Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Length Contraction spaceship Problem

  1. Mar 9, 2010 #1
    1. The problem statement, all variables and given/known data
    A spaceship passes you at a speed of .750c. You measure its length to be 28.2 m. How long would it be when at rest?

    2. Relevant equations
    I think the equation that is relevant is L=L(sub0)xsqrt(1-v^2/c^2)

    3. The attempt at a solution
    Apparently, the answer is 42.6 m. I can't even imagine how to get 42.6 from that.
  2. jcsd
  3. Mar 9, 2010 #2


    User Avatar
    Homework Helper
    Gold Member

    I think you just got your variables swapped. If you are using

    [tex] L = L_0 {\sqrt{1 - \frac{v^2}{c^2}} [/tex]

    then [tex] L [/tex] is 28.2 m, and you need to solve for [tex] L_0 [/tex], the length when the relative velocity is 0. [tex] L [/tex] is the length when traveling at speed [tex] v [/tex].
  4. Mar 9, 2010 #3
    Then by that logic isn't the answer 64.457?

    I did 28.2=L(1-.75^2). Is that wrong?
  5. Mar 9, 2010 #4


    User Avatar
    Homework Helper
    Gold Member

    You forgot to take the square root. :tongue2:
  6. Mar 9, 2010 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    You forgot the square root.
  7. Mar 9, 2010 #6
    Yay! Thank you very much!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook