# vacuum confusion

I bet you will know the answer to this one then. You know when a rocket is
moving in the space between planetary atmospheres, what do thier thrusters thrust against to change direction? Alway`s puzzled me, this one! :eek:

The thrusters push against the rocket itself. By sending a force one way it pushes the rocket the other direction. Action=reaction, according to Newton’s laws.

nitpick action=**-**reaction

That does n`t sound right too me. So the thrust is pushing against the physical presence of the rocket, but space is a vacuum , so what is it pushing against?

It works the same way the main motor propels the vehicle in the vacuum of space. The steering rocket is thowing mass at high velocity sideways relative to the axis of the vehicle. If the mass of the burned rocket fuel is going one direction the rocket must go the other direction. That’s conservation of momentum.

If you are on ice skates with little friction to the ice and you throw a hammer forward, you will go backward. Same thing

It doesn’t have to push against anything.

Imagine a stationary rocket floating in space. It’s momentum is zero. When you turn on the engines the fuel turns into hot gas and shoots out the back of the rocket at very high speeds. But the momentum of the entire system has to remain zero. (That’s “conservation of momentum” — a very basic rule of physics.) To balance out the high momentum of the gas shooting out the back, the rocket moves forward.

Thank`s for that lads. My brain can just about deal with that. But , her`s a similar problem I have; if earth is moving through space at 36,000 mph[so i`ve read!], and your average rocket does about 17,000mph, why doesn`t the rocket get left behind, like a pigeon being thrown out of a car window! Not that i`d suggest anyone would do that with a pigeon! p.s. Do you have pigeons over there? p.p.s I`m in good old England, you see!

For every action there is an equal and opposite reaction.
Have you ever fired a gun? The kick is the opposite reaction. The bullet goes forward, the gun goes backward.
have you ever seen one of those desk top frames with the five hanging balls? You pull one ball back and let if fall, and 1 ball off the other end flies into the air. When it comes down, the first ball moves again.
You can do it with 2 balls also.
Equal and opposite reactions.

Because the rocket is also moving at 36,000 mph when it’s at its rest position. You just don’t count that because in its rest position it’s motionless relative to the Earth.

When is it at a rest position travelling at 17,000mph?

so my analogy with the pigeon and the car does n`t apply then?

There is no such thing as absolute velocity. Speeds are always measured relative to something else. This isn’t intuitively obvious because down here on Earth we’re used to comparing the speed of everything to Earth itself.

When a rocket is in space you can measure it’s speed relative to the Earth, or relative to the Sun, or even relative to the center of the galaxy. Each will give you a different value, and none of them is more “right” than the others.

The analogy of the pigeon and the car doesn’t work because of air resistance. When you throw the pigeon out of the car it slams into the stationary air around the car, causing it to slow down abruptly. If you were driving in a vacuum and threw the pigeon out it would continue to sail along beside you.

That`s freaky that mate, but `ll accept it. Do you want to go and join them on the Moon Mystery now, there having trouble.

Why would it? You’re essentially throwing the pigeon out of the car into a blob of soup that your vehicle happens to be traveling through (air). Try throwing the pigeon out of a vehicle traveling through a vacuum and observe the difference.

Apart from the whole explosive decompression thing. A mushy, sublimating blob that used to be a pigeon (the best sort of pigeon in my opinion) will continue to sail along beside you

Si

Seriously? I don’t mean to be unkind, but do you honestly expect us to take a moon landing conspiracy argument seriously, coming from someone who started a thread like this one?

No, but I was curious about the response tho`, and it`s kinda proved that you yanks have too
much of a vested future in proving conspiracy theories like this wrong!

I’m not an American, but all evidence points to a few of them landing on the moon in the late 60s early 70s. The supposed evidence against is relatively simple to debunk if one understands a little bit about, among other topics, physics.

If you throw a rocks from the back end of a small boat, you’ll start moving forwards.

If you confuse velocities in different reference frames with each other, you’ll get all kinds of funky results. Such as: If I’m sitting in a train moving at 100 km/h, and I get up and start walking forward at my maximum walking speed of 8km/h, why am I not thrown backwards to smash into the rear door of the car?

Yes, we have pigeons over here. And we also have normal apostrophes. And so do you.

We’re not all American here, you know. And I reckon if someone presented evidence of a genuine conspiracy, they’d probably prevail. Moon landing woo woos have been done to death though, that’s all.