h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

If you roll a coin along a tabletop, does the lower half move in the opposite direction?

Yes

really? Or does it pivot around the lower point that touches the table?

Is this an ongoing question and answer, or can you give me a definate "yes or no"?

I assume you were looking for the answer 'none; it is travelling from London, not to London'.

I was trying to help you get there, CD, because you're very close.

There are two answers, but CD is close to the best one.

I think every bit of the train travelling towards London, otherwise some bits would not ultimately arrive.

But at the start the engine has to have a fair amount to pull to get the rest of the train to move, and given the way the carriages continually bounce against each other another answer could be - about half the train at any one time, though which half in constantly shifting.


Geggs

Over the whole journey, yes, every bit arrives at Edinburgh - even those parts which were travelling towards London, which you are yet to identify.

That's a fair point about carraiges bumping, and you'd see that at low speed. There's another which you could identify at moderate speed, but the one CD's close to would happen at any speed.

If you run your finger around a circle, it goes from left to right and then right to left or vice-versa.

Yes, but if the circle's travelling to Edinburgh at 100mph, your finger will only be travelling to Edinburgh at a slightly faster/slower rate.

But we're not talking circles or coins - this is a train...

I believe that the wheels would turn in such a way to make a force in the London-bound direction, pushing the train forward in accordance to the 3rd Law of motion. Therefore, it could be argued that the wheels moved toward London.

Hmmm, I don't think I quite follow you there, mikey. I can imagine a frictional force on the rails, but I'm just looking for a part of the train itself which is moving towards London.

No takers for this? If not, then I'll post the answer in a little while...

I doubt that this question is one that can be calculated or deduced logically (without knowing far too much about train wheels).

I think I've found an answer, but if anyone wishes to think on about the problem don't follow the link:

http://www.compadre.org/precollege/items/detail.cfm?ID=3955 Click on 'Rolling: The Physics of Wheels' and scroll down to the bottom.

That's the correct answer, by the way, and you see now why Czar Dene was very close.

I think it's quite easy to envisage, if you consider a wheel as 'pivoting' about the point where it touches the rail.

The alternative answer, which isn't so good, is that certain components in the engine could happen to be spinning faster than the train is moving (maybe there are flywheels or something - every engine's different). If that's the case, then the edges of these rotary components will momentarily be moving backwards.

So, who's going to post the next one?

OK, then, I'll post the next one.

Q. Name three olympic sports (not necessarily current) in which the winners travel backwards.

Rowing, backstroke and tug-of-war(?)

correct! - your turn

Let's try a simple one:
During WWII, there was a bridge connecting Germany and Switzerland, and on the German side, there was a sentry tower with a guard in it. He would come out every three minutes to check on the bridge, and he had orders to turn back anyone who tried to get into Germany, and shoot anyone trying to escape without a pass. There was a woman who desperately needed to get into Switzerland, and she knew she didn't have time to get a pass. It would take her at least six minutes to cross the bridge, but she managed to do it. How?

Ah yes, I saw this in the movie Hannibal Brooks, starring Oliver Reed.

He crosses the alps with Lucy the elephant in tow, and without being noticed by the Germans. At the film's climax, they run a rope around the base of the sentry tower and Lucy pulls the lot down.