Waves high school question -- Book answers are totally rubbish

[Herman Trivilino]

I found this in a paper written by Ronald Edge and published in the May 2001 edition of The Physics Teacher journal.

Theory shows that in the deep sea, the wave cannot have a sharper crest angle than 120°, which is too small a slope for conventional surfing¹² [see Fig. 2(a)]. As a wave approaches the shore, the depth at the crest is larger than at the trough, where shallow-water wave theory shows the speed is lower, so the crest overtakes the trough, the peak sharpens, and the wave ultimately breaks.

And then he discusses "extending the ride" by surfing in a direction that's not perpendicular to the wave front ...

The surfer must then start to ride the wave at the point where the water is starting to rise preparatory to breaking, just ahead of the vertical portion and under the breaking curl.

The article goes into a little detail about the difference between deep water and shallow water waves, but the transition between them is treated only qualitatively. So, I think that it's possible to surf a wave before it breaks, and is in at least some cases desirable to do so, but it's when the wave is transitioning from deep water to shallow water, a time when it's preparing to break but hasn't done so yet.

 

[Barclay]

STATEMENT 2 An observer is standing on the bank of the river. The wind is blowing from left to right and waves moving from left to right. However, he sees a small piece of wood that is moving from right to left as it floats in the middle of the river.

Hello, I need to drudge this thread up again. I'm not understanding wave motion. The statement in the book says that the waves move left to right. BUT I've read in this thread that waves move in a circular motion and oscillate up and down. I've read also that waves to not move left to right but energy transfers left to right.

Has the book got it wrong in the statement above when it says "waves moving left to right"?

 

[haruspex]

Hello, I need to drudge this thread up again. I'm not understanding wave motion. The statement in the book says that the waves move left to right. BUT I've read in this thread that waves move in a circular motion and oscillate up and down. I've read also that waves to not move left to right but energy transfers left to right.

Has the book got it wrong in the statement above when it says "waves moving left to right"?

No, what you were told in this thread is that if you focus on a parcel of water that participates in the wave then that parcel moves in a circular fashion. The wave itself is, in a sense, an optical illusion. Think about a spot moving on a TV screen. In fact, the pixels do not move,they just turn on and off, but the illusion is that a spot moves along.

 

[Ray Vickson]

Hello, I need to drudge this thread up again. I'm not understanding wave motion. The statement in the book says that the waves move left to right. BUT I've read in this thread that waves move in a circular motion and oscillate up and down. I've read also that waves to not move left to right but energy transfers left to right.

Has the book got it wrong in the statement above when it says "waves moving left to right"?

No, you have it wrong when you mis-read what people wrote. Waves move---there is not doubt about that. However, the waves travel through water, and the water itself does not move horizontally with the wave: the water itself just moves up and down, and a little bit back and forth.

Think of another example: sound waves in air. The wave travels faster than a commericial jet plane, but you are not blasted out of your shoes by faster-than-hurricane winds when a sound wave passes. The air itself just barely moves, even though the wave moves through it (i.e, through the air).

 

[Barclay]

Once again I have a fragile understanding and I'm going to write down what I'm seeing before I lose it:

Energy moves from say left to right and causes the 1st particle of water that receives the energy to oscillate up and down ... immediately the next particle along receives the remaining energy and too oscillates but because every particle along the chain receives energy a fraction of a second later they are not in synchrony and their position in space looks like a wave when they are all put together. Eventually there is no more energy at the very end of the chain (at the extreme right and the wave breaks)

 

[haruspex]

Once again I have a fragile understanding and I'm going to write down what I'm seeing before I lose it:

Energy moves from say left to right and causes the 1st particle of water that receives the energy to oscillate up and down ... immediately the next particle along receives the remaining energy and too oscillates but because every particle along the chain receives energy a fraction of a second later they are not in synchrony and their position in space looks like a wave when they are all put together. Eventually there is no more energy at the very end of the chain (at the extreme right and the wave breaks)

Getting there, but a wave does not break because it runs out of energy. If anything, it's because it has too much energy.
Waves on the shore break because of the reducing depth. This concentrates the energy into a smaller height, making the wave front too steep.
But you won't really understand waves in water until you take on board the circular motion of the water elements. The best I can suggest there is to keep watching and studying the videos I linked to way back in this thread.

 

[Herman Trivilino]

Energy moves from say left to right and causes the 1st particle of water that receives the energy to oscillate up and down ...

