# If everything moves fast then....

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#### HorseSpace

##### Guest
So we are thought that the earth moves at X speed through space, the solar system moves at Y speed, the galaxy moves at Z speed and the galaxy cluster moves at W speed.

Does that mean we actually from an outside view (outside of the galaxy cluster) move x + y + z + w?

And if we are outside the galaxy cluster, does that part also "move"?

And if that part does not move at a certain speed, cant you then consider that once leaving the moving parts they will start to move away very fast, so that your spaceship cannot get back in the "bubble"?

I cant make this fit with the whole moving thing, and what actually moves all this?

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#### ramparts

##### Guest
Well, yes, velocities add up like that, as long as they're all pointing in the same direction. So if I move left on a train at 10 mph (relative to the train), and the train moves left relative to the ground at 10 mph, someone from the ground will see me move at 20 mph. If I'm moving at 10 mph to the right on this train, then someone on the ground will see me walking in place and not moving anywhere Same thing for objects in space (as long as they're not moving near the speed of light, in which case the math gets trickier).

The one thing to keep in mind is that all motion is relative as long as it's at a uniform speed. So if the Earth moves at a constant speed x relative to the Sun, then you can just as easily say that the Sun is moving at the same speed relative to the Earth. There's no absolute motion or absolute stillness.

Does that help?

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#### Shpaget

##### Guest
HorseSpace":1pmgrb2j said:
And if that part does not move at a certain speed, cant you then consider that once leaving the moving parts they will start to move away very fast, so that your spaceship cannot get back in the "bubble"?

Not exactly.
They won't start moving very fast. You do it by accelerating and leaving the star system/galaxy....
If you do reach something that doesn't move at all (and you wouldn't have any evidence or reason to believe it does not move since it's impossible to determine a non-moving state if you don't already have something that you know doesn't move to compare it with) you would need to stop your spaceship. Otherwise you just zoom through.
If you are able to change your speed from the stand still (relative to Earth) to some speed needed to reach distant object to complete standstill (or the one that is standstill relative to something else) there is no reason to believe that you wouldn't be able to get back to Earth, unless you run out of fuel.

In other words, if you can accelerate in one direction, there is no physical law that makes it impossible to do it in opposite.

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#### skeptic

##### Guest
ramparts":34arskea said:
Well, yes, velocities add up like that, as long as they're all pointing in the same direction.

Well no, velocities do add up like that and Ramparts, you should know better. The way velocities add and this is for all velocities not just theoretical examples in SR is:

(x + y) / (1 + xy/c^2)

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#### origin

##### Guest
skeptic":2zna0dbb said:
ramparts":2zna0dbb said:
Well, yes, velocities add up like that, as long as they're all pointing in the same direction.

Well no, velocities do add up like that and Ramparts, you should know better. The way velocities add and this is for all velocities not just theoretical examples in SR is:

(x + y) / (1 + xy/c^2)

Oh come on, even at an additive velocity of a 1,000,000 mph you are talking in the range of (X + Y)/1.0000000. So based on the question in the OP I think that his answer was perfect. Why complicate the answer. It is like using GR to calculate first semester physics projectile questions.

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#### ramparts

##### Guest
Yeah, I think I covered for that possibility:

(as long as they're not moving near the speed of light, in which case the math gets trickier)

But it's possible I've misread this thread But yeah, in non-relativistic cases where all velocities point in the same direction, then they add up linearly.

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#### aaron38

##### Guest
This reminds me of a sci-fi story, one of Niven's maybe, with some scientist characters driving in a car, who are suddenly transported to an alien planet. One of the scientists says "Hey, we were going 60mph, what happened to our momentum?" The alien says "Yeah well, you were also moving at 10 miles per second with respect to this surface, we took it as a lump sum."

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#### neilsox

##### Guest
Velocities add if pointing in the same direction. That is usually a close approximation, except the directions are typically different in 3 dimensions, so the total is less than the sum, possibly less than any of the parts. We can have stationary = stopped in one reference frame, but rarely in two or more reference frames; not even close in all reference frames. Neil

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#### bdewoody

##### Guest
This is why in Sci Fi movies the dumbest statement from a captain is "All Stop". Relative to what? Everything is always moving in some direction relative to something.

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