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

  • Thread starter eldrun
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In summary, the two spaceships moving towards each other at a speed of 0.9c will have a relative speed of 1.8c as measured by an outside observer. If one spaceship measures the other approaching at 0.9c, then the other spaceship will measure the same approaching speed. This is known as the velocity-addition formula.
  • #1
eldrun
2
0
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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  • #2
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.
 
  • #3
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?
 
  • #4
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.
 
  • #5
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:

What is the speed of the two spaceships?

The speed of the two spaceships is 0.9 times the speed of light (c), which is approximately 270,000 kilometers per second.

Will the two spaceships collide?

It depends on the distance between the two spaceships and their relative speeds. If they are moving towards each other at the same speed, they will collide. However, if their speeds are different or if they are able to adjust their course, they may avoid a collision.

What would happen if the two spaceships collided?

If the two spaceships were to collide, it would result in a catastrophic impact with intense heat and release of enormous amounts of energy. The impact would likely destroy both spaceships and cause damage to the surrounding area.

Can the two spaceships travel faster than the speed of light?

No, according to Einstein's theory of relativity, the speed of light is the ultimate speed limit in the universe. It is not possible for any object, including spaceships, to travel faster than the speed of light.

How does the concept of time dilation apply to the two spaceships?

As the two spaceships are moving at high speeds towards each other, time dilation would occur. This means that time would appear to pass slower for the spaceships than for an outside observer. This effect becomes more significant as the speed of the spaceships approaches the speed of light.

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