As long as you're not thinking of energy as a substance. The medium is a substance. Energy is one of its properties. We use the word "elastic" to describe the medium, because it has an equilibrium configuration, and when it's disturbed there are so-called restoring forces that tend to return it to that equilibrium configuration. But because the medium also has mass, it continues to move once it reaches that equilibrium configuration, overshooting, where the restoring force slows it down, causes it to move back towards equilibrium, and so on. These oscillations are a description of how the medium moves. The wave, on the other hand, is simply a propagation of this disturbance from equilibrium.

The motion of the medium can, alternatively, be described in terms of energy rather than force. The medium stores potential energy by virtue of its position, and possesses kinetic energy by virtue of its motion. There is an ongoing exchange of potential and kinetic energy.

 

[Barclay]

Getting there.. ...

So the energy moves left to right and this is captured by the water molecules which oscillate in sequence and the wave does indeed travel left to right but this is really an optical illusion because it is formed from vertically oscillating particles

 

[haruspex]

So the energy moves left to right and this is captured by the water molecules which oscillate in sequence and the wave does indeed travel left to right but this is really an optical illusion because it is formed from vertically oscillating particles

I know Mister T is saying vertically oscillating, but this is an interesting difference between waves on a string and surface waves in a liquid.
Sound waves in air are purely longitudinal, the molecules oscillate back and forth in the direction of travel of the wave.
A vibrating string oscillates purely(?) transversely.
Surface waves in a liquid are somewhere between the two. The molecules undergo a circular motion. Consider a cork floating on the water as a wave passes through. Start with the cork in a trough. At this point the cork is in its lowest position and moving in the opposite direction to the wave. Next, it rises on the leading edge of the next wave. Half way up, it is moving vertically. At the crest, it is moving with the wave, but at a lower speed, so the wave passes under it. Half way down, it is moving vertically again.
Viewed from the side, with the waves going left to right, the cork executes roughly a circle, clockwise.

There is a good reason why it is different from waves in a string.
All waves result from an ebb and flow of energy between two forms. In light, it is the magnetic and electric fields. In a string, it is kinetic energy of the sideways movement and the elastic energy of tension. Surface tension in water is nowhere enough to do that. Instead, we have the gravitational potential energy and the kinetic energy. But if it were purely vertical motion there would be no mechanism to transfer the energy along the surface, so it must involve horizontal kinetic energy.

 

[Tom.G]

Barclay.
See if this thought experiment (or real experiment) helps with waves. As others have noted it's not exactly as in a water wave but I've found it to be an easy starting point.

1) Consider a rope, maybe fifteen feet long. Tie one end of it to a doorknob or other fixed object. Take ahold of the free end and walk away until the rope is freely suspended in the air but not pulled tight. Now shake your end of the rope up and down. You will see a wave travel from your hand to the tied-off end of the rope. If the rope was traveling along with the wave it would be yanked out of your hand. The up and down DISTURBANCE of the rope is what we call a wave. This disturbance travels along the rope, but the rope itself doesn't rush to the doorknob. This is roughly what the wind does to water to make waves.

2) Now consider doing the above experiment in a train car. The rope is tied at one end of the car and you are standing in the aisle holding the other end of the rope. Repeat Step 1), above. You will get the same result. Even if the train is moving, you will get the same result. The moving train is the water current in the river..

 

[Barclay]

Surface waves in a liquid are somewhere between the two. The molecules undergo a circular motion. Consider a cork floating on the water as a wave passes through. Start with the cork in a trough. At this point the cork is in its lowest position and moving in the opposite direction to the wave. Next, it rises on the leading edge of the next wave. Half way up, it is moving vertically. At the crest, it is moving with the wave, but at a lower speed, so the wave passes under it. Half way down, it is moving vertically again.
Viewed from the side, with the waves going left to right, the cork executes roughly a circle, clockwise.

I've got an awareness of the circular motion in water waves ... and I'm glad of it ... though it is beyond my requirements at the moment. All the information that you have provided in tjis thread .has been very useful. Thank you

 

[Barclay]

Barclay.
1) Consider a rope, maybe fifteen feet long. Tie one end of it to a doorknob or other fixed object. Take ahold of the free end and walk away until the rope is freely suspended in the air but not pulled tight. Now shake your end of the rope up and down. You will see a wave travel from your hand to the tied-off end of the rope. If the rope was traveling along with the wave it would be yanked out of your hand. The up and down DISTURBANCE of the rope is what we call a wave. This disturbance travels along the rope, but the rope itself doesn't rush to the doorknob. This is roughly what the wind does to water to make waves.

2) Now consider doing the above experiment in a train car. The rope is tied at one end of the car and you are standing in the aisle holding the other end of the rope. Repeat Step 1), above. You will get the same result. Even if the train is moving, you will get the same result. The moving train is the water current in the river..

Thanks. Useful images to help me see what is going on