# B Knowing if the elevator is moving or not

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1. Dec 25, 2017

### ubergewehr273

Consider a hypothetical scenario where you are present in an elevator which is either at rest or is moving with constant velocity (ignore cases of acceleration when elevator stops/starts moving). Now how will you determine if the elevator is moving or not given that there are no windows in the elevator. You may use any instrument of your choice to determine the case.

2. Dec 25, 2017

### Staff: Mentor

All motion is relative. So the elevator is always still in one reference frame and moving in different reference frames. So the answer is that you can't measure it, nor can you define it in a frame invariant manner.

The computer on your desk is moving right now in the reference frame of the moon. Can you measure that?

3. Dec 25, 2017

### ubergewehr273

Ok let's say that I wanted to know if the elevator is moving or not in the frame of reference of the earth.

4. Dec 25, 2017

### Staff: Mentor

The answer is still no.

5. Dec 25, 2017

### ubergewehr273

Why? I made the reference frame specific.

6. Dec 25, 2017

### Staff: Mentor

Your instrument would need to know what the reference frame is. You might as well measure nothing and twist the instrument dial to read whatever speed you want it to show.

So the answer is still no.

7. Dec 25, 2017

### PeroK

You could use a GPS.

8. Dec 25, 2017

### PeroK

Measure changes in the gravitational field strength.

Measure the Coriolis force.

Use the Earth's magnetic field.

9. Dec 25, 2017

### Staff: Mentor

Any of these could, in principle, be used to determine whether the elevator is moving relative to the earth (although it would be a good exercise to calculate the strength of these effects - there's a reason why we use GPS signals instead of these other effects for navigation).

The important point that OP must understand is that all of these techniques, whether practical or not, are not answering the question "Is the elevator moving?", and in fact that question is altogether meaningless. They are answering a different question, namely "Are the elevator and the earth moving relative to one another?"

10. Dec 25, 2017

### jerromyjon

Not so much meaningless, as it is the "ah-ha" moment when you realize you CANNOT tell it is from the inside, meaning there is no preferred reference frame.

Last edited: Dec 25, 2017
11. Dec 25, 2017

### PeroK

Does that imply that you could tell it is moving from the outside?

In terms of the original question, the closed elevator is a red herring. You could be in a glass elevator on the outside of a skyscraper and the same argument applies. The question of whether you are absolutely in motion is meaningless.

12. Dec 25, 2017

### sophiecentaur

This is a typical PF conversation. It starts with a very basic scenario and then it gets more and more complicated. The problem arises because the initial question is seldom specified sufficiently. If it were then the odds are that the OP would already have thought out the answer.
The basic answer to the basic question has to be 'No' but, the elevator could be moving very fast and over a great distance (unlikely) and then the changes in g and air pressure could give a clue and so could Coriolis. - "Any instrument of your choice" makes things more interesting and much harder.

13. Dec 26, 2017

### Tom.G

Read some af the works by Albert Einstein. Among other things, he pointed out that to be an unanswerable question.

14. Dec 27, 2017

### A.T.

Do you literally just mean that there no windows, or do you mean that no measurements relying on interaction with the outside are allowed?

15. Dec 27, 2017

### sophiecentaur

I have read some of his helpful pictures and models. Some of them are excellent but I think he suffered from being very much brighter than his audience and he didn't always realise just how literal they could be. If he had specified all those pictures as tightly as really necessary, the story would have been longer and more convoluted than the original bit of theory he was trying to get across.
Can't blame him, though.

16. Dec 27, 2017

### Vanadium 50

Staff Emeritus
Can't you just count the number of chimes as you cross floors?

17. Dec 27, 2017

### sophiecentaur

Is the correct answer.

18. Jan 9, 2018

### ubergewehr273

Not every building with an elevator has the facility of chimes.

19. Jan 9, 2018

### Nik_2213

Hmm. Getting adequate GPS signals indoors is problematic at best...

May we assume that the shaft / cab combo are sufficiently ventilated to prevent a 'Tube Train' piston 'Whoosh' ?
Some out-door watches & GPS devices also have a barometer function; If you ride cab for half a dozen floors, there should be a non-trivial change. This may not be accurate enough to choose the floor, but surely proof of concept.

20. Jan 9, 2018

### jbriggs444

Sounds like the old "how many ways can you use a barometer to measure the height of the Empire State Building" puzzle... Bribe the elevator operator with a barometer to have him tell you whether it's moving.

Or ask an accountant how fast it is moving. And let him ask you what you want the answer to be.

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