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Einstein's relativity theory demands that all meaningful physical objectsshould be defined covariantly, i.e. in a coordinate independent way. Conceptsof relative velocity, acceleration, gravity acceleration and gravity potentialare fundamental in Newton's theory and they are imprinted in everyone'sphysical intuition. Unfortunately, relativistic definitions of them are notcommonly known or appreciated. Every now and then some confused authors usewrong, non-covariant, definitions of velocity, acceleration and gravity, basedon their vague Newtonian intuitions and hidden in a superficial, often purelysemantic, relativistic disguise. A recent example of such a confusion (Gorkavyi& Vasilkov, 2016) is discussed at the end of this Note.

Velocity, acceleration and gravity in Einstein's relativity