[shamino]

Thanks, I think I finally get it - and the experiment would definitely confirm your point.

How does our inner ear chooses to uses the ground vs a point on the treadmill as it's starting point? I keeping wanting to think that I'm in motion with respect to a point on the treadmill.

What if your inner ear were located on your feet ? Would you then not be able to tell the difference between treadmill and sidewalk ? Your feet are actually traveling in the same motion on the treadmill as it would the sidewalk ?

[chrisseaton]

You are born onto the earth, so your reference point starts as the earth.

In reality the earth is moving through the solar system, so your starting reference point is in motion already.

If you get into a vehicle and start moving at speed relative to the earth, then that vehicle becomes your new reference point when the speed becomes stable.

When you step onto a treadmill your speed relative to the earth, your previous reference point, doesn't change, so the reference point doesn't change. If someone were to drag the treadmill behind a vehicle with you surfing on it, then it would become your reference point.

It's confusing to talk about your inner ear being in your feet because they're in a running motion, so they're being shaken up all the time. They'll experience constant acceleration and deceleration relative to the earth as your foot goes back and forward in a running motion. If you had little dice on strings in your feet you wouldn't be able to tell what was going on with them as the dice would be flapping all over the place.