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It's unbalanced forces you feel, not unbalanced acceleration per se. If you have a constant acceleration, it's because there's a consistent force on you. It's why this part of the article is wrong: > Humans are also terrible at perceiving velocity at a constant speed. [not perceiving sway] is why, when you’re traveling on a train at a steady fifty miles an hour, your body believes you might as well be sitting perfectly still. Humans in a train moving steadily are enclosed in an environment where everything is moving at the same speed as them - seats, air, everything. There are no unbalanced forces to feel in the first place. Just the same as we don't perceive the earth's movement around the sun (well, in a moving-body sense) or the sun's movement around the galaxy (which is blisteringly fast on a human scale); because our frame of reference moves with us. Of course, in the real world, trains do sway side-to-side and also up and down a little where the rails meet up (click-click click-click...), and we all feel that. But we're talking about a theory train here, and only looking at forward velocity :) Edit: Wikipedia says the sun orbits the galaxy at 220km/s. Monty Python's "Galaxy Song" says 40,000mph, which works out to 18km/s. Either way, it's pretty zippy for us humans. Galaxy Song: this video