Starship Asterisk*

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.

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

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.

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

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.

"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.

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.

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

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.

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.