Forces inside a moving elevator

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  • #1
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For some reason this is something I have really hard time wrapping my head around.

Suppose you are sitting on a chair inside a completely closed elevator. The elevator might be moving vertically at a completely constant speed, or it might be completely stationary.

It is my understanding that regardless of which is the case, you'll feel pretty much the same. If you are completely still, it will be impossible to tell if the elevator is stationary or moving at a constant speed. If you drop a ball, it will accelerate towards the floor exactly the same regardless. It's impossible to tell by measuring the movement of the ball if the elevator is moving or not.

All that feels logical. However, then, you stand up and... you'll feel the difference while doing that movement. If the elevator was stationary, it will feel like always. If the elevator is moving downwards at a great speed, you'll find that standing up is extremely easy, almost like you were a lot lighter (almost weightless, if the elevator is moving at a great speed). Conversely, if the elevator is moving upwards at a great speed, you'll feel like you weigh a ton, and it will be harder for you to stand up. Climbing a ladder inside the elevator will be much easier or much harder depending on which vertical direction the elevator is moving.

That kind of makes sense... and kind of doesn't make sense. If you are completely still, you can't tell if the elevator is moving or not, but if you stand up (or sit back down), you'll be able to. It's hard to understand the mechanics behind this.

I'm also assuming that with the dropping the ball experiment, how much the ball will bounce will also depend on the speed of the elevator, for a similar reason (assuming the ball has some elasticity to it).

Could someone explain the mechanics behind this?
 

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  • #2
Doc Al
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However, then, you stand up and... you'll feel the difference while doing that movement. If the elevator was stationary, it will feel like always. If the elevator is moving downwards at a great speed, you'll find that standing up is extremely easy, almost like you were a lot lighter (almost weightless, if the elevator is moving at a great speed). Conversely, if the elevator is moving upwards at a great speed, you'll feel like you weigh a ton, and it will be harder for you to stand up. Climbing a ladder inside the elevator will be much easier or much harder depending on which vertical direction the elevator is moving.
Why do you think this is true?
 
  • #3
jbriggs444
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Guessing at some cognitive dissonance here. The idea would be that a rider standing up in a rising elevator is "doing work" to elevate his body while a rider sitting in a chair is somehow not. An obvious (but flawed) idea would be to multiply force applied by vertical distance moved up the shaft, yielding (hypothetically) the work supplied by the rider.

But, of course, the floor of the elevator is moving too and the rider is doing negative work across that interface. If the book-keeping is done right, it all balances out and no extra effort is required.
 
  • #4
Nugatory
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I'm also assuming that with the dropping the ball experiment, how much the ball will bounce will also depend on the speed of the elevator, for a similar reason (assuming the ball has some elasticity to it).
Same question as @Doc Al asks above about another part of your post: Why do assume this? You’re talking about the speed of the elevator relative to something. What? The surface of the earth? The center of the Earth? The sun at the center of the Earth’s orbit? Or something else?

This might also be a good time to think about standing on the flat and level floor of a building. Is taking a step to the east easier, harder, or the same as taking a step to the west? The building and the floor are moving east with a speed of many hundreds of kilometers, but why should that matter?
 
  • #5
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This might also be a good time to think about standing on the flat and level floor of a building. Is taking a step to the east easier, harder, or the same as taking a step to the west? The building and the floor are moving east with a speed of many hundreds of kilometers, but why should that matter?
In that case you are not moving against or in the direction of gravity.

I was explicitly stating that the elevator is moving vertically. If it's moving horizontally, then there would be no discernible difference.
 
  • #6
Vanadium 50
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Why do you think this is true?

You really should answer Doc Al's question if we are to make progress,
 
  • #7
Nugatory
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In that case you are not moving against or in the direction of gravity.

I was explicitly stating that the elevator is moving vertically. If it's moving horizontally, then there would be no discernible difference.
The point of my rhetorical question was to get you to think about why it might or might not matter whether you’re moving with or against the force of gravity.
 
  • #8
A.T.
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...it will be impossible to tell if the elevator is stationary or moving at a constant speed...
Right
...If the elevator is moving downwards at a great speed, you'll find that standing up is extremely easy, almost like you were a lot lighter (almost weightless, if the elevator is moving at a great speed). Conversely, if the elevator is moving upwards at a great speed, you'll feel like you weigh a ton, and it will be harder for you to stand up. ....
Wrong. Are you confusing velocity with acceleration?
 
  • #9
Janus
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In that case you are not moving against or in the direction of gravity.

I was explicitly stating that the elevator is moving vertically. If it's moving horizontally, then there would be no discernible difference.
The real question is why you think there would even be a discernible difference just because you were moving vertically at a constant speed.
 
  • #10
russ_watters
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All that feels logical. However, then, you stand up and... you'll feel the difference while doing that movement. If the elevator was stationary, it will feel like always. If the elevator is moving downwards at a great speed, you'll find that standing up is extremely easy, almost like you were a lot lighter (almost weightless, if the elevator is moving at a great speed). Conversely, if the elevator is moving upwards at a great speed, you'll feel like you weigh a ton, and it will be harder for you to stand up. Climbing a ladder inside the elevator will be much easier or much harder depending on which vertical direction the elevator is moving.
In order to stand up or climb up a ladder, your legs apply a force to your torso. What, EXACTLY, determines the magnitude of that force?

And to be obvious: can you feel the difference just holding your arms out in front of you? Same why or why not: what exactly would cause them to feel heavier?
 
  • #11
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As only A.T. stated it unambiguously, I want to make it perfectly clear that your premise is wrong. There will be no discernible difference in your ability to stand up. It doesn’t matter if you are traveling vertically, horizontally, or any other direction. If there is no acceleration the physics does not change.

All the other posts with hypothetical questions and counter examples etc. are intended to help you realize this.
 
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