Question

Why the length contraction occurs when the observer and the object are in the same relative motion?

Answer 1

in physics, length contraction is a phenomenon of a decrease in length measured by an observer of a objects which are travelling at any non- zero velocity relative to the observer. this contraction more formally called  Lorentz contraction or Lorentz–FitzGerald contraction after hendirck lorentz and george firtzgerald) is usally only noticeable at a substancial fraction of the speed of light.