Razaelle
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Posted - 2004.04.11 00:30:00 -
[1]
 Quote:
Ok, I follow you just fine. But it doesnt sound right is all. I dont see how it'd be at all possible for a rocket to accelerate past the velocity of the propelant used to propel it. If the particles leaving the engine are traveling at 100,000km/s relative to the rocket, then when the rocket reaches 100,000km/s the particles are actually traveling at 0km/s. Basically just ejecting burnt fuel into space. Perhaps I am wrong, but isnt there some "law" about "for every action there's an equal and opposite reaction". Basically you push one way to go the other. How hard you push depends on how far/fast you go. Given space, it only effects how fast. So how could you possibly go faster in one way, than your pushing in the other? That would mean a Class C model rocket engine could propel a planet to near light speed if given the fuel and time(ignoring the whole mass problem).
Let's try it in another way, then. You actually are confusing the 2 referentials used to measure the speed of your propeller, so let's clarify that.
We have in this problem 2 means to measure the speed of the propeller. Relative to the earth (the ground), and relative to the rocket itself.
In any case, you are measuring the speed of rocket when looking from the ground. When you say 'the rocket goes at 100.000 km/s', it seems obvious that this is the speed of the rocket relative to the ground. Obviously, the speed of the rocket relative to itself is always 0 km/s.
Now the speed of the propeller. We have 2 ways to know it :
- relative to the rocket, it would always been 100.000 km/s, regardeless of the speed of the rocket
- relative to the ground, it would be 100.000 km/s minus the speed of the rocket (that's what make you say the propeller would go at 0 km/s when the rocket reach 100.000 km/s.)
So, that's right. when your rocket hit the 100.000 km/s, your propeller would go at 0 km/s when measured from the ground. But what's accelarate your rocket is the speed of the propeller relative to the rocket, not relative to the ground. This speed is a constant 100.000 km/s, so the acceleration of the rocket is constant, regardeless of rocket's speed relative to the ground.
In fact, the speed of the rocket relative to the ground as absolutely no influence on the rocket speed and acceleration. It's absolutely valid to say that the rocket is stationnary, and that is the ground that is moving. That is the fact, by the way. Earth is moving in space, orbiting around the sun. In fact, you can not say that the ground is a better point of view for the purpose of measuring speed, than the sun, for example. Or the moon. Or the center of the galaxy. Your rocket will have a different speed each time you choose another of these objects as reference, and so the propeller speed will differ as well. The only relevant fact here is that the propeller go at 100.000 km/s when measured from the rocket perspective.
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