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Time dilation problem

by spXq
Tags: dilation, time
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spXq
#1
Nov13-12, 05:34 AM
P: 4
The distance to Alpha Centauri is 4,3 light years. How fast would a space ship have to travel to get there in 10 years, according to the crew?

The answer *should* be 0,395c. So far I've gotten all sorts of answers but not much close, so I seem to be approaching the problem the wrong way.
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Doc Al
#2
Nov13-12, 06:13 AM
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Show what you've done so far.

Hints: What's the basic idea of time dilation? What's the definition of velocity?
spXq
#3
Nov13-12, 06:20 AM
P: 4
The first part of the assignment asks for the time required when traveling at 30% of light speed. Given a gamma factor of 1,048 I get ~13,7 years, which is correct (so the gamma factor must be correct as well).

velocity = distance / time

But...

for t = 10 and d = 4,3

v = 4,3 / 10
v = 0,43, which is wrong

taking length contraction into account gives

4,3/1,048 = 4,1

v = 4,1 / 10
v = 0,41, which is still wrong

Doc Al
#4
Nov13-12, 06:22 AM
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Time dilation problem

Quote Quote by spXq View Post
Edit: velocity = distance / time
Good. Keep going.

From Earth's viewpoint, you have the distance. What's the time?
spXq
#5
Nov13-12, 06:26 AM
P: 4
Quote Quote by Doc Al View Post
Good. Keep going.

From Earth's viewpoint, you have the distance. What's the time?
Hmm... i don't know. :/

The distance is 4,3 ly but I don't know the speed, so how do I find the time?
spXq
#6
Nov13-12, 06:29 AM
P: 4
Hang on... the time for the crew is 10 years so the time for the observer is t/gamma

10/1,048 = 9,54 years

Edit: no that's not right....... I have no idea what I'm doing

Edit2: the time from earths viewpoint is d * gamma = 4,3 * 1,048 = 10,48 years, while the time is 10 years for the crew
Doc Al
#7
Nov13-12, 07:21 AM
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Quote Quote by spXq View Post
Hang on... the time for the crew is 10 years so the time for the observer is t/gamma
You have that reversed. If the ship time is 10 years, then to earth observers it will be longer: t*gamma, not t/gamma.

10/1,048 = 9,54 years
In addition to what I already pointed out, do not use the gamma from the previous part of the question.

So continue with that velocity equation, v = d/tearth.


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