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# special relativity watch

1. Hi,

I'm not sure what I should use as the proper time/length in this question:

an observer on spaceship A see's spaceship B fly past at 0.8c . 5*10^-6 seconds later spaceship A see's another spaceship, spaceship C.

In spaceship A's frame, how far has B travelled when it see's C?

How far does an observer on B think it has travelled once A see's C?

Thanks a lot for any help
2. (Original post by jobo3)
Hi,

I'm not sure what I should use as the proper time/length in this question:

an observer on spaceship A see's spaceship B fly past at 0.8c . 5*10^-6 seconds later spaceship A see's another spaceship, spaceship C.

In spaceship A's frame, how far has B travelled when it see's C?

How far does an observer on B think it has travelled once A see's C?

Thanks a lot for any help

Well if A sees B travelling at 0.8c and C comes past 5 x 10^-6 seconds later, B has travelled 0.8c x 5 x 10^-6 metres = 0.8 x 300 x 5 = 1200m

For the second part, the time is dilated and length is contraced by a factor: so the distance should be scaled down by this factor: 720m.

Does this seem sensible?

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