[chrisseaton]

The fluffy dice on a string.

Do you understand that when you accelerate in a car the dice moves toward the back of the car, so that the string leans backwards?

This happens because the car moves forward, but the dice wants to stay where it is. So it kind of gets dragged. This is a fundamental property of the universe we live in, called the first law of motion, and I can't explain that further as nobody knows why the universe is like the but it is.

Do you understand that this doesn't happen if you hold a fluffy nice on a string and run on a treadmill?

This happens because the dice wants to stay where it is, and it is staying where it is, so nothing happens. Whatever you legs are doing beneath the fluffy dice is irrelevant.

Your ear effectively has a tiny fluffy dice on string inside it, and tiny hairs that feel whether or not it is leaning backwards. You don't need to see or feel anything else to sense this, and it doesn't matter what your legs are doing. That's why the treadmill and your eyes don't matter. Your legs are running on it but the little dice in your ear knows what is really happening.

That is how you know that you are really accelerating.

And, going back to the original point, is the whole problem. There is no way to poke this fluffy dice to fake the sensation of acceleration, as it is sealed inside your head.

The fluffy dice in your head thing is obviously not quite true, but it really isn't that far from the truth.

You can do an experiment to understand this better (or at least prove it to yourself). Get a glass of water and half fill it. Get in a car and get a friend to accelerate somewhere legal and safe. The water will slosh to the back of the glass. This is like the dice. Now get a treadmill and turn it up high. Put the glass of water on the skateboard. Lower the skateboard on to the treadmill while it's still running. The 'acceleration' here will be huge as the skateboard will go from 0 to whatever mph instantly (it's not really acceleration, that's the whole point). The water will not slosh (if you put the skateboard down carefully).

[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.