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enter image description hereI've a spring and I give it a single impulse towards left. A compression zone is produced as is shown in the picture, then this compression moves ahead to the left of the spring and we see a compression zone in the middle of the spring .

My question is that why doesn't this compression in the middle of the string move both to the left and right as it relaxes (as I've drawn on the picture)? I have seen that the correct thing is that the compression pulse will move towards the left only, however we can argue that the compression in the middle of the spring will push on both sides of the spring and hence produce pulses going both towards left and right.

Could anyone please help me in understanding as to why the compression pulse will go only to the left and not to the right.

Also are there going to be any rarefactions produced in the spring if I just push it and leave it there?

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  • $\begingroup$ Hint: read up on group velocity of waves. $\endgroup$
    – John Alexiou
    Commented Dec 31, 2020 at 6:59
  • $\begingroup$ Thank you for your recommendation. I did study it but I'm afraid I couldn't find it relevant. Could you explain it a little? $\endgroup$
    – Kashmiri
    Commented Jan 3, 2021 at 16:47

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The third example in your image is a snapshot of the pulse. Unfortunately the snapshot can only show us the deformation (or potential energy) in the spring. It cannot show us the velocity of the elements in the spring.

If the compression is created, held statically in place, and then released, the waves will move in both directions as you show in the final example.

But when created from a compression from the right, the elements of the spring will also have a velocity. This velocity when combined with the potential energy of the compression serve to cancel out the pulse to the right and only allow the pulse to the left to propagate.

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  • $\begingroup$ So , in the third example the compression pulse has some velocity to the left and the force that this compression applies onto it neutralises the wave trying to go out to the right? $\endgroup$
    – Kashmiri
    Commented Dec 31, 2020 at 9:47
  • $\begingroup$ Since i posted this question I've seen many videos, which show a rarefaction as well as compression, will any rarefaction be produced in this case? $\endgroup$
    – Kashmiri
    Commented Dec 31, 2020 at 9:48
  • $\begingroup$ @BowlOfRed Very interesting--Are there any web links (or text books) for this 'velocity explanation' ? $\endgroup$
    – user45664
    Commented Dec 31, 2020 at 16:56
  • $\begingroup$ @user45664, i believe you like wave physics, I'd be glad if you could help me here physics.stackexchange.com/questions/603812/…? noredirect=1#comment1359353_603854 and here physics.stackexchange.com/questions/603864/… $\endgroup$
    – Kashmiri
    Commented Dec 31, 2020 at 17:32
  • $\begingroup$ See researchgate.net/publication/340085346 $\endgroup$
    – user45664
    Commented Jan 1, 2021 at 16:57

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