# Phonons (kintetic waves) in a plasma?

1. Jun 27, 2011

### treehouse

What I am imagining is a thruster in space igniting a fuel to create an explosion to push a spaceship and phonons (mechanical waves) pushing the spent fuel to the sides of the thruster where vents vacuum it back into the spaceship for recycling. What do you think of this?

Last edited by a moderator: Jun 27, 2011
2. Jun 27, 2011

### ardie

slight problem there, phonons cannot propogate through vacuums

3. Jun 27, 2011

### treehouse

An explosion is not a vacuum.

4. Jun 27, 2011

### ZapperZ

Staff Emeritus
Can you provide a citation where phonons exist in plasma? There is a reason why there is a "plasmon".

Zz.

5. Jun 27, 2011

### nasu

Besides the problem of using proper terms (not any mechanical wave means phonons), it seems you'll end up with the same speed as before you ejected the gases.
Assuming no forces external to the spaceship, if the gases are sucked back inside you will end up with the same speed as before ejecting them (or less), just from conservation of momentum. Same final mass, same momentum...

6. Jun 27, 2011

### treehouse

I'm not being sarcastic when I tell you "I want to be lectured to on this."

But isn't momentum vectored? Can't the explosion push the spaceship one direction and the mass of the spent fuel going back into the spaceship push it in basically the same direction? Besides, when an electric car accelerates doesn't its momentum increase even though its resting mass doesn't change?

Last edited: Jun 27, 2011
7. Jun 27, 2011

### Staff: Mentor

The reason an explosion can push something in a direction is because it is ejecting mass away from the vehicle. If there was no mass ejected then there wouldn't be any acceleration.

The electric motor is using energy from the batteries. The charged battery has more mass than the empty battery, with the vehicle losing mass through friction and heat and such. If you run the vehicle until the battery is dead and then stop, the vehicle will have less mass.

8. Jun 27, 2011

### treehouse

But isn't it the force of the explosion pushing all things within its range away from its center that pushes both the mass and the vehicle?

9. Jun 27, 2011

### Staff: Mentor

Yes. I think you are saying the same thing we are, but in a different way. If I were to "shoot" high velocity particles at a rocket nozzle in space, the vehicle it was connected to would be accelerated. The same effect is happening when you use a rocket. Ideally the velocity of the exhaust would be zero,(I think) as that would mean that 100% of the energy of the combustion would be input into acceleration of the rocket, but that is not reachable.

I just want you to understand that the "eplosion" IS the fuel. If no fuel exhaust went backwards, there would be no explosion in that direction.

10. Jun 27, 2011

### nasu

Yes, the momentum is a vector.
In order for the spaceship to move forward, the gases ejected must have a velocity with at least some component in the opposite direction.
If you want to somehow bring the gas back in the spaceship you need to reverse the direction of the momentum of these gases. The effect of this will be a deceleration of the spaceship.
The components of the momentum of the gas particles transverse to the direction of the motion are irrelevant. They cancel out all the time. Once the gas is ejected, no internal process can change its overall momentum.

A car of any kind moves due to the interaction with the road and the Earth. If you consider the system car-road, the motion of the car forward results in a change in the momentum of the Earth (or maybe just part of it) in the opposite direction.
Imagine you are in the middle of long boat. You can move towards one end of the boat and in the process the boat will move in the opposite direction because you are pushing it with your feet. You gain some forward momentum (relative to the water) and the boat an opposite momentum. If you replace the boat with the Earth, the velocity gained by the Earth is so much times smaller that we usually neglect it. But it is there.
Going back to the boat, your idea would be similar to a scenario in which once you reach one end of the boat, you try to pull the boat back (under your feet) so you are again in the middle and start all over.

Regarding phonons, you can read any book on introductory solid state or read a little on Wikipedia: http://en.wikipedia.org/wiki/Phonon

11. Jun 28, 2011

### treehouse

What about solar sails? Don't stars get more massive over time?

12. Jun 28, 2011

### nasu

What about them? I am not sure what point are you trying to get to.
And from what I know, the stars, if anything, will loose mater and energy over time. Why would you think they get more massive? And how is related to the topic (whatever that is)?

13. Jun 28, 2011

### treehouse

Some stars get less massive; but some stars which emit photons that can push solar sails turn into black holes - and when stars get less massive it is because as part of a nuclear reaction generating an immense amount of energy they eject matter which was previously relatively stationary inside the star.

Last edited: Jun 28, 2011
14. Jun 28, 2011

### treehouse

15. Jun 28, 2011

### nasu

This statement has quite a few problems.
Most of the stars emit photons and any photons can be used to push solar sails, in principle (there are no special photons for this). And how is the star becoming a black-hole related to photons pushing the sails??
And by the way, when a star becomes a black-hole does NOT became more massive.

In rest, I still don't understand your goal. Do you aim to find a case when the conservation of momentum does not hold?