Waves high school question -- Book answers are totally rubbish

[Barclay]

Gravity is creating the current, not the wind.

Hello. So what does wind do? Does the wind create the waves?

Wind in this question is pushing the wood along right to left.

 

[Herman Trivilino]

Wind creates the waves. Those waves move in the direction of wind.

Current moves the block of wood in the opposite direction.

 

[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.

Wind creates the waves. Those waves move in the direction of wind.

Current moves the block of wood in the opposite direction.

Okay so there is NOT a local gust of wind moving the wood from right to left in the middle of the river. I think you're saying that a heavy wind is creating waves and moving them all left to right AGAINST the current. The current is created by gravity and trying to push the waves right to left (because the Earth is round and spinning in anticlockwise) ... BUT in a small area in the river there must be a NULL POINT where the heavy wind does not act and the wood follows the current (created by gravity) right to left

 

[haruspex]

Okay so there is NOT a local gust of wind moving the wood from right to left in the middle of the river. I think you're saying that a heavy wind is creating waves and moving them all left to right AGAINST the current. The current is created by gravity and trying to push the waves right to left (because the Earth is round and spinning in anticlockwise) ... BUT in a small area in the river there must be a NULL POINT where the heavy wind does not act and the wood follows the current (created by gravity) right to left

Better, but still a few points...
It doesn't require a heavy wind to create waves that move oppositely to the current. If the current moves left at speed c and the wind is to the right at speed w then the wind has speed w+c relative to the water. All that has to do is generate waves faster than c relative to the water, so that to you on the bank the waves move to the right.
I don't understand the reference to Earth's spin. The only sense in which the current 'tries' to carry the waves leftwards is that the waves are waves in the water as a medium, so their natural speed is relative to the medium.
But most importantly, the waves themselves will not tend to push the wood anywhere. The movement of the wood results from two forces: drag from the wind and drag from the current. It won't take much current to overwhelm the drag from the air. There is no need to invoke any special locations in the river. That said, very near the shore the current will be weakest, so there could be a narrow band in which the wind wins.

 

[Barclay]

I don't understand the reference to Earth's spin.

Mr T said that current is created by gravity so I thought it must be something to with the planet spinning. Why does the water flow in a river ... the current ... I thought was to do with the wind ... Mr T said NO and said was caused by gravity so I thought must be something to do with spinning Earth. Water gets to the top of the hill with the spin and then when the Earth rotates further the water rolls down the hill ... I sense I've got it wrong.

I'll rephrase my answer to STATEMENT 2:
There is NOT a local gust of wind moving the wood from right to left in the middle of the river. A heavy wind is creating waves and moving them all left to right AGAINST the current. The current is created by gravity and trying to push the waves right to left ... BUT in a small area in the river there must be a NULL POINT where wind does not act and the wood follows the current (created by gravity) right to left.

 

[Herman Trivilino]

Mr T said that current is created by gravity so I thought it must be something to with the planet spinning.

Gravity is attributed to Earth's mass, not its spin. (Earth's spin does have a small effect on the apparent strength of Earth's gravity, but it's only about one-third of one percent).

Basically, all I'm saying is the water runs downhill, and that's what causes the current in the river. That current carries the block along with it, despite the fact the wind happens to be blowing in the opposite direction.

 

[haruspex]

There is NOT a local gust of wind moving the wood from right to left in the middle of the river. A heavy wind is creating waves and moving them all left to right AGAINST the current. The current is created by gravity and trying to push the waves right to left ... BUT in a small area in the river there must be a NULL POINT where wind does not act and the wood follows the current (created by gravity) right to left.

I cannot understand why you keep inventing this null point. There is no need for it. A current of a given speed will exert much more drag on the wood (assuming typical density, not e.g. balsa wood) than a wind of the same speed. So it is not unusual for the current to win, pushing the block into the wind, even if the wind is somewhat faster than the current.

Mister T answered your confusion over what causes the current, though it can be more complex than simply a stream flowing downhill. Could be tidal, or caused by density or temperature differences in the ocean, or ...

 

[Barclay]

Hello, thanks for sticking with this thread. I'm sounding dense but I know but I'm on the verge of cracking this puzzle.

... the water runs downhill, and that's what causes the current in the river. That current carries the block along with it, despite the fact the wind happens to be blowing in the opposite direction.

(i) Okay that block in the middle of the river is being carried right to left by the current in the river. The river is running downhill right to left.
Got that bit. ... Correct ? YES or NO

The wind is blowing from left to right and waves moving from left to right.

