[ADSSDA]

I don't think you understand the arguments people are making.

Your inner ear does not experience any acceleration in your example, which is what is required to avoid sickness. What's happening relative to the surface of the treadmill doesn't matter, the acceleration of your inner ear does, and that's not changing.

[shamino]

How does my inner ear know I'm accelerating on the treadmill or the ground ? What if I blindfolded myself ?

The treadmill is just like the ground, for all intents and purposes.

[ADSSDA]

Is your head accelerating while you're on the treadmill? I've never seen a treadmill where that happens, the whole idea of a treadmill is running in-place.

The most common VR treadmill-ish device I'm aware of is the Virtuix Omni: http://www.virtuix.com/, where your head is clearly not accelerating.

What are you picturing when you claim your head (and hence inner ear) is accelerating on a treadmill?

[shamino]

I'm starting to understand yours and everyone replies. I think what I had in mind is that I'm accelerating relative to a point on the treadmill, and therefore my entire body is accelerating. I can't wrap my head around the fact that we're still considering the ground as the frame of reference.

I know that the treadmill moves on its own, but what if I push the surface of the treadmill to make me move forward. Wouldn't I not know the difference between a sidewalk and the treadmill ?

[ADSSDA]

The ground as a point of reference isn't really relevant, it's your previous position that matters (new position relative to your old position). Acceleration requires a change in trajectory, and no matter what the treadmill is doing, if you are not changing your trajectory you are not experiencing acceleration (ignoring the effects of gravity).

[shamino]

Thanks for taking the time with me. Acceleration for me is a change in a velocity - isn't that what is going on ?

Velocity is a change in distance, and to me it seems there must be a change in distance - since there is energy being spent (we're "traveling" as we run, while putting in the force of our feet against the treadmill - each step is an "acceleration" forward).

[chrisseaton]

> Acceleration for me is a change in a velocity - isn't that what is going on ?

Yes that's what it is. But you aren't changing velocity when you start running on a treadmill. When you are on a treadmill your velocity is the same as a person standing still on the ground next to you. We may talk about you 'running at 5 mph', but this is just a casual way of talking unless you're doing a physics problem. In reality, your legs are moving in a similar way as if you were running at 5 mph, but your velocity, relative to when you started, is zero the whole time.

> it seems there must be a change in distance - since there is energy being spent

If you stand still and flap your arms you're spending energy aren't you? But you're not going anywhere. Next imagine running on the spot. Still spending energy but not going anywhere. Next imagine that on a treadmill. Still spending energy, still not going anywhere.

> we're "traveling" as we run, while putting in the force of our feet against the treadmill

Here's another way to think about it from scratch that may help.

You are running forward, going at 5 mph. The treadmill underneath you is going backward, at 5 mph. Your actual velocity is therefore 0, because the two cancel out.

When you run on the ground you are running forward at 5 mph, the ground is staying still, so the result is actually 5 mph.

People casually say that your velocity on a treadmill is 5 mph, but really they mean that you are moving your legs fast enough that you would be going 5 mph, if the treadmill was't working in the opposite direction against you. If you were doing a physics exam you would be more careful with your terminology and you would say that the velocity of someone on a treadmill is 0.

> each step is an "acceleration" forward

No, each step forward on a treadmill is compensation for the treadmill going backward. All of the progress forward that step gives you is used up balancing out that the treadmill is going backwards, and there is no acceleration.