A question in the Feynman Lectures on physics,chapter 15-4

In summary, Feynman tried to explain why the rod perpendicular to the motion does not shrink. He said that by symmetry, the two marks must come at the same y- and y'- coordinates, and that we can not tell which system is moving by doing experiments except looking outside.
  • #1
Haixu
6
0
In 15-4,Feynman tried to explain why the rod perpendicular to the motion does not shrink."How do we know that perpendicular lengths do not change? The men can agree to make marks on each other's y-meter stick as they pass each other.By symmetry, the two marks must come at the same y- and y'- coordinates,since otherwise when they come together to compare results,one mark will be above or below the other,and we could tell who was really moving."
The last sentence means that we can not tell which system is moving by doing experiments except looking outside.But is comparing the y'- with y- a kind of "looking outside"?? As y- is lying outside actually. So why?
 
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  • #2
Haixu said:
In 15-4,Feynman tried to explain why the rod perpendicular to the motion does not shrink."How do we know that perpendicular lengths do not change? The men can agree to make marks on each other's y-meter stick as they pass each other.By symmetry, the two marks must come at the same y- and y'- coordinates,since otherwise when they come together to compare results,one mark will be above or below the other,and we could tell who was really moving."

The last sentence means that we can not tell which system is moving by doing experiments except looking outside.But is comparing the y'- with y- a kind of "looking outside"?? As y- is lying outside actually. So why?
It took me awhile to find your quote because it is on page 15-6, not 15-4.

The point of "looking outside" is to determine who is really moving and who is really stationary by seeing a difference. But for this comparison, even when they do look outside, since it is a symmetrical situation, they still cannot determine which is the one that is moving and which is the one that is stationary.

An example of determining which one is really moving by not looking outside would be if they each had some kind of identical apparatus that worked one way for one of them and worked a different way for the other one. Then you could use this apparatus as an absolute speedometer that could tell you how fast you were going without looking outside. Of course you would look outside to compare all the space ships with these speedometers to show that it was not symmetrical and you would have to show that the readings were consistent with the relative speeds of the different space ships. But if everyone's apparatus registers the same result even when you know there is a relative speed, then it's not working as a speedometer.
 
  • #3
ghwellsjr said:
It took me awhile to find your quote because it is on page 15-6, not 15-4.

The point of "looking outside" is to determine who is really moving and who is really stationary by seeing a difference. But for this comparison, even when they do look outside, since it is a symmetrical situation, they still cannot determine which is the one that is moving and which is the one that is stationary.

An example of determining which one is really moving by not looking outside would be if they each had some kind of identical apparatus that worked one way for one of them and worked a different way for the other one. Then you could use this apparatus as an absolute speedometer that could tell you how fast you were going without looking outside. Of course you would look outside to compare all the space ships with these speedometers to show that it was not symmetrical and you would have to show that the readings were consistent with the relative speeds of the different space ships. But if everyone's apparatus registers the same result even when you know there is a relative speed, then it's not working as a speedometer.


Thanks for your help, you explained it very clear!
 

1. What is the main topic of chapter 15-4 in the Feynman Lectures on physics?

The main topic of chapter 15-4 is magnetism. Specifically, it discusses the principles and properties of magnetism and how it relates to electricity.

2. What is the Feynman symbol used in this chapter and what does it represent?

The Feynman symbol used in this chapter is the symbol for a magnetic field, which is represented by the letter B. This symbol is used to represent the strength and direction of a magnetic field at a specific point in space.

3. How does the concept of magnetic flux relate to the behavior of a magnet?

Magnetic flux is a measure of the number of magnetic field lines passing through a given area. The behavior of a magnet is directly related to its magnetic flux, as the strength and direction of the magnetic field lines determine how a magnet will interact with other magnets or charged particles.

4. What is the difference between a permanent magnet and an electromagnet?

A permanent magnet is a material that naturally produces its own magnetic field, while an electromagnet is a coil of wire that produces a magnetic field when an electric current is passed through it. The strength and direction of an electromagnet's magnetic field can be controlled by adjusting the amount of current flowing through the wire.

5. How is magnetism related to the phenomenon of induction?

Magnetism is closely related to the phenomenon of induction, which is the production of an electric current in a conductor when it is exposed to a changing magnetic field. This is because a changing magnetic field creates a changing electric field, which can induce a current in a nearby conductor.

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