1. The problem statement, all variables and given/known data This is an introductory special theory of relativity short question from university question paper that carried 2marks. Does the density of an object change as its speed increases? if yes by what factor. 2. Relevant equations m=m0/√[1- (v/c)2] L=L0√[1-(v/c)2] 3. The attempt at a solution My guess is yes . If 1/√[1-(v/c)2]=γ lorentz factor ρ=m/L3=m/volume(V)=(m0/V0) *γ4 ρ/ρ0=γ4. No text book has exclusively covered this problem except for one. There it is written for proper mass density ρ/ρ0=γ and it is γ2 for mass-energy density. And V=V0/γ. The author only mentioned the relations.Did not derive them either concisely or in detail. I am now confused.How come lorentz factor has the same power for both length and volume? Why and how does proper mass density and mass-energy density have different factor?