When a train moves forward there is always a part of it that moves backward relative to the ground. What part is it?

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Well, one answer is the outer part of the wheel at the point when it's at the bottom of it's rotation. The bit that sits on the rail probably doesn't move backwards, but the extended bit that sticks out as a guide will be going backwards a tiny bit because it has a bigger diameter than the rolling part of the wheel. 

Don't know if that's the answer you had in mind.

 

7 minutes ago, mistermack said:

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Well, one answer is the outer part of the wheel at the point when it's at the bottom of it's rotation. The bit that sits on the rail probably doesn't move backwards, but the extended bit that sticks out as a guide will be going backwards a tiny bit because it has a bigger diameter than the rolling part of the wheel. 

Don't know if that's the answer you had in mind.

 

That's the only part I know of. +1

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In the old days of steam, that I remember well, there was so much torque on the big driving wheels that they would often spin backwards as the train was trying to pull away, so I thought of that first, but that's a special case. 

 

Edited May 22, 20232 yr by mistermack

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Train engine cylinders are mounted horizontally or near-horizontal and parallel to the rails, IIRC.  When the engine first starts, before the forward power has transferred to the cars and they are in motion, some of the pistons and piston rods will move backwards relative to the ground.  

There is a potential second answer.  But I'm not sure it's "always."  

 

 

 

14 minutes ago, mistermack said:

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In the old days of steam, that I remember well, there was so much torque on the big driving wheels that they would often spin backwards as the train was trying to pull away, so I thought of that first, but that's a special case. 

 

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I've seen it in movies. In the OP, I've mention always to eliminate it.

 

3 minutes ago, TheVat said:

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Train engine cylinders are mounted horizontally or near-horizontal and parallel to the rails, IIRC.  When the engine first starts, before the forward power has transferred to the cars and they are in motion, some of the pistons and piston rods will move backwards relative to the ground.  

There is a potential second answer.  But I'm not sure it's "always."  

 

 

 

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The second hidden contents doesn't pass the always part. The first hidden contents doesn't show anything  

 

PS. The mention of "always" in the OP means, "while the train moves forward."

On 5/22/2023 at 7:31 PM, Genady said:

When a train moves forward there is always a part of it that moves backward relative to the ground. What part is it?

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From the correct answer above I have two possible alternatives. (Correct or not may be depend on translation and definition )
1: The rear facing seats (an anyone seated in them) on the train will always move backwards relative to the ground (but not relative to the train).
2: Electricity from the current collector, supplying anything behind the current collector, will move backwards relative to the ground (and relative train)

Note: All trains aren't electrical. All trains don't have rear facing seats. Also, as far as I know there are/were trains without flanges on running wheels.

 

45 minutes ago, Ghideon said:

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From the correct answer above I have two possible alternatives. (Correct or not may be depend on translation and definition )
1: The rear facing seats (an anyone seated in them) on the train will always move backwards relative to the ground (but not relative to the train).
2: Electricity from the current collector, supplying anything behind the current collector, will move backwards relative to the ground (and relative train)

Note: All trains aren't electrical. All trains don't have rear facing seats. Also, as far as I know there are/were trains without flanges on running wheels.

 

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I don't know if this puzzle has been ever translated, but the assumed definition is that the direction of the train relative to the ground is forward, and the opposite direction of movement relative to the ground is backward. Thus, the alternative 1 wouldn't fit.

I'm not sure how the direction of electricity is defined, but perhaps in some definition your alternative 2 is correct. +1

I don't know about railroad trains without guides on the wheels. What stops them from sliding off the rails?

 

 

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7 hours ago, Genady said:

I don't know about railroad trains without guides on the wheels. What stops them from sliding off the rails?

 

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For instance the Locher rack system, also used to propel the train. Used on trains on rack railways 

Found a reference: https://en.wikipedia.org/wiki/Pilatus_Railway

 

24 minutes ago, Ghideon said:

 

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For instance the Locher rack system, also used to propel the train. Used on trains on rack railways 

Found a reference: https://en.wikipedia.org/wiki/Pilatus_Railway

 

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This rail with teeth runs in the middle, but there are two normal rails outside. The wheels on these rails are normal railroad wheels with guides, I think.

 

 

 

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18 minutes ago, Genady said:

This rail with teeth runs in the middle, but there are two normal rails outside. The wheels on these rails are normal railroad wheels with guides, I think.

 

Today yes, initially not necessarily so  (if the source above is correct)

"The system was also capable of guiding the car without the need for flanges on the wheels. Indeed, the first cars on Pilatus had no flanges on running wheels, but they were later added to allow cars to be moved through tracks without rack rails during maintenance." https://en.wikipedia.org/wiki/Pilatus_Railway  

 

Edited May 25, 20232 yr by Ghideon

2 minutes ago, Ghideon said:

 

 

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Today yes, initially not necessarily so  (if the source above is correct)

"The system was also capable of guiding the car without the need for flanges on the wheels. Indeed, the first cars on Pilatus had no flanges on running wheels, but they were later added to allow cars to be moved through tracks without rack rails during maintenance." https://en.wikipedia.org/wiki/Pilatus_Railway  

 

Oh, I see.

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I don't know much about train engines but if there are pistons and a crankshaft, part or all of them will inevitably go backwards unless they are oriented perfectly perpendicular to the tracks.

 

3 hours ago, npts2020 said:

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I don't know much about train engines but if there are pistons and a crankshaft, part or all of them will inevitably go backwards unless they are oriented perfectly perpendicular to the tracks.

 

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Yes, backwards relative to the train, but not necessarily relative to the ground when the train goes fast forward relative to the ground.

 

On 5/26/2023 at 3:00 AM, Genady said:

Yes, backwards relative to the train, but not necessarily relative to the ground when the train goes fast forward relative to the ground.

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It would be surprising to me if pistons and cams didn't move backwards relative to the ground most or all of the time on their backstroke. How fast would the train have to be moving to be going faster than those things move?

 

22 minutes ago, npts2020 said:

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It would be surprising to me if pistons and cams didn't move backwards relative to the ground most or all of the time on their backstroke. How fast would the train have to be moving to be going faster than those things move?

 

I really don't know, and you might be right. In this case, +1.

Now, let's consider an electric train.

27 minutes ago, Genady said:

Now, let's consider an electric train.

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LOL. Any rotating or reciprocating parts unless aligned as specified above will run into the same "going backwards" scenario. However, at some point the train will be moving fast enough that even that "backward" movement relative to the train will still be going forward relative to the ground, albeit at a much slower rate. The whole proposition rests on whether that point is with the operating parameters of a current trains, which I know nothing about. It would be easy enough to compute if you knew the shaft diameter and/or piston stroke length and rpm but I have little idea of those parameters on a train. BTW, the range I got was from fast walking speed to well beyond the velocity of any current trains. (was hoping someone had a better idea of the size and speed of such things)

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6 hours ago, npts2020 said:

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LOL. Any rotating or reciprocating parts unless aligned as specified above will run into the same "going backwards" scenario. However, at some point the train will be moving fast enough that even that "backward" movement relative to the train will still be going forward relative to the ground, albeit at a much slower rate. The whole proposition rests on whether that point is with the operating parameters of a current trains, which I know nothing about. It would be easy enough to compute if you knew the shaft diameter and/or piston stroke length and rpm but I have little idea of those parameters on a train. BTW, the range I got was from fast walking speed to well beyond the velocity of any current trains. (was hoping someone had a better idea of the size and speed of such things)

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There is a part of the train that moves backward relative to the ground when the train moves forward at any speed.