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# Progressive waves explanation needed please

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1. Progressive waves explanation needed please
http://store.aqa.org.uk/qual/gce/pdf...W-QP-JUN09.PDF
question 5b
Could someone explain to me why the displacement at Y is up, during its first 1/4 of the period? Why is it opposite to what you would logically think, i.e along the movement of the string hence downwards, is it due to progressive waves being transverse? So the vibrations are perpendicular to the direction of motion? Hence the displacement being up instead of down?
2. Re: Progressive waves explanation needed please
The particle does not move along the string, it stays in the same vertical plane. Rather
That said imagine that the wave was moved forwards 1/4 of the period. Now the particle Y would be at the top of the wave.

Imaging the particle y being a floating ball in the sea. As the wave move forwards under it, then it will be swept upwards.
3. Re: Progressive waves explanation needed please
(Original post by Nick_)
The particle does not move along the string, it stays in the same vertical plane. Rather
That said imagine that the wave was moved forwards 1/4 of the period. Now the particle Y would be at the top of the wave.

Imaging the particle y being a floating ball in the sea. As the wave move forwards under it, then it will be swept upwards.
Thanks a lot The analogy is brilliant! Thats why (someone earlier trying to explain this to me ) was saying if the wave moves from left to right you must look at the part of the wave to the Left of the displacement point (Y in this case). If it was right to left we would look at the right side of the displacement point?
4. Re: Progressive waves explanation needed please
(Original post by coolstorybrother)
Thanks a lot The analogy is brilliant! Thats why (someone earlier trying to explain this to me ) was saying if the wave moves from left to right you must look at the part of the wave to the Left of the displacement point (Y in this case). If it was right to left we would look at the right side of the displacement point?
yep

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