What is speed in space really?

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Toshis8
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What is speed in space really?

Post by Toshis8 » Tue, 21. Mar 23, 18:14

I was wondering a bit, its a relative thing, depends on your point of reference. It could be one speed when you look from earth, another when you look from the point of sun, yet another if you look from mars for example or from another star system. Like there is really no one true speed, it keeps changing depending on how you look. Something to think about.

For example, there is a comet flying in our system, we measure its speed from the point of earth or the sun? What is the correct way?

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Re: What is speed in space really?

Post by abisha1980 » Tue, 21. Mar 23, 18:48

speed do not exist, Light travel at 0 meters a second.
information travels at speed limit of 300.000 m/s

anything else, it's just relative.
hope i did not shatter your brain.
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Re: What is speed in space really?

Post by EGO_Aut » Tue, 21. Mar 23, 20:12

As you can see from the unit m/s, speed = distance / time.

The distance indicates how great the distance is between two points along the path during a movement

PS: c = speed of light = ~ 300 000 000 m/s :roll:

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Observe
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Re: What is speed in space really?

Post by Observe » Tue, 21. Mar 23, 20:29

In other words, if I was born on a photon, my lifespan would be zero. :D

Toshis8
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Re: What is speed in space really?

Post by Toshis8 » Tue, 21. Mar 23, 20:31

EGO_Aut wrote:
Tue, 21. Mar 23, 20:12
As you can see from the unit m/s, speed = distance / time.
If i am walking 5km/h in a train, which is driving 200 km/h, which is rotating around center of the earth at 1670 km/h, which is moving around the sun at 107000 km/h, etc. What is my speed? How do you tell speed of something in a system, where everything is moving?

For example, "Voyager 1" is the fastest man made object, which is moving at 61500 km/h away from earth. "Alpha Centauri" is the closest star system, which is getting closer to us at a speed of about 79200 km/h. If "Voyager 1" was flying directly into "Alpha Centaury" and i was in the "Alpha Centauri", from my point of view, Voyager's speed would be 140700 km/h. So, what distance does Voyager 1 cover per one hour, 61500 km or 140700 km?

Toshis8
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Re: What is speed in space really?

Post by Toshis8 » Tue, 21. Mar 23, 20:46

Probably an interesting way to think about it - imagine boiling water. A blob of boiling water surrounded by nothing. There is no static point of reference for the whole system. How to tell speed of individual water molecule in a system, where everything that exists is moving?

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Re: What is speed in space really?

Post by fiksal » Tue, 21. Mar 23, 21:26

Toshis8 wrote:
Tue, 21. Mar 23, 18:14
I was wondering a bit, its a relative thing, depends on your point of reference. It could be one speed when you look from earth, another when you look from the point of sun, yet another if you look from mars for example or from another star system. Like there is really no one true speed, it keeps changing depending on how you look. Something to think about.

For example, there is a comet flying in our system, we measure its speed from the point of earth or the sun? What is the correct way?

speed is a scalar value of velocity vector.

It's all relative to the observer because there's no fixed / absolute reference in space that we can point to.

So what velocity is, is rate of change of distance between two points over time.
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Re: What is speed in space really?

Post by Alan Phipps » Tue, 21. Mar 23, 21:32

All that matters is the estimated time of appropriate arrival at the next point of interest or decision and the effect of that journey on limited resources. In the small scale straight line speed may rule, but in greater and more complex navigations there may be significant other factors at play.

Consider 2 air fighters in combat. Both maintain a constant actual airspeed yet in one case they approach head on, and the other is more a tail chase pursuit. All that matters is when they can lock on and realistically engage, evade, or have to break off to refuel.

Now consider a spacecraft wishing to rendezvous with an orbiting item. Perhaps it can go on a fastest straight line to a predicted intercept point by turning and braking halfway, or perhaps it has to enter orbit and adjust that orbit to catch up or slow down to rendezvous, or perhaps it can use a slingshot around the body being orbited to increase speed and reduce fuel usage.

The most effective and timely solutions may not depend so much on actual speed but on strategy, navigation, and fuel usage.
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Re: What is speed in space really?

Post by Falcrack » Tue, 21. Mar 23, 21:39

Okay, so speed is relative to your frame of reference. What about angular velocity though, which is how fast an object is rotating? Is that also relative, as in, could you say if you were spinning in space that you were actually not rotating and the universe was rotating around you?

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Re: What is speed in space really?

Post by alt3rn1ty » Tue, 21. Mar 23, 21:45

You are all wrong, it depends on total glasses of rum, more rum = floor hits your face faster. You look like a panda bear in the morning. So, Panda Bears are the best measure of speed imho .. cheers!, hic.
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Re: What is speed in space really?

Post by mr.WHO » Tue, 21. Mar 23, 21:54

Falcrack wrote:
Tue, 21. Mar 23, 21:39
Okay, so speed is relative to your frame of reference. What about angular velocity though, which is how fast an object is rotating? Is that also relative, as in, could you say if you were spinning in space that you were actually not rotating and the universe was rotating around you?
It's all relative and depends on point of reference.

