SR length contraction question

[ehasan]

A rod moves along x-axis with speed u and has length L relative to Inertial Frame S. What is its length L' relative to the other inertial S' that moves with speed v with respect to S.

I know the famous length contraction formula L=L' * sqrt(1-square(v/c))
but I confuse how to include velocity of the rod (u) in this formula...
thanks

 

[HallsofIvy]

The speed of the rod relative to inertial system S' would be $(u+ v)/(1+ uv/c^2)[/math]. Put that into the contraction formula rather than v.$

 

[ehasan]

The speed of the rod relative to inertial system S' would be $(u+ v)/(1+ uv/c^2)[/math]. Put that into the contraction formula rather than v.$

$<br /> Thanks a lot for making it clear to me$