Why when stepping on/off a still escalator or moving track, you still feel a jolt due to the velocity change?


You are walking and about to go up an escalator. The escalator is not in function and so there is no speed difference in the ground.
However, upon stepping on it, a common experience is to “feel” the jolt as if the speed differential is there.
The same happens when stepping off.
The effect seems to happen to a large range of people and persists even if one walks with their eyes closed.
Why is this?

In: Engineering

If you’re expecting the change in velocity (ie, you’re not paying attention and don’t notice the escalator isn’t moving) but don’t feel it, you’ll get that jolt because you’ve braced yourself for it. This is a bit like the “phantom step,” when you think there’s a step but there isn’t, so your foot falls very awkwardly.

Another potential cause is that an escalator’s steps are not the same height as normal steps, so the first step or two will catch someone off guard. Additionally, since the steps raise/lower at each end of the escalator, there will be steps of very atypical height at the beginning and end.

You may have noticed at some point that humans bob up and down as we walk. Well, we actually bob forwards and backwards too. We lean very slightly backwards in anticipation of taking another step. This is one of the ways we keep our balance when moving. Normally in life when we’re moving the ground is functionally stationary in relation to itself, and when you take a step we don’t have to worry about the ground moving at a different speed where we’re stepping.

When you step onto an escalator, the ground is moving at a speed very different to what it was. And forwards. Suddenly, since you were leaning backwards to take the next step, the lean becomes more pronounced, and your body feels like it’s falling as opposed to walking. The jolt you feel is the muscles in your body quickly reacting to prevent the fall.

TLDR: Monkey brain does a frighten when the ground moves.

The treads of an escalator are about 1 cm or so below below the surface of the floor. Step off from (or onto) a 1cm board and you’ll feel the same jolt as your foot falls farther than (or stops sooner than) your brain expected it to.