# Homework Help: Relativity and observed length of a moving object

1. Dec 2, 2008

### goWlfpack

Relativity and observed length of a moving object!!

1. The problem statement, all variables and given/known data
A box is cubical with sides of proper lengths L1 = L2 = L3 = 2.3 m, as shown in the figure below, when viewed in its own rest frame. This block moves parallel to one of its edges with a speed of 0.89c past an observer.
What is the length of each side as measured by this observer? Assume that the side that the block is moving parallel to is L1.

2. Relevant equations
L = Lp square root(v^2/c^2)

3. The attempt at a solution
so i got the answer to L2 and L3 which are the sides of the cube perpendicular to the direction of travel. However i dont know how to find the length of the parallel side. anybody know why?

2. Dec 2, 2008

### Staff: Mentor

Re: Relativity and observed length of a moving object!!

Look up "length contraction" in special relativity.

3. Dec 2, 2008

### goWlfpack

Re: Relativity and observed length of a moving object!!

thats where i found the equation, the v squared/ c squared came out so small that 1 minus that came to just 1.... and then the length is the same... but that was wrong

4. Dec 2, 2008

### Staff: Mentor

Re: Relativity and observed length of a moving object!!

Show exactly what you did. (The equation in your first post is not correct.)

5. Dec 2, 2008

### goWlfpack

Re: Relativity and observed length of a moving object!!

ok, i took the proper length, which is 2.3 and multiplied it by the square root of (1 - v^2/c^2) i used .89 as v and 2.99e8 for c

when i did v^2 / c^2 i got 8.86e-18
when i do 1 - that answer, the change is so insignificant that it just counts as one. the square root of 1 is 1 and 1 multiplied by 2.3 is 2.3 .. but that is not right

6. Dec 2, 2008

### Staff: Mentor

Re: Relativity and observed length of a moving object!!

v = 0.89c, not 0.89.
So what's v/c?

7. Dec 2, 2008

### goWlfpack

Re: Relativity and observed length of a moving object!!

got it! green check! :) i always miss little things like that! thanks so much!