Wave motion

First of all, how can P be displaced upwards but be moving downwards ?

This doesnt make sense.

Could someone explain this preferably with a diagram ? Please and thank you !

Before the wave moves along the rope, P is at its regular position. When the waves start to move along the rope, P will oscillate up and down.

First it moves upwards. It is displaced upwards, and is moving upwards.

Next, it reaches its highest point. It has been displaced upwards, but is no longer moving upwards or downwards; it is stationary.

Then, it starts to move back down again. It has still been displaced upwards, because it is above its regular position, but it is moving downwards.

And so on. Sorry for the lack of a diagram, but I hope this has clarified enough so that you can have a go at the question.

Reply 2

Ari Ben Canaan

OP

16

Original post by Pangol

Before the wave moves along the rope, P is at its regular position. When the waves start to move along the rope, P will oscillate up and down.

First it moves upwards. It is displaced upwards, and is moving upwards.

Next, it reaches its highest point. It has been displaced upwards, but is no longer moving upwards or downwards; it is stationary.

Then, it starts to move back down again. It has still been displaced upwards, because it is above its regular position, but it is moving downwards.

And so on. Sorry for the lack of a diagram, but I hope this has clarified enough so that you can have a go at the question.

So what would be your answer to the question ?

Reply 3

Pangol

15

Original post by Ari Ben Canaan

So what would be your answer to the question ?

What would be your answer to the question?

I've tried to clarify the bit that you say doesn't make sense. It's best that you see if you understand this by having a go at the question yourself.

Reply 4

Ari Ben Canaan

OP

16

Original post by Pangol

What would be your answer to the question?

I've tried to clarify the bit that you say doesn't make sense. It's best that you see if you understand this by having a go at the question yourself.

Fairplay.

Option C.

I drew a diagram.

What do you get ?

Reply 5

Peel

10

You're getting velocity and displacement mixed up by the sounds of it.

Don't have a diagram, but at any point in a wave crest, the displacement relative to the mean elevation will be positive. Just before the wave crest, the velocity (or movement) will be positive towards the wave crest, at the wave crest the velocity will be zero, and after the wave crest, the velocity will be negative causing the wave to move downwards towards a trough, but it will be above the mean elevation so the displacement at this point is still positive.

It might help to just pick a point on a wave and imagine how it moves vertically.

Reply 6

Ari Ben Canaan

OP

16

So I have marked P and Q on this wave I got of the web.

Q is displaced downwards and has velocity in the downward direction.

Is this correct ?

Your diagram is a displacement / distance diagram, and you have P and Q in the correct places with referrence to each other. If P is moving as described in the question, your wave will be moving to the left; the question tells you that the waves are travelling from P to Q, i.e. to the right. So, your points should be in a slightly different place, and your conclusion will therefore change, but you have all of the right ideas.

Reply 8

Ari Ben Canaan

OP

16

Original post by Pangol

Your diagram is a displacement / distance diagram, and you have P and Q in the correct places with referrence to each other. If P is moving as described in the question, your wave will be moving to the left; the question tells you that the waves are travelling from P to Q, i.e. to the right. So, your points should be in a slightly different place, and your conclusion will therefore change, but you have all of the right ideas.

So if a sketch a graph showing the sinosuidal curve displaced slightly to the right I should have P moving down but still having a positive displacement ?

Reply 9

Pangol

15

Original post by Ari Ben Canaan

So if a sketch a graph showing the sinosuidal curve displaced slightly to the right I should have P moving down but still having a positive displacement ?

That's what you're aiming for. If you had your point P just before the crest that it is just after, that would be the right place.

Reply 10

Ari Ben Canaan

OP

16

Okay, now I think I've got it. Q has displacement upwards and Velcoity upwards; this is evident if I sketch a graph showing the curve a few seconds later.