The density of a stationary body is equal to ρ0. Find the velocity (relative to the body) of the reference frame in which the density of the body is η=25% greater than ρ0.
1.369. v=cη(2+η)/(1+η)=0.6c, where c is the velocity of light. The definition of density as the ratio of the rest mass of a body to its volume is employed here.