How Long Does It Take for One Rocket to Pass Another at Relativistic Speeds?

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ZanyCat
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I do have a specific example/problem, but my actual question is more so conceptual (I'm sure that seeing someone confused by relativity is a first around here!).

The problem:

Two rockets are each 1000m long in their rest frame. Rocket Orion, traveling at 0.900c relative to the earth, is overtaking rocket Sirius, which is poking along at a mere 0.700c. According to the crew on Sirius, how long does Orion take to completely pass?
That is, how long is it from the instant the nose of Orion is at the tail of Sirius until the tail of Orion is at the nose of Sirius?

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Okay, so I'm thinking that I'll need to find a) the observed velocity of O in S's FOR and b) the observed length of O in S's FOR, and go from there.

My confusion is coming from the velocities. I'm taking S as my Frame of Reference, so from S's FOR, is O moving past at 0.200c? Or do I need to transform the velocity?
 
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ZanyCat said:
Okay, so I'm thinking that I'll need to find a) the observed velocity of O in S's FOR and b) the observed length of O in S's FOR, and go from there.
Good.

My confusion is coming from the velocities. I'm taking S as my Frame of Reference, so from S's FOR, is O moving past at 0.200c?
No. To find the velocity of O with respect to S, you'll need to use the relativistic addition of velocity formula.
 
I'm struggling with that part. Do I need to consider the Earth as my stationary FOR, then?
 
Never mind, got it sorted, thanks Doc!