That's begging the question a little bit, as acceleration is absolute only because we restrict ourselves to inertial (i.e. lacking acceleration) reference frames. If you allow non-inertial reference frames, acceleration can be relative - Coriolis acceleration, for instance, may be nonzero relative to a rotating reference frame but zero in inertial coordinates.
@freemo @sda @mandlebro @mngrif
What you've written is true in the case where all reference frames are inertial. It is *not* true if the reference frames themselves are allowed to accelerate relative to one another.
Let's take a 75kg man standing in a windowless cube as an observer. He can stand up, and let's say there's a 735N normal force between his feet and the floor. Even with accurate measurements of his mass and this normal force, he still cannot determine his acceleration. Perhaps he is in an otherwise empty universe, and the box is being driven "upward", accelerating at 9.8m/s. Perhaps he is in a uniform gravitational field of 9.8N/kg pulling him "downward", and the box is stationary. Since he can't distinguish between these cases to determine his own acceleration, there is no way he can guarantee that he and an outside observer would agree on its value - it's relative to the observer.
@khird @sda @mandlebro @mngrif
Your response only indicates that a person can not determine the SOURCE of their acceleration, moreover that apparent motion also is not an indicator of acceleration.
You also seem to be mistaken about what a non-inertial and inertial frame is. In the example I gave in my last response I explained both inertial frames and non-inertial frames
Second a person appearing to stand still ont e surface of a planet but undergoing gravity is, in fact, experiencing acceleration, it just doesnt look like it to us because we can not see space time accurately with our eyes. But one of the biggest discoveries that is central to general relativity is that a spaceship accelerating in space is equivalent to a person standing still on the surface of a planet.