# eli5: At constant speed and no vision, are you supposed to be able to tell if you are travelling forward or backwards? How?

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Was on the train and it was pitch black outside. Thought I was sitting facing the direction of travel but turned out I wasn’t.

Even when I felt the acceleration/decelerating, I didn’t know which it was.

In: 16

To achieve constant speed you need to balance out the forces going forwards and the forces going backwards, which means there’s no acceleration. This is part of Newton’s first law of motion: every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. So with no visual aid you wouldn’t be able to tell whether you were stationary or acceleration.

If you have no visual cues, or wind rushing by, then no, you’ll be unable to tell in which direction you are moving, or how fast, or whether you’re even moving at all. In fact, even if you can see your surroundings, you can’t tell if it’s you that’s moving, or the surrounding landscape is moving past you. Think hard enough about this for long enough, and you might invent the theory of relativity.

As for acceleration, you can feel acceleration as a force. On a train, it will be a force pushing you back into your seat, or pulling you forward. Since you can’t tell which direction you’re already moving, you won’t know if the acceleration is making you faster, or making you slower (sometimes called deceleration)

No, you aren’t. Your body can only detect changes in motion. So you’d have to undergo acceleration of some sort or feel some force on your body.

OP – did you mean to post this twice?

Acceleration in one direction feels like deceleration in another direction. That’s why you couldn’t tell which direction you were facing even when your speed was changing. Beyond that, you can’t feel yourself moving at a constant speed unless air is hitting you or something like that, you can only feel changes in speed.