Bullets shot from the gun spin, but it you're shot at, you're not interested in bullet spin, just the movement towards you.
Still bullet spin make an impact of bullet trajectory, which is important in some long range, precise shots, so if you're sniper, you might need to take bullet spin into account (plus other things like Earth grativy and even Earth spin in relation to your geolocation).

This is basically how physics works - for simple things you can use simple models and approximation and outright ignore minor things.
Yet for bigger and more complex things, you need to build more complex models, measurements and take more things into account.

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Re: What is speed in space really?

Post by abisha1980 » Tue, 21. Mar 23, 21:56

Falcrack wrote:
Tue, 21. Mar 23, 21:39
Okay, so speed is relative to your frame of reference. What about angular velocity though, which is how fast an object is rotating? Is that also relative, as in, could you say if you were spinning in space that you were actually not rotating and the universe was rotating around you?
if i was not mistaken angular momentum is localized and falls under classical physics.
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Re: What is speed in space really?

Post by Toshis8 » Tue, 21. Mar 23, 22:24

fiksal wrote:
Tue, 21. Mar 23, 21:26
Toshis8 wrote:
Tue, 21. Mar 23, 18:14
I was wondering a bit, its a relative thing, depends on your point of reference. It could be one speed when you look from earth, another when you look from the point of sun, yet another if you look from mars for example or from another star system. Like there is really no one true speed, it keeps changing depending on how you look. Something to think about.

For example, there is a comet flying in our system, we measure its speed from the point of earth or the sun? What is the correct way?

speed is a scalar value of velocity vector.

It's all relative to the observer because there's no fixed / absolute reference in space that we can point to.

So what velocity is, is rate of change of distance between two points over time.
If you simplify system to just two points, then i agree. But if you expand system to multiple moving points, velocity becomes ambiguous thing, where each point could be attributed multiple different velocities.

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Re: What is speed in space really?

Post by Toshis8 » Tue, 21. Mar 23, 22:35

I wonder, imagine you're in spaceship and there is nothing around. You activate your engines, would you have velocity? Logically i would think no, if there is only one point in system, BUT if you feel inertial forces when activating engines, you should be moving, right (but there is nothing to measure your distance off)?

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Re: What is speed in space really?

Post by euclid » Tue, 21. Mar 23, 22:47

There is no empty space but for the sake of argument, you could drop a beacon and then observe the distance increase to obtain a velocity.

And concerning multiple points (your reply to fiksal's contribution) it makes no difference as you can always take snapshots at t and t +dt to obtain the relative velocities of all points.

Cheers Euclid
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Re: What is speed in space really?

Post by fiksal » Tue, 21. Mar 23, 22:48

I am not sure about the above question.

Angular momentum from what I remind myself is a vector too.

Like all it has a point in relative space and direction
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Re: What is speed in space really?

Post by EGO_Aut » Tue, 21. Mar 23, 22:50

Toshis8 wrote:
Tue, 21. Mar 23, 22:35
I wonder, imagine you're in spaceship and there is nothing around. You activate your engines, would you have velocity? Logically i would think no, if there is only one point in system, BUT if you feel inertial forces when activating engines, you should be moving, right (but there is nothing to measure your distance off)?
If you feel inertial force you accelarate. a = v / t (simple formula)

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Re: What is speed in space really?

Post by Toshis8 » Tue, 21. Mar 23, 22:54

euclid wrote:
Tue, 21. Mar 23, 22:47
There is no empty space but for the sake of argument, you could drop a beacon and then observe the distance increase to obtain a velocity.
No beacon, nothing is outside your spaceship, but you feel inertial force. Are you moving?

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Re: What is speed in space really?

Post by fiksal » Tue, 21. Mar 23, 23:09

Toshis8 wrote:
Tue, 21. Mar 23, 22:54
euclid wrote:
Tue, 21. Mar 23, 22:47
There is no empty space but for the sake of argument, you could drop a beacon and then observe the distance increase to obtain a velocity.
No beacon, nothing is outside your spaceship, but you feel inertial force. Are you moving?

you can't measure your own inertia / momentum in this context, without observing the surroundings.

and to that point, "moving" is not an accurate thing to say

you can measure force however on yourself, that force is changing your momentum. You can calculate change of velocity from any previous moment in time (which is the acceleration, or can be expressed as force)


if you measure such force on yourself, you know you not at the same spot relative to an outside observer


I should amend myself. I think angular momentum can be measured from our own perspective. I am a bit fussy on mathematical details
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Re: What is speed in space really?

Post by euclid » Wed, 22. Mar 23, 12:31

Toshis8 wrote:
Tue, 21. Mar 23, 22:54
euclid wrote:
Tue, 21. Mar 23, 22:47
There is no empty space but for the sake of argument, you could drop a beacon and then observe the distance increase to obtain a velocity.
No beacon, nothing is outside your spaceship, but you feel inertial force. Are you moving?
Short answer: Yes.

If you feel "heavy" then you are accelerated and EGO_Aut already provided a simple formula for it.

Cheers Euclid
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