Physics Discussion Thread

[rsingh]

Ahh my ten year old is postulating about existence of "white hole" as an counterpart to Black hole. Other day kids were discussing "parallel world" on back seat.In my time one mullah depressed whole class when he asked if any body new chemical formula of hot water..........

[Amber G.]

As many know or have suspected, Q1, Q2, P3 etc are fairly famous, and are connected with Richard P Feynman. (One of the finest teacher and Physicist)
For Q2 - I can not do better than let the master explain it.
Q2

For Q1 You can see from the same master -
Q1

For P3, I am going to make some comments later but in the meanwhile, another Video -
P3


As many have said before, the answer of P3 is by no means obvious (there are many scientific papers have been published even in recent years - simple google search will show that) - As famous Wheeler (Feynman's thesis adviser) said: "Feynman has absolutely convinced him the day before that it went around backwards, and he absolutely convinced him today that it will go around forward, and he did not yet know what he will convince him on the next day" (Source - Gleick, Feynman's biography - Genius as Rahul M posted a few posts ago)

(When Feynman tried to demonstrate this in Princeton Lab - something went wrong with the vacuum pump and there was a big explosion - scattering glass and water all over and getting Feynman into trouble )

[Amber G.]

Ahh my ten year old is postulating about existence of "white hole" as an counterpart to Black hole. Other day kids were discussing "parallel world" on back seat.In my time one mullah depressed whole class when he asked if any body new chemical formula of hot water..........

Reminds me of my kids when they were little, they had a song

"Bol bol bol... ..
Mister Gol Matol
Just how massive is a big black hole?

Half the radius
Divide by G
and Multiply by the
square of C ...."

(One of the little kid is now a grad student of Physics and funny part is he did get one problem about black hole in one of the exam where he knew the formula)

[vina]

As many know or have suspected, Q1, Q2, P3 etc are fairly famous, and are connected with Richard P Feynman. (One of the finest teacher and Physicist)
For Q2 - I can not do better than let the master explain it.
Q2

Amber G, I know that between me and RPF, the chances of me being right and he being wrong are infinitesimally small, however, I do submit that RPF is mistaken. The engineer in me screams that he is wrong. I think He has given a "Scientist's" answer and not an "Engineer's" answer.

RPF has taken a "hypothetical" condition and arrived at a "hypothetical" answer. His answer is correct only if you assume that the rail track is like a line cross section like "|" and the "side of the cone" shaped wheel rim rests on it and hence left wheel and right wheel traveling different distances under rotation and therefore keeps course.

Now, in real life, the rail has top surface is more like "I" and not "|" and that is for engineering reasons. You can actually make a "|" shaped track take the load of the wagon . But no engineer work his salt , except maybe a Paki from the Lal Masjid Univ engineering dept would engineer it that way. The bearing surfaces would be a "point contact" and not a "line contact" if engineered that way. The life of the rail wheel and the tracks would be in hours and not a few decades as it is in real life in a "point contact" design. Imagine the contact point of your car tyre to be a point, instead of a line. Your tyre would last a few days at best before wearing out.Similar scene.

ChandraS is right. The bearing surfaces achieve "line contact" and that is why the sleepers are angled (by 5 deg he says for IR). Now you may argue but that line contact still doesn't change RPF's arguement about wheels on either side traveling differnent distances if the wagon undergoes sideways motion. Two arguments against that. 1) The frictional forces resisting that will be will be enormous and will quickly move the wagon back to the center and 2) If i recall what I observed of the wagon wheel and rail (head on for stationary wagons, where you can see the wheel making contact with the rail), I dont recall seeing any significant play/gap for it to ride up and down sideways.

A more serious argument I would make against RPF's explanation is this. Assuming that the brakes are applied in a panic situation and the wheels lock up and throw sparks ..basically the train is sliding on the tracks and wheels are not rotating (like in that WWII movie where the train is on the verge of being attacked by planes and the train goes into the tunnel and the slam the brakes and you can see the wheels lock up and train slides on the track throwing sparks.. was the movie Von Ryan's Express) , so in this case of a sliding train under emergency braking, if there was a disturbance in the transverse direction, by RPF's explanation the train would not stay on the tracks , while we know it does!.

I think us engineers (me and ChandraS) have it right (we work in the real world after all) The restoring forces and the weight of the wagons would set up forces and moments that keep the wagons on "track", with the middle being stable equilibrium (min energy position).

