Length contraction.

1. Jul 20, 2011

sphys4

1. The problem statement, all variables and given/known data
A spaceship moves past you at speed v. You measure the ship to be 300 m long, whereas an astronaut on the ship measures a length of 445 m. Find v.

3. The attempt at a solution
I tried to substitute my numbers into L1=Lsqrt(1/(v^2/c^2)) and keep getting a negative wrong answer!
445=300sqrt(1-(v^2/3E+8^2)) ... what am i doing wrong?

2. Jul 20, 2011

Pi-Bond

In your formula L (on the right hand side) is the proper length. Are you sure you have substituted the proper length correctly?

3. Jul 20, 2011

sphys4

Well L1 is supposed to be the length observed by an observer in relative motion with respect to the object, which would be 445?

4. Jul 20, 2011

Pi-Bond

But the astronaut is at rest relative to the spaceship. Who is the one measuring the moving spaceship?

5. Jul 20, 2011

sphys4

I guess that means i am, so i switch 300 and 445?

6. Jul 20, 2011

Pi-Bond

Yes. The right hand side of your equation deals with proper length, or the length measured by an observer in the rest frame of the object. (Astronaut in this case) The left side deals with what an observer in relative motion will measure as the length. (you in this case)

7. Jul 20, 2011

sphys4

aha i see, thank you so much!