# I have a few questions, this one is special relativity.

1. Dec 10, 2006

### chevycamaro1987

1. On his 25th birthday, the astronaut twin leaves on a long space journey at an average speed of v/c= 0.98. He returns after 20 earth-years have elapsed, to celebrate his earth-twin's 45th birthday. Compare their biological ages.

2. 1/ square root 1- v^2/c^2, My teacher is using v/c instead of giving me a velocity which is throwing me off, I'm not sure if I square the .98 or not.

3. 1/ square root 1- v^2/c^2, so what I was doing was putting the .98 and squaring it, which gives me .9604. I subtracted the 1 from .9604, and got .0396. I inverse the .0396 and get an answer of 25.25...Do I add that 25.25 to the original age of the astronaut which would give me an answer of 50.25 or is the astronauts age just 25.25...?

Last edited: Dec 10, 2006
2. Dec 10, 2006

### chroot

Staff Emeritus
Welcome to PF. We will not do your homework for you. Please show us your work, and discuss why you are unable to proceed. We will help by pointing you in the correction direction.

- Warren

3. Dec 10, 2006

### chevycamaro1987

i edited it chroot, is that better?

4. Dec 10, 2006

### chevycamaro1987

sorry i dont know how to use latex!

5. Dec 10, 2006

### chroot

Staff Emeritus
You're making this way too hard, I think.

Find gamma for v=0.98c. That's 1/Sqrt[1 - (0.98)^2].

The number of years elapsed onboard the spaceship is shorter than the time elapsed on earth by a factor of gamma. So, the number of years elapsed on board the spaceship is 20/gamma.

- Warren

6. Dec 10, 2006

### chevycamaro1987

So i tried the formula you gave me (i was using the same one i thought but i got 50, i dont know what happen) and got an answer of 5.03. i dont understand your last part, the 20/gamma. since the 20 years elapsed, i'm dividing it by gamma= 5.03 and then do i add that to the astronauts original age giving him 30 years compared to his twin's 45th? this is just way too much for my understanding. sorry if im being a pain

7. Dec 11, 2006

### chroot

Staff Emeritus
In the twenty years on earth, one twin's age increased by 20. Since he was 25 to begin with, he's 45 at the end of that time period.

In the same twenty of years on earth, the twin is flying around, and has aged only 5.03 years. The twin was also 25 when the experiment started, so now he's approximately 30 years old.

- Warren