If two spaceships are moving toward each other at the 0,9c

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Discussion Overview

The discussion revolves around the relative speed of two spaceships moving toward each other at 0.9c, exploring the implications of relativistic velocity addition. Participants consider how to measure this speed from different reference frames, including a stationary observer and the moving ships themselves.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant questions the relative speed of two ships moving toward each other at 0.9c, suggesting a calculation that leads to a closing speed of approximately 0.4c when viewed from a stationary reference frame.
  • Another participant asserts that the speed measured by each ship of the other is the same, indicating that they can serve as mutual references without needing a third ship.
  • A participant proposes a formula for calculating the relative speed using the relativistic velocity addition formula, arriving at a result of approximately 0.9945c.
  • One post summarizes that the closing speed of the two ships, as perceived by each other, should mathematically lead to a speed of 1.8c, which is then adjusted using the velocity addition formula to arrive at 0.9945c.

Areas of Agreement / Disagreement

Participants express differing views on the relative speed calculations, with some supporting the use of the relativistic velocity addition formula while others question the initial assumptions about speed measurements. The discussion remains unresolved regarding the interpretation of the results and the implications of relativistic effects.

Contextual Notes

There are limitations in the assumptions made about reference frames and the interpretation of speeds in relativistic contexts. The discussion does not fully resolve the mathematical steps or the implications of the results presented.

eldrun
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Hi,

I wonder, if we have two spaceship and they are moving tovard each other at the same 0,9c speed. what is the relative speed as seen in the other ship?

if our referance is another motionless ship. i think, ship's 0,9c speed seen ~0,4c and we're seeing they're closing each other, 0,4c speed. (or, am i wrong?)

but, we're sitting the traveling ship. so, what is the other ship's relative speed?

(sorry for my bad english)

Thanks.
 
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Whatever speed the first ship sees or measures of the second ship is exactly the same speed the second ship will see or measure of the first ship. They can be each other's reference, you don't need a third ship to serve that purpose.
 
Hmm,

i think i found the right equation

v2 = -0,9c
v1 = +0,9c

v2' = (v2-v1) / (1-(v1*v2/c²)

v2' = 0,994475... c

is that right?
 
That's the how fast each of the first two spaceships will approach each other if they are both approaching a third spaceship at 90%c.
 
eldrun said:
Hi,

I wonder, if we have two spaceship and they are moving tovard each other at the same 0,9c speed. what is the relative speed as seen in the other ship?

if our referance is another motionless ship. i think, ship's 0,9c speed seen ~0,4c and we're seeing they're closing each other, 0,4c speed. (or, am i wrong?)

but, we're sitting the traveling ship. so, what is the other ship's relative speed?

(sorry for my bad english)

Thanks.

As a summary (it has been more or less answered already):

- If you see two spaceships moving toward each other, each at a speed (according to you) of 0.9c, then you will measure their closing speed to be 1.8c.
It cannot be otherwise (you can easily check for yourself that this a mathematical necessity).

- The speed with which one spaceship (with its independent measurement system) will measure the other spaceship coming closer:
1.8c / (1 + 0.81) = 0.9945 c

See also http://en.wikipedia.org/wiki/Velocity-addition_formula

Harald

PS welcome to physicsforums:smile:
 
Last edited:

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