[ChandraS]

The bearing surfaces achieve "line contact" and that is why the sleepers are angled (by 5 deg he says for IR). Now you may argue but that line contact still doesn't change RPF's arguement about wheels on either side traveling differnent distances if the wagon undergoes sideways motion. Two arguments against that. 1) The frictional forces resisting that will be will be enormous and will quickly move the wagon back to the center and 2) If i recall what I observed of the wagon wheel and rail (head on for stationary wagons, where you can see the wheel making contact with the rail), I dont recall seeing any significant play/gap for it to ride up and down sideways.

RPF is not wrong. The gage between the rails is slightly increased on curves depending on radius of the curve and the speed to be sustained. This is usually a few mm only. RPF's argument is the basis of this. The wheels do ride up and down slightly, leading to the differential travel of the two wheels. No doubt, the friction forces are huge but are be surmounted by the momentum and loco power. Recall the velocity vector is tangential to the curve.

A more serious argument I would make against RPF's explanation is this. Assuming that the brakes are applied in a panic situation and the wheels lock up and throw sparks ..basically the train is sliding on the tracks and wheels are not rotating (like in that WWII movie where the train is on the verge of being attacked by planes and the train goes into the tunnel and the slam the brakes and you can see the wheels lock up and train slides on the track throwing sparks.. was the movie Von Ryan's Express) , so in this case of a sliding train under emergency braking, if there was a disturbance in the transverse direction, by RPF's explanation the train would not stay on the tracks , while we know it does!.

You are correct here. Emergency braking is mainly done on a straight stretch of track. That's why train engineers are instructed not to perform emergency braking on curves. The train will simply jump tracks!! Of course, depending on speed, the engineer may do so if the situation so warrants. Von Ryan's Express is a pretty good movie but can hardly qualify to be an argument against RPF

I think us engineers (me and ChandraS) have it right (we work in the real world after all) The restoring forces and the weight of the wagons would set up forces and moments that keep the wagons on "track", with the middle being stable equilibrium (min energy position).

Yep, reality is very different from the textbooks as I find out everyday at work. The caveats & disclaimers that are put out with every result or report will make people wonder what do we really know!!

[Amber G.]

Vina, ChandraS - I am traveling and have not read your whole posts carefully, but I will make a brief comment anyway FWIW here are some points:

See Wiki
http://en.wikipedia.org/wiki/Rail_adhesion


and if the train is turning (or accidental veered of- course on a straight track)


Here is a picture of wheel click

(I actually looked (and measured) at the wheel in Indian train wheel (quite a long time ago) and the tapering was quite significant - more like 15-20 degrees.

Note that track has convex (top) surface click which make the contact area with the wheel smaller.

Also FWIW the engineers (specially mechanical Engineers and also who have worked for railways) whom I asked this question knew the answer given by Feynman .. (the problem of course is very old - much before Feynman )

Anyway - hope this helps - more on this in a few days after the travel.

[vina]

RPF is not wrong. The gage between the rails is slightly increased on curves depending on radius of the curve and the speed to be sustained. This is usually a few mm only. RPF's argument is the basis of this.

Ah..! Thanks ChandraS. BRF Rocks!. There is no subsitute to actually having people who are hand on and do it. No amount of googling would give out information like that gauge between the rails is changed on curves! So that change is how you get the "toe in" forces in railways and get the train to start turning.. the gauge change , which gets the train started in the turn , along with the "cant" or "tilt" (the well know tan thetat yada yada in high school) is what takes the train round the curve!. From you I learnt that 1) That tracks are not perpendicular but have an inclination (I thought that the wheel shape was "concave "and the rail shape was "convex" so that they have a good line contact at the bearing surfaces. I would have never thought that the engineering practice would be to tilt the track, while I think it would be far easier to maufacture a track with a surface shape that will fit the conical side of the wheel !

How you get the toe in forces in automobiles is that the suspension tilts the wheel. When you turn the wheel left, the wheels in addition to turning left, also tilt ever so slightly to the left, getting the "toe in" forces which starts the turn. If not it will require a huge amount of force (you will have to work against friction) to turn. Same is the reason why all wheel independent suspension cars will have better handling, becuase in that case the rear wheels too can "toe in" in addition to the front wheels.

ChandaS, could you take some time to think over and rack your brains on this to recollect old thoughts?. I would think that to get the train to turn to the left, what you want to do is to get the train to ride in the "narrower" cone portion of the left wheel and make it ride on the "wider" cone portion of the right wheel, so that it turns left. For that, you would actually start by decreasing the gauge initally from the right side to start the turn, while simeltaneously getting the "bank" going to get the centrifugal forces in play to keep it turning.

Von Ryan's Express is a pretty good movie but can hardly qualify to be an argument against RPF

Oh. Von Ryan's express served pretty well. It is only in old movies like that yous see the trains slide and sparks flying under emergency stop . That scene (if it was that movie) was well filmed. Obviously not something you get to see in everyday life.

However, I think the "victorian 19th century" / Feynman explanation based on kinematics is flawed, because it assumes point contact /kinematics as the reason why the train stays on "track" and is self steering. Like you explained, steering a train is carefully engineered , it doesnt do that by itself

Think of a real world case, if you think that Von Ryan express emergency braking/sliding train is all Hollywood special effects. If the "Feynman" explanation is correct, what will happen if some guy engineered a train where the left and right wheels are not connected by a solid shaft and hence able to rotate at independent speeds, like the rear wheels of a front wheel drive car or the front wheels of a rear wheel drive car ?. The "Feynman explanation" would say that the wagon wont be able to stay on track in a straight line! Will you call that designer a Paki ?

If so , how do you explain this bogie design ?


Check out this link For full size.

So, by the "classic" kinematic explanation, the tracks and wheel shapes should be different for this design and such a train cannot run on the same tracks as a "conventional" train, right ?

If so, check out this Amtrak Train which is from the company Talgo, which uses the bogie design as above!. It runs as the same track as the normal conventional train!. Now when the bottom is knocked out of the "Kinematic explanation" ie that fixed shaft connecting the wheels is what gives straigth line stability, you know that it is lot more complex than that!. .. Engineers intuition right I guess! .

But seriously, BRF rocks. Who would have thought that someone like me, the closest I have come to rail tracks and wheels is at railway crossings and at stations would get so much insight into how the trains actually stay on track ,all in the space of two "cigarette breaks" and discussion with a bunch of very smart folks like you and Amber G and learn non intuitive stuff like tracks are actually "tilted" and that gauges "change", and then the opportunity to dig deeper and examine stuff critically and google/reasrch for stuff that can prove the "conventional wisdom" as inadequate. A real mental workout, makes you feel like you are 16 again and a bright spark with great inquisitiveness!

So .. Is RPF ("classical Kinamatic explanation") wrong ? Are the Talgo bogie and train designers Pakis?

Hint
Feynman / Classical Kinematic explantion is right of course and the Tolgo Train designers are Spanish and NOT Pakis! . But how can both be right ?.
Becuase there are TWO questions 1) How does the train "auto guide" ie steer itself through corners setc and the 2) How is the train laterally stable, ie if there is a disturbance in the transverse direction, it comes back to it's stable position, even if the wheels are not rolling like in Von Ryan's express . The answers to both is the shape of the wheels. Yes, if the wheels are independent, the train will not "auto guide" , which in all fairness is what RPF talked about in the video. The answer to the 2nd question is what keeps the train laterally stable, and that is what I answered. The confusion comes in the way the question was framed as " What keeps the train on its tracks"? by Amber G , ie. how does the train know where the center of the track and therefore it can follow curves. That really is two questions with the same aswer. But are the Talgo guys Pakis?.. coz, if Feynman is right, their train while being laterally stable, will not be able to go around curves! No they are Spanish, because they provided active steering for their train!.
ChandraS, your answers were spot on. Thanks,you helped me a great deal in undertanding that there were 2 problems actually. I threw the Talgo train at you hoping that you were not familiar with it and could throw your core understanding in doubt! Did you go to that Railway Engg school that is somewhere in the Eastern part of the country .. In any case, the Railways obviously do a very good job in training their folks. A really nice thing to know

[ChandraS]

Vina - I will reply in detail once I have a chance to go through the articulated axle stuff you have posted.

Regarding turning the train to the left, you actually turn the left rail a little bit tighter - increasing the gauge. Since the train is in forward motion, the narrower portion of the left wheel will be in contact and similarly the right wheel will travel on the wider portion. This results in the differential motion of the wheels making the train turn.

The track shape can be made to fit the conical profile of the wheel. But this would results in huge stress concentration at the flange/web root resulting in a more beefier profile. It would also mean higher fabrication costs since the regular I section is easier to cast than an I with slant top surface. It is easier - economical - to tilt the track to fit the wheel profile.

more later.

[ArmenT]

Here's a classic physics question:

In a washing machine or dryer, when a drum spins at high speed, the clothes are flung towards the wall of the drum due to centrifugal force. The same effect can be observed in a speed governor. There are several such examples in the real world.

Now observe what happens when you stir a cup of tea. You would think that the tea leaves would be flung outwards towards the walls of the cup, but in real life the tea leaves gather towards the center of the cup. Why does this happen?

[negi]

There is a catch there in case of a dryer the entire cage spins ; the friction between the cage and the clothes couples the force to the clothes and they too start to spin and we have a case for centrifugal force .

In case of Tea cup when the spoon is stirred one is actually churning a suspension of water with tea leaves ; one would observe that the tea level actually rises on the periphery and a vortex is created at the center .Now I believe tea leaves being lighter get trapped in the vortex and hence get confined to the centre , I presume if we put heavier particles eg. ball bearings etc then one should hear the sound of bearings hitting the cup walls.

[John Snow]

In case of the washer the lid is closed.
In case of tea cup its open to atmospheric pressure, and tea is in liquid, now when we start stirring at the center, the fluid close to the stirrer is at a higer velocity and a partial vaccum is created and the atmospheric pressure pushes the liquid surface down increasing the levels at the inner rim of the cup, also the fluid that is closer to the rim is slower because of friction with the cermaic walls.

If you ever help SHQ make dosa dough in summet you will see the same happening if the jar lid is open, if you see the plastic jar lid also has a small air vent for the partial vaccum not to make the removal of the lid and you spill the dough much to annoyance of SHQ ( she love it , to yell at you 'yenna appa.... doas ye ...... )

[vina]

Here's a classic physics question:

In a washing machine or dryer, when a drum spins at high speed, the clothes are flung towards the wall of the drum due to centrifugal force. The same effect can be observed in a speed governor. There are several such examples in the real world.

Now observe what happens when you stir a cup of tea. You would think that the tea leaves would be flung outwards towards the walls of the cup, but in real life the tea leaves gather towards the center of the cup. Why does this happen?

What happens when you spin a centrifuge is that that the "heavier" particles go outwards, while the "lighter" particles congregate to the center. In plain Inglees / Ingirisu .. particle with the higher density will be near the wall, while the lighter particles will be in the center of a mixture spinning in a centrifuge. Just like Xerox Khan in Kahuta separating the lighter U235 out of U238

Now "tea leaves" are more dense than water . duh. I know that coz when I make tea, I find the tea leaves in the bottom as I pour the tea out in the strainer. So basically , though the water is spinning, the tea leaves have sunk to the bottom of the cup and are below the layers of spinning water (basically the spinning water that is stirred has a parabolic shape) and layers below the lowest point of the parabola are not spinning, so they stay in the middle (assuming a normal cup.. not a very tall cup with a limited amount of water so that you can spin the water for the vortex bottom to touch the bottom of the cup).

In fact, in ocean waves, even during huge storms, there is hardly any movement at depths beyond 1.5 times the max wave heights. That is why in a storm, a surface ship will get tossed around like well.. " a ship in a storm" , while a submarine just a few meters below the surface will not feel any effect and ride the storm out with great tranquility with "peace , progress and prosperity" like our Chini neighbor.

While in a washing machine, the clothes are actually being spun

[ChandraS]

Coming back to the articulated axles in railways.
Based on my reading of the references and other literature, I can only surmise that their primary motive is to increase speeds on curves and to mitigate the centrifugal forces due to such speeds for a given curvature. I am still not convinced as to how they violate the principle stated by Feynman. The gist of his explanation is that you need the wheels to traverse different distances to be able to turn. For a solid axle rotating at a constant speed, the conical shape of the wheels is the solution. For independent axles, the conical shape is not necessary due to their ability to spin at different speeds. It will be fine running with flat profile wheels. But given that the existing infrastructure is geared towards the solid axles with tilted trackes and all, straight line stability requires a 'rigid' connection between the wheels. The restoring forces and weights will do the job on keeping it in a straight line similar to solid axles. The Talgo technology, shown in the pic earlier, achieves this by looping over the wheels and creating a connection. Also it enables the 'toe in' forces without having to tilt the wheel slightly as done in automobiles. Anyways, this explanation may not answer all your queries and I apologize for my inability to explain myself more clearly.

[ChandraS]

Here's a classic physics question:

In a washing machine or dryer, when a drum spins at high speed, the clothes are flung towards the wall of the drum due to centrifugal force. The same effect can be observed in a speed governor. There are several such examples in the real world.

Now observe what happens when you stir a cup of tea. You would think that the tea leaves would be flung outwards towards the walls of the cup, but in real life the tea leaves gather towards the center of the cup. Why does this happen?

It is the difference between a free vortex and forced vortex. The washing machine drum and its contents spin as a solid body. This creates the centrifugal forces pushing the clothes outwards. This is a free vortex. For the tea cup, the stirring creates a vortex. It is similar to the whirlpool drawing the tea leaves towards the center. This is a forced vortex. If the tea cup wer to be placed on a turntable and spun, the tea leaves would be pushed outwards.
Check this out for a more mathematical explanation Vortex

[SaiK]

http://www.hindu.com/2009/01/07/stories ... 502200.htm
The new measurements showed that our solar system is hurtling along at 960,000 kmph, 160,000 kmph faster than thought. “These measurements are revising our understanding of the structure and motions of our galaxy,”

Astronomers believe the crunch to end all crunches could happen around the same time our sun is due to burn up the last of its nuclear fuel, within the next 7 billion years. It is highly unlikely that planets and stars will collide. Instead the two galaxies will merge to form a new, large galaxy.

“One thing we don’t know yet is whether Andromeda will hit us square on, or whether it will be a glancing blow.”

Other astronomers at the meeting reported an updated map of the Milky Way’s spiral arms. It shows two prominent and symmetrical arms spiraling our of the galaxy’s core, which then branch into four separate arms. Earlier observations had confused astronomers by revealing different numbers of spiral arms reaching out from the galaxy’s centre

now if the spiraling arms are moving in a certain sphere, and further if the measurements indicate speeding from 960,000 kmph, got extra speed of 60,000 kmph since the last measure.. it could be also, that (imagine a black hole suction) towards the center that has increased the speed and hence the difference?

[vina]

Coming back to the articulated axles in railways.
... The gist of his explanation is that you need the wheels to traverse different distances to be able to turn. For a solid axle rotating at a constant speed, the conical shape of the wheels is the solution. For independent axles, the conical shape is not necessary due to their ability to spin at different speeds. It will be fine running with flat profile wheels.

Well, what I found is that very high speed trains like TGV, ICE etc have nearly flat profiles with rigid axles and that is done to mitigate the "dynamic hunting" that is inherent in the kinematics of the conical section wheels. There will still be a conical section though, however slight.

The Talgo technology, shown in the pic earlier, achieves this by looping over the wheels and creating a connection. Also it enables the 'toe in' forces without having to tilt the wheel slightly as done in automobiles. Anyways, this explanation may not answer all your queries and I apologize for my inability to explain myself more clearly.

No.. No. The talgo bogie is the like the rear wheels of your Maruti car. Both spin at different speeds.They are connected to the chassis that is all. In the convnetional bogies the solid rod forces the left and right wheel to spin at same speed!. Talgo bogie need not have a conical profile. But it still does !. Why?

Remember, if the wheel profiles are flat, the train moves left and right, over coming friction, it will be in neutral and not stable equilibrium, it wont come back to the center .The potential energy curve will be a flat line , not a curve with a minimum .

Okay, I will make the "hint" back to normal size. (it is visible even at the small size in my Opera browser at home though, but doesnt show up in Mozilla at work, shows up in IE as very small font). Check that out.

[vina]

Hint
Feynman / Classical Kinematic explantion is right of course and the Tolgo Train designers are Spanish and NOT Pakis! . But how can both be right ?.
Becuase there are TWO questions 1) How does the train "auto guide" ie steer itself through corners setc and the 2) How is the train laterally stable, ie if there is a disturbance in the transverse direction, it comes back to it's stable position, even if the wheels are not rolling like in Von Ryan's express . The answers to both is the shape of the wheels. Yes, if the wheels are independent, the train will not "auto guide" , which in all fairness is what RPF talked about in the video. The answer to the 2nd question is what keeps the train laterally stable, and that is what I answered. The confusion comes in the way the question was framed as " What keeps the train on its tracks"? by Amber G , ie. how does the train know where the center of the track and therefore it can follow curves. That really is two questions with the same aswer. But are the Talgo guys Pakis?.. coz, if Feynman is right, their train while being laterally stable, will not be able to go around curves! No they are Spanish, because they provided active steering for their train!.
ChandraS, your answers were spot on. Thanks,you helped me a great deal in undertanding that there were 2 problems actually. I threw the Talgo train at you hoping that you were not familiar with it and could throw your core understanding in doubt! Did you go to that Railway Engg school that is somewhere in the Eastern part of the country .. In any case, the Railways obviously do a very good job in training their folks. A really nice thing to know/quote]

[Satya_anveshi]

Guys,

This thread reminds me of a program called "Quest" that used to be aired on DD long time ago. There were two professors from Calcutta (Prof. Bagchi and Prof. Bose(?)) who used to take turns and ask questions to 3-4 member teams.

Enjoying it.

[John Snow]

Ahh brings back the days of Swaying couple (when talking about trains)

[ChandraS]

Vina,

Thanks for the explanation about the wheel profiles and the newer Talgo technology. I use Firefox at home and work, hence couldn't see the hint! The part about the automobiles was also informative and very much appreciated. I didn't really think of straight line stability in terms of energy and equilibrium. Now, it all makes sense. I am glad to know you were only jousting with me. Having followed your posts for long and admired your knowledge and erudition, I was aghast to see you talk about RPF and Paki in the same line Thought you had ingested some herbs and zam-zam cola

BTW, I am not associated with the Railways in any manner. I am just a brick and mortar engineer who went on to work with steel, concrete, earthquakes, etc. Railways was part of the transportation class in BE nearly 8 years ago. It has really been nice engaging with you and get back into some of the basics learnt in engg

[negi]

Hm.. the only reason why TGV or ICE might have a more subtle conical profile wheels must be due to the high tolerances during the manufacturing as well as with which the tracks are laid in EU, also the overall diameter and the width of the wheel base used in TGV/ICE might be a governing factor i.e. in case the diameter is smaller and width still comparable to Indian counterparts then a more subtle inclination is self explained. Lastly I do not think the rail network in EU requires trains to negotiate turns as tight as we have in India.

[Amber G.]

Here's a classic physics question:

In a washing machine or dryer, when a drum spins at high speed, the clothes are flung towards the wall of the drum due to centrifugal force. The same effect can be observed in a speed governor. There are several such examples in the real world.

Now observe what happens when you stir a cup of tea. You would think that the tea leaves would be flung outwards towards the walls of the cup, but in real life the tea leaves gather towards the center of the cup. Why does this happen?

Related problem: If one is holding a helium balloon in a car and while car turns to the right, which way will the balloon will go? (Of for that matter, if the car suddenly brakes, will the balloon go forward or backward?)

[SaiK]

in a washer dryer, the cloths are free to move (air medium)and get throw to the sides cause of high rotational spin speed. in a tea cup, the leaves are not free to move around, but has a fluid medium, hence have to follow along with fluid flow and gets into the bottom like a whirl pool. may be the centrifugal force is not forceful enough to the counter force blocking force generated by the curvature of the tea cup., causes a whirlpool.

i dunno!

[Amber G.]

in a washer dryer, the cloths are free to move (air medium)and get throw to the sides cause of high rotational spin speed.

..

Just to add, if clothes were lighter than air (say helium balloons) they would be "thrown" towards the axis of rotation... (That's the basic principle for gas centrifuges used for enriching uranium -- The gas - containing U235 is lighter (than U238) and thus moves towards and is near the center of the axis...

Basically one would not go wrong (actually that's the whole premise of Einstein's General theory of relativity - ) if one just assumes the "centrifugal force" will behave just like gravity...Iron sinks, wood floats in a lake ... in a centrifuge filled with water.. iron will go towards the wall and wood towards the center.

[Amber G.]

Vina -Thanks --- I still have not digested all what you have posted, but few comments on earlier post ....
FWIW (My opinion only etc... Sorry if it has already been discussed etc...)

so in this case of a sliding train under emergency braking, if there was a disturbance in the transverse direction, by RPF's explanation the train would not stay on the tracks , while we know it does!.

Actually while train is braking, there is more chance of derailing (precisely because tapered wheels will not help) and the flanges come into play for second line of safety..
In any case, it is easy to check how the train wheels actually look (and run tests with model train (without flanges and no tapered wheels to check the validity of explanation given) … One additional comment ..

I think us engineers (me and ChandraS) have it right (we work in the real world afeter all) …

Yes, RPF was good in theoretical Physics (having won Noble prize etc) but don’t put him down as an Engineer...He was very good in fixing electronics and mechanical things, expert locksmith and safe-cracker, for war efforts he worked for US arms force as (highlrated) mechanical engineer designing gear assembly for targeting system, before he was recruited for atomic bomb…. and he was VERY well known for fixing things and solving Engineering problems. Don’t forget places where he taught or learned - Caltech , MIT (and Cornell for that matter) are institutes known as first-class Engineering schools – and as an investigator for challenger disaster he learned pretty quickly and in the end knew engineering behind Shuttle motors/engines more thoroughly than many/most professional engineers working in the field.

(The train wheel problem is quite old - RPF first heard, like many others, in his MIT days)

[ArmenT]

Regarding the tea leaves, it is funny that Einstein was mentioned, because he was actually the first one to explain what was going on. It is called the:
Tea Leaf Paradox

The above article has an explanation, complete with diagrams. As some of you have surmised, near the bottom and edges of the cup, the liquid is slowed down due to friction against the walls of the cup (which isn't moving). In this situation, the centrifugal force is less important and the pressure difference is more significant.

Einstein first explained it in a paper in 1926 where he showed why river banks in the northern hemisphere have more erosion on the right bank and rivers in the southern hemisphere have more erosion on the left bank. If I recall correctly, this might have been the same paper where he noted that in all major meandering rivers, the ratio of the river's actual length to the straight line length ("as the crow flies" distance between river source and mouth) is approx. 3.14

This phenomenon is also used to seperate red blood cells from plasma.

[John Snow]

Watch simpsons in australia, Bart wondering why the toilet water spins opposite direction in North America.

(serously doesnt corriolis component add to that?)

[vina]

Ahh brings back the days of Swaying couple (when talking about trains)

"Swaying Couple" as young couples in coupe's in IR ?.. Naughty Naughty..

John Snow Gaaru. This is "Fyzzics" thread and a phamily phorum.

[vina]

Actually while train is braking, there is more chance of derailing (precisely because tapered wheels will not help) and the flanges come into play for second line of safety..
In any case, it is easy to check how the train wheels actually look (and run tests with model train (without flanges and no tapered wheels to check the validity of explanation given) … One additional comment ..

I think us engineers (me and ChandraS) have it right (we work in the real world afeter all) …

Agree . Nothing like a good experiment to get ''facts" in place. However, I would put my money on the flanges not coming into play. Why , coz if it did, the Talgo train would wear the tracks and flanges out in a couple of hours of operation.

.....He was very good in fixing electronics and mechanical things, expert locksmith and safe-cracker, for war efforts he worked for US arms force as (highlrated) mechanical engineer designing gear assembly for targeting system, before he was recruited for atomic bomb…. and he was VERY well known for fixing things and solving Engineering problems. Don’t forget places where he taught or learned - Caltech , MIT (and Cornell for that matter) are institutes known as first-class Engineering schools – and as an investigator for challenger disaster he learned pretty quickly and in the end knew engineering behind Shuttle motors/engines more thoroughly than many/most professional engineers working in the field.

Yes. I am well aware of that fact. You see, if you went to IITM anytime in the past 2 decades, you were a Feynman fan by near "default" . One of the most respected faculty in the Physics dept, Dr V. Balakrishnan was Feynman's student and I think a close friend as well. From the IIT grape vine I heard from long long ago, that he had gone to attend RPF's funeral for a final send off. In fact the Indian edition's Feynman's Lectures in Physics foreword is /was by Dr Balki. I still have those volumes after all these years and it going around the world with me!. I did read "Surely You are Joking Mr Feynman" as an undergrad and I remember vividly about his work with gears in building mechanical stuff and he even talks about tables and how to select gears . That is why I was flummoxed on how he put out that point contact explanation.

Truth be said, Feynman was trying to explain only one thing here and he was very expressive in using his hands in the video in explaining that as well. That question is "How does the train auto steer?" ..If suppose the interviewer had followed up with a hypothetical question.. as "Mr Feynman, but how does the Talgo train know how to go in a curve ?. So, will it fall off the tracks"?.. I think Feynman would have answered. "Falling off the tracks is a different question! .. I just talked about how it goes around curves. Now the Talgo train cannot go around the curves by itself. Just like a car, you need to steer!. So it will be too much for a driver to do it, the Talgo train guys have come up with an automatic system that allows the train to safely steer the wheels and go around a curve!"

[Amber G.]

(serously doesnt corriolis component add to that?)

No, coriolis is negligible and contrary to some popular belief water will flow in the same direction in an ordinary experiment. For all practical purpose - effects due to residual (due to initial spin introduced by non-symmetry when water flows in) spin are *much* more than coriolis due to earth's spin.

[Amber G.]

You see, if you went to IITM anytime in the past 2 decades, you were a Feynman fan by near "default"

Proverbially true for other places too.. even when (many more decades ago ) when I was in IITK. (I chose not to go to Caltech for graduate work right after IIT, but have met him and heard him many times.)

[Bade]

vina, the famous prof who wrote the preface to the Indian edition of the lecture volume was not RPF's student. It is very likely just a BTech folklore. Will leave things at that before the rumuor spreads further. I have also noticed a trend in India to associate with famous people who may have lectured one on a few occasions in the context of summer and winter schools. Being a student of someone has an entirely different meaning as having worked closely with someone for a PhD or during postdoctoral years.

