GRED: Fast train cars connected by string

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What happens to the strings connecting the train cars?

The strings droop.
71
20%
The strings break.
76
21%
The strings remain the same.
216
60%
 
Total votes: 363

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RJN
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GRED: Fast train cars connected by string

Post by RJN » Thu Jul 01, 2010 3:04 pm

GRED: Guess the Result of the Experiment of the Day: Fast train cars connected by string

Train cars sit on a circular track connected by taut strings. The train cars all begin to circle the track at once, faster and faster, eventually reaching relativistic speed. What happens to the strings?

Please do NOT post any answers or comments with spoilers here. Answers and comments with spoilers are encouraged in GRED Answer post here: http://asterisk.apod.com/vie ... 32&t=20063 . Please check back there later -- what I believe to be the correct answer will be posted there a few days after this initial post.

Comments or questions here -- without spoilers -- are OK. In particular, questions about the experimental setup are OK.

GRED editor wanted: I would like GREDs to be a regular feature on the Asterisk where people send in their GRED suggestions to an editor who picks the good ones, posts them, and oversees GREDs generally. If you are interested in being that editor, please contact RJN.

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Henning Makholm
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Re: GRED: Fast train cars connected by string

Post by Henning Makholm » Fri Jul 02, 2010 1:22 am

There are at least three cars, right? Evenly spaced around the circle?
Henning Makholm

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alter-ego
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Re: GRED: Fast train cars connected by string

Post by alter-ego » Fri Jul 02, 2010 3:22 am

We should assume the reference frame at rest wrt the train track correct? I've answered my own question: Both reference frames are important in order to arrive at an acceptable answer.
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RJN
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Re: GRED: Fast train cars connected by string

Post by RJN » Fri Jul 02, 2010 5:24 pm

Henning Makholm wrote:There are at least three cars, right? Evenly spaced around the circle?
Thanks for your question. However, I don't think the number or spacing of the train cars is a key factor here, so long as the strings are taut to begin with. It is important that the relative spacings of the cars do not change, though. Therefore, all cars have the same speed at all times, and what happens to one string will happen to all of the strings. - RJN

Cancer

Re: GRED: Fast train cars connected by string

Post by Cancer » Fri Jul 02, 2010 6:54 pm

This sort of question has been known to cause arguments heated enough to get a group of tenured (and untenured) physics professors thrown out of a restaurant.

stevev

Re: GRED: Fast train cars connected by string

Post by stevev » Sat Jul 03, 2010 1:03 am

"Train cars sit on a circular track connected by taut strings. The train cars all begin to circle the track at once, faster and faster, eventually reaching relativistic speed. What happens to the strings?"

Do you mean that there is a complete circle of cars around the track, or just a few cars taking up a small section of the track?

How is acceleration applied? Is there an "engine" that pulls the train, or is acceleration applied equally to all the cars?

How rapidly is acceleration applied? In particular if there is a lead "engine" car and it accelerates sufficiently rapidly, it has to break the string between it and the following car. (That case does require some truly extreme acceleration.)

How long is the circular track? One might assume that with a sufficiently large circular track, there could be an arbitrarily small centripetal force required to keep the train cars going in a circle. Conversely a shorter track could require incredible forces to keep the trains on the track.

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alter-ego
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Re: GRED: Fast train cars connected by string

Post by alter-ego » Sat Jul 03, 2010 2:12 am

This is entirely a "thought" experiment, so:
stevev wrote:"Train cars sit on a circular track connected by taut strings. The train cars all begin to circle the track at once, faster and faster, eventually reaching relativistic speed. What happens to the strings?"

Do you mean that there is a complete circle of cars around the track, or just a few cars taking up a small section of the track?
Consider the cars equally space around the track, ALL tied together with equal lengths of "massless" string
How is acceleration applied? Is there an "engine" that pulls the train, or is acceleration applied equally to all the cars?
Irrelevant how acceleration is applied, EXCEPT don't think of one lead car in a push/pull situation; consider uniform acceleration applied to all cars.
How rapidly is acceleration applied? In particular if there is a lead "engine" car and it accelerates sufficiently rapidly, it has to break the string between it and the following car. (That case does require some truly extreme acceleration.)
How fast acceleration occurs is irrelevant; as I said, no lead engine
How long is the circular track? One might assume that with a sufficiently large circular track, there could be an arbitrarily small centripetal force required to keep the train cars going in a circle. Conversely a shorter track could require incredible forces to keep the trains on the track.
All irrelevant, disregard all forces. The question posed deals with a very fundamental principles in trying to use special relativity involving an accelerating (non-inertial) reference frame, and forces can be ignored.
A pessimist is nothing more than an experienced optimist

Moe Death

Re: GRED: Fast train cars connected by string

Post by Moe Death » Sat Jul 03, 2010 3:11 am

With no engine to pull the cars, they can't accelerate to any speed, let alone relativistic. Those strings are swinging in the breeze...

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Henning Makholm
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Re: GRED: Fast train cars connected by string

Post by Henning Makholm » Sat Jul 03, 2010 3:28 am

Moe Death wrote:With no engine to pull the cars, they can't accelerate to any speed, let alone relativistic. Those strings are swinging in the breeze...
They are self-propelled units.
Henning Makholm

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wonderboy
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Re: GRED: Fast train cars connected by string

Post by wonderboy » Mon Jul 05, 2010 9:00 am

This isn't hard to understand people. I hope moe death posted his reply at about the same time you posted yours Alter-ego.

If not, he clearly never read your post above his, I shall now try and answer this as best I can in the other topic


Thank you


Paul.
"I'm so fast that last night I turned off the light switch in my hotel room and was in bed before the room was dark" Muhammad Ali, faster than the speed of light?