Register to reply 
Length contraction problem 
Share this thread: 
#1
Nov1603, 11:33 PM

P: 58

suppose you had a planet with a train going all the way around the circumference. The train is a complete loop so you just have an endless train. now speed the train up to near c.
Because of length contration would the train begin to squeeze the planet? 


#2
Nov1603, 11:38 PM

P: 837

http://math.ucr.edu/home/baez/physic...igid_disk.html 


#3
Nov1903, 12:20 AM

P: 58

If an object were to go exactly c would it lose its 3rd dimention?
what i mean is, would it have any length whatsoever? 


#4
Nov1903, 09:24 AM

P: 837

Length contraction problem
A massive body cannot travel at c, but its length can contract to arbitrariliy small values as it comes arbitrarily close to c.



#5
Nov2003, 04:23 PM

P: 124




#6
Nov2003, 05:28 PM

Sci Advisor
PF Gold
P: 2,226

It's another way of saying as v > c, l > 0. The word 'arbitarily' is used as whatever theoretically possible value of l we choose there will always be another value of theoretically possible value of l' closer to 0 and whatever theoretically possible value for v we choose there will always be another theoretically possible value of v closer to c (but l can never equal 0 and v can never equal c).



#7
Nov2003, 05:45 PM

P: 124




#8
Nov2103, 08:24 AM

P: 406

This is a very valid point. Due to the small curvature of the earth, each section of the train is, at an instant, contracted in its direcetion of motion.
This means that the total length of the train must be foreshortened. We need not think of the train as travelling at a speed very close to c. Say it travels at 1080km/h = 0.30km/s = 300m/s Let it 'rest' length = L' Its contracted length is now L=L'*sqrt(1v^2/c^2) = L'*sqrt(1  (300/3*10^8)^2) = L'*sqrt(1 (10^6)^2) = L'*sqrt(1  10^12) L = L'(0.999999999999499999999999875) A negligable amount? The circumference of the earth is approximatly 40,000km or 40,000,000m Thus the length of the train would be L' = 40,000,000m Thus L = 40,000,000 * 0.999999999999499999999999875 L = 39,999,999.999979999999999995m This is still less than a millimeter of a difference. And the train would have to travel at 1080km/h to obtain even this contraction. But this speed is approximatly the speed of rotation of the earth about the equator. Well the earths rotational speed is actually 1600km/h. So the earths circumference is contracted by less than a millimeter. It would be interesting to see what happens for other planets. But seriously, relativity says nothing about the streaths of materials. So the train might squeeze the earth 


#9
Nov2103, 10:20 AM

Sci Advisor
PF Gold
P: 2,226

Obsessive, in Newtonian mechanics it's perfectly possible to have a perfectly rigid body that doesn't expand or contract, this is impossible in relativity as a wave through this material would be instanteous.



#10
Nov2303, 09:29 PM

P: 58

a perfectly riged train and perfectly rigid planet would pose problems for relativity. the train would contract with infinite strength, while the planet would repel the squeazing with infinite strength.



#11
Nov2403, 02:15 PM

P: n/a

While you're on technicalities.
Since the train is a 3 dimensional object only the top of the train would be able to travel at the speed of light. So the train would have to squeeze itself before it could start to squeeze the planet. 


#12
Nov2503, 04:11 PM

P: 58

If a rocket were traveling at c5mph, and a passenger on board were to run to the front at 10mph, would he be traveling at c+5mph? 


#13
Nov2603, 11:34 AM

P: n/a

Even though this is purly hypothetical, it is impossible. As soon as the man running hit 5mph he would be traveling at c. Once c is obtained time stops, then the man would no longer be able to run. Here's the next one for you.
Because the man has to move independantly to obtain c, when he reachs c he cannot move anymore. So would he slow down, or always be stuck going c until some outside force slowed him down? 


#14
Nov2603, 03:48 PM

P: 837

http://math.ucr.edu/home/baez/physic.../velocity.html 


#15
Nov2603, 07:06 PM

Astronomy
Sci Advisor
PF Gold
P: 23,108

this would tend to stretch each car of the train and create strain in the couplings connecting one car to the other if the train did not break apart by this tension, then it would be stretching out like taffyand then the contrary effect of SR lengthcontraction would be factored in. So, depending on the young's modulus or strength of the traincars, the two effects could even cancel???? At 1/2 the speed of light the centripetal acceleration (1/4) 9E16/6.4E6 3.5 E9 meters per second per second. So if each traincar masses E4 kilogram, each car must be experiencing a downwards force of 35 trillion newtons. This translates into tension throughout the trainwhich therefore needs supernatural tensile strength not to snap and even so it would stretch. This is before we introduce the SR length contraction. The SR length contraction is only down to 0.87 of original length at that speed. I am thinking the train would be lucky not to have already stretched out more than enough to compensate for that! 


#16
Nov2703, 10:33 PM

P: 58

I understand that the train would have in real life ripped appart, but then again in real life the train wouldnt be going at relative speed. also the length contraction would not neutralize the centrifical force at all, the tension on the train would still remain the same even if the train did contract a considerable amount, and so the train would ripp appart either way.
But now I have another question to do with light, and the effect of length contraction. When an objects undergoes length contraction, its mass is not effected and so it has the same number of atoms. So all of the atoms that it had prior to contraction exist within less space. If light comes from exited electrons jumping from one orbit to another and back, then because of the length contraction the length of the electrons orbit jump is also contracted in the direction of the movement. The frequency of light is related directly to how far the electron jumps, so any light shone in the direction that the rocket is traveling will have an altered frequency. Is that true, or is there something that I overlooked? Also if this were true, would it be a red, or blue shift? 


#17
Dec603, 05:52 PM

P: 21

Well, of course, if the train around the equator was earthbound and travelling westwards it would experience a special relativity length expansion., though presumably not enough to take it off the rails. ;)
Dennis Revell 


Register to reply 
Related Discussions  
Length Contraction problem  Introductory Physics Homework  4  
Length contraction and wave length transformation  Special & General Relativity  0  
Length contraction  Special & General Relativity  6  
Err, did I do this length contraction problem right?  Introductory Physics Homework  6  
Probably a very easy length contraction problem..  Introductory Physics Homework  6 