing and re-emitting enough light that the above scenario (with the black sail) happens again.

 

[DrPizza]

So, if an object (such as a matter-antimatter collision) is naturally emitting photons, the photons impart a force opposite the direction of their motion to the emitter?

Photons carry momentum. Conservation of momentum, so, yes.

 

[gophins72]

So, if an object is naturally emitting photons, the photons impart a force opposite the direction of their motion to the emitter?

i dont believe so, photons have no mass but have momentum, so whatever is putting those photons into motion is imparting a force on it, and then by conservation creating an opposite force. it is hard to say since we're talking about a made up machine. i'm also guessing -- i'm not a physicist.

also, solar wind consists of electrons and protons (which have mass) as well as photons. i'm assuming your light also is emitting more than just photons.

 

[ussfletcher]

Alright guys thanks to the last 3 posts and the Mössbauer effect I have my answer.BTW, I was basically just looking for someone to justify my thoughts without just giving me a generic 'Newton's laws' explanation. Also, I did not actually suspect the OP to be a workable situation, it was simply a thought experiment for the forces exerted by the emissions of photons.

 

[Patranus]

What happens when you attach a fan to a sailboat?

 

[ussfletcher]

What happens when you attach a fan to a sailboat?

I addressed that in the OP.

 

[silverpig]

What if it's on a treadmill?

 

[marvdmartian]

Like Alan Dean Foster (author of many a sci-fi/fantasy book) invented, the KK drive. Basically, he mounted a device on the front end of a ship, that could generate the gravity field of a small star. Because the ship is attracted to it, it moves toward the gravity field, which is then pushed away by the structure of the ship, which then attracts the ship, etc, etc, etc. So long as the field is on, you have pretty much perpetual motion! :hmm: