 1. dmitry_pivovarov says:

In the interpretation of General Relativity, the uniform acceleration of a body is equivalent to the action of a gravitational field on the body. And gravity is the curvature of space-time. The curvature of space-time, in turn, cannot depend on the choice of reference frame.

On the other hand, velocity is the world line of a body in four-dimensional space-time. Depending on how exactly we project the world line onto three-dimensional space, the velocity values will be different in different reference systems. Something like the projection of a cylinder on a 2d plane – it is a circle or rectangle, depending on the choice of plane.

There is one less dimension in 3d than in 4d spacetime. And it is this difference that we see as the relativity of velocity.

2. vadim_romansky says:

No, acceleration can also be relative. The question is how the reference frames in which they are measured relate to each other.

If we take two CO at rest, but with different coordinate origins, we will have a relative coordinate, but the same speed and acceleration.

Take two uniformly moving relative to each other – you get a relative speed, and the acceleration is absolute (but only in the non-relativistic limit. In the relativistic case, the acceleration will also change)

Take those that are moving at an accelerated rate , and then the acceleration will be relative.

3. vildyan_yanbikov says:

According to my ideas, the speed is always absolute relative to Absolute outer space. The velocity of a material particle can only be relative if it is measured relative to the velocity of another material particle. The same applies to acceleration.