Though this thread is interesting and has it merits, I know of very productive physicists who are never challenged or bothered by physics problems being discussed here. Just an observation. All the jihadi UGs who swear by these types of problems rarely get their hands on to the harder and more challenging problems in physics and usually fizzle out in life very early on itself. What a waste of talent. All that fancied approach of the swamys and balkis of the world have not helped nurture that talent pool. To me having seen it all first hand, it is all but a passing fad on young impressionable minds. Good time pass.

[vina]

All the jihadi UGs who swear by these types of problems rarely get their hands on to the harder and more challenging problems in physics and usually fizzle out in life very early on itself.

Ah Bade Saar . I had already posted that studying some random "convexities" is a total waste of time. If you want to study convexity, you are better off studying the convexity of Pamela Anderson's very real "assets" .

If Swami Vivekananda remarked, "You are closer to God when you play football than while praying" , I submit that you are closer to unlocking the "real mysteries" of the universe while gazing at said Maam's "convexities" on Bay Watch, than studying those in a lab or doing some theoretical work on a black board!..

What a waste of talent. All that fancied approach of the swamys and balkis of the world have not helped nurture that talent pool. To me having seen it all first hand, it is all but a passing fad on young impressionable minds. Good time pass.

Ah, But Physics itself, whether you address "hard" or "easy" problems a massive time pass? . Nearly every Fyzzicist worth his salt, continues doing it because of the "kicks" he/she gets out of it. If it was a "rational" decision, you would head straight to wall street and ponder over the "negative convexity" of Mortgage Backed Securities!. So point is, why deprive the lil ones of their harmless kicks ?.The alternative to that would be those "shops" where you get the strong stuff from "Idukki" in Vales.

[ramana]

A tribute to Amal Kumar RayChaudhri

[anishns]

Hint: No...it is NOT the flanges. (flanges are for extra safety not the primary reason)

Flanges cannot be only for extra safety.....I mean would the train be able to switch tracks without the flanges?

[SaiK]

What spoiled the CERN party

[vsunder]

An excellent article on cloaking and invisibility.

Cloaking

By the way ref [20] in the above is an old article of mine which is completely linked to cloaking though I did not call it that 20 years ago. Here is the article:

Identifiability of Conductivity

Essentially the idea in cloaking is totally elementary. You want to cloak something. So surround it by a material
so that the voltage of current map on the boundary is the same as that of a homogenous material on the inside.
Its then a simple change of variable formula and the nature of the change of variables allows one to construct
the metamaterials that Pendry and company do.

[Sanjay M]

Here's a classic physics question:

In a washing machine or dryer, when a drum spins at high speed, the clothes are flung towards the wall of the drum due to centrifugal force. The same effect can be observed in a speed governor. There are several such examples in the real world.

Now observe what happens when you stir a cup of tea. You would think that the tea leaves would be flung outwards towards the walls of the cup, but in real life the tea leaves gather towards the center of the cup. Why does this happen?

In the case of the teacup, the dominant effect is the fluid flow. The tea leaves travel with the fluid, and in this case, the fluid is traveling towards the centre. The reason the fluid is traveling towards the centre is because the stirring has created a vortex. A vortex drives fluid towards the centre because the pressure there is lower, according to Bernoulli's Principle:

http://en.wikipedia.org/wiki/Vortex

[SaiK]

Kepler mission - nice video.
http://www.nasa.gov/multimedia/nasatv/o ... bcast1.asx

question: dr. koch says kepler would look for transits and blocking the light. what if kepler is not aligned to the plane of the target orbit? i.e, perpendicular to it?

??

[Amber G.]

question: dr. koch says kepler would look for transits and blocking the light. what if kepler is not aligned to the plane of the target orbit? i.e, perpendicular to it?

It is looking at 100,000's of stars, hopefully for a few of them, the orientation, and actually more important, timing would be right.

BTW the transits (specially Venus transits around SUN) are/have been VERY important events to watch.. Venus transits were (before radars etc were invented) the ones, which gave accurate measurements of the distance of sun from earth. Next Venus transit is in June 2012 so be ready for it
(They are very rare - they come in pair but after around 240 years - 1882 transit was watched by many --- 2004/2012 transits lost some of the charm because one can use radar to measure the distance much more accurately --- for distance stars, the measurement of time for transit can give accurate data on the orbit of planet which is not easily available otherwise)