Since time is relative to the observer's velocity, does that mean that velocities are also relative, since the same distance is covered in a different time, depending on your reference frame? If velocities are relative, does that mean that kinetic energy is also relative via $E_{kinetic} = \frac{1}{2} mv^2$? And if kinetic energy is relative, does that mean that mass is relative, via $E = mc^2$ ?
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1Closely related/possible duplicate: https://physics.stackexchange.com/q/133376/50583 – ACuriousMind Sep 13 '21 at 16:41
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2Does this answer your question? Why is there a controversy on whether mass increases with speed? – Dale Sep 13 '21 at 16:43
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1Note: the approximation $E_\text{kinetic} \equiv (\gamma-1)mc^2 \approx \frac12 mv^2$ is only valid in the nonrelativistic limit $v\ll c$. – rob Sep 13 '21 at 17:06