m=m0√(1-v2/c2),
m/m0=√(1-v2/c2),
(m/m0)2=1-v2/c2,
v2/c2=1-(m/m0)2,
v2=c2-(mc/m0)2, or c2(1-(m/m0)2),
v=√(c2-(mc/m0)2). This can also be written:
v=(c√(m02-m2)/m0 or (c√(1-(m/m0)2).
(This is the relativistic equation showing the apparent increased mass m of an object with rest mass m0 moving at a constant speed of v, and where c is the speed of light in vacuo. By observing/measuring the apparent increase in mass (usually of a small particle) through its interaction with its environment, its relative speed can be estimated because c is a natural physical constant and a property of many forms of radiation, not just light.)