# Thread: Jet propulsion via conservation of momentum

1. ## Jet propulsion via conservation of momentum

I've been wondering how jet propulsion in space works. Since space is a vacuum, there is nothing for the thrust to act against. For example, when you swim, you propel yourself forward by pushing against the water when you paddle.

Jet propulsion in space works by the law of conservation of momentum, see http://en.wikipedia.org/wiki/Momentu...inear_momentum.

I think this is amazing.

2. Which is how those ion propulsion systems work. Incredible...

3. Yep, force of thrust on the spacecraft not the thrust on anything behind it is what drives it. I was also saucer eyed when I was taught it.

4. Yep, the more basic explanation for the lay-man would be Newton's 3rd law, "Every action has an equal but opposite reaction". This is true because of the conservation of momentum.

5. To me, this is just proof that we're making the universe up as we go along. Big chunks of metal full of people shouldn't be able to fly either.

6. Originally Posted by Frikkenator
Yep, the more basic explanation for the lay-man would be Newton's 3rd law, "Every action has an equal but opposite reaction". This is true because of the conservation of momentum.
This is the correct one AFIK

7. Don't they teach Newton's laws in school any more?

8. Originally Posted by ponder
Don't they teach Newton's laws in school any more?
Lol, I actually remember out science teacher asking us how a rocket is propelled, even after we've learned about Newton's laws. People came up with the grandest theories, only to realise how simple it actually is!

We were lucky though, had probably one of the best science teachers around!

9. Jet propulsion doesnt work in space. Jet propulsion requires a fluid. Jets push against the atmosphere. They don't use conservation of momentum. Rockets use conservation of momentum.

A jet engine draws sucks in air, compresses it and then explosively blows it out the back.

You can't suck enough in space.

Rockets don't need to suck in anything.

http://www.physicsforums.com/archive.../t-108114.html

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