(ii) So here the waves are traveling against the current ... Correct ? YES or NO
(iii) So the wind is makes waves that propagate left to right ... Correct ? YES or NO

(iii) So the wind must be relatively strong if it is to make waves that propogate against the current ... This is INCORRECT according to:

It doesn't require a heavy wind to create waves that move oppositely to the current. If the current moves left at speed c and the wind is to the right at speed w then the wind has speed w+c relative to the water. All that has to do is generate waves faster than c relative to the water, so that to you on the bank the waves move to the right.

But I don't understand that.

What makes waves move? A surfer rides a wave because it moves. Yet theory that I've discussed here says that points on a wave oscillate up and down and energy moves right to left or left to right in the wave. But I can see a wave moving when a surfer surfs

 

[Herman Trivilino]

What makes waves move?

The medium has a property called elasticity. Which is a fancy way of saying it has an equilibrium shape, and when disturbed it tries to return to that shape.

A surfer rides a wave because it moves. Yet theory that I've discussed here says that points on a wave oscillate up and down and energy moves right to left or left to right in the wave. But I can see a wave moving when a surfer surfs

Those are breaking waves. In other words, that wave is in the process of being destroyed when the surfer is riding it. If the wave were out in the deep water it couldn't be surfed.

 

[Barclay]

Those are breaking waves. In other words, that wave is in the process of being destroyed when the surfer is riding it. If the wave were out in the deep water it couldn't be surfed.

Great.

So when waves look like they're moving in the middle of an ocean they are NOT ... they are actually just bobbing up and down (oscillating) ... it is an optical illusion when I think they're moving.

A bit of wood on the wave will be carried by the current created by gravity when the water rolls down the hill. The wind creates the waves. Wind is like a pebble dropped on a still pond that creates a ripple (waves) in the water. Gravitational potential energy of the pebble from its height is transferred into kinetic energy when if falls ... is converted into wave energy. When wave energy gets depleted the wave BREAKS.
Wind is not a single pebble ... wind is infinite pebbles creating waves allover the place.

 

[Herman Trivilino]

So when waves look like they're moving in the middle of an ocean they are NOT ... they are actually just bobbing up and down (oscillating) ... it is an optical illusion when I think they're moving.

It's an optical illusion when you think the water itself is moving in the direction of the wave motion. The waves actually do move, that is not an illusion. What is moving between source and observer is a disturbance of the elastic medium, not the elastic medium itself. The medium oscillates a small distance, the wave propagates a large distance.

Another example is the speaker on your television set and your eardrum. The speaker membrane vibrates back and forth, that disturbs the air. The disturbance propagates across the room and sets your eardrum vibrating. But the air that's disturbed at the surface of the speaker does not propagate from the speaker to your eardrum.

 

[haruspex]

Those are breaking waves. In other words, that wave is in the process of being destroyed when the surfer is riding it. If the wave were out in the deep water it couldn't be surfed.

i'm not a surfer, but my understanding is that it is possible to ride unbroken waves. http://www.surfing-waves.com/catchingwaves.htm
Indeed, the standard technique seems to be to catch the wave shortly before it breaks.
What allows a surfer to be carried forward by a wave is the trick of catching the wave, that is, to stay on the front of the wave.

@Barclay A parcel of water at the front of a wave is moving in the direction of the wave, but the wave, as an entity, overtakes the water. The parcel finds itself now on the back of the wave, where it slides back to its previous position.

 

[Barclay]

I just need to end this thread and anser

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.

Okay that block in the middle of the river is being carried right to left by the current in the river because the river that is running right to left.

The water waves at the bank of the river are traveling against the current, being propagated propagate left to right by the wind (a localized area of wind that has enough energy to create the waves against the current).

Voila ??

 

[Herman Trivilino]

i'm not a surfer, but my understanding is that it is possible to ride unbroken waves. http://www.surfing-waves.com/catchingwaves.htm
Indeed, the standard technique seems to be to catch the wave shortly before it breaks.
What allows a surfer to be carried forward by a wave is the trick of catching the wave, that is, to stay on the front of the wave.

@Barclay A parcel of water at the front of a wave is moving in the direction of the wave, but the wave, as an entity, overtakes the water. The parcel finds itself now on the back of the wave, where it slides back to its previous position.

So, I wonder if this can be done only with waves that are about to break. In other words, could it be done out in the deep water where the waves show no sign of breaking any time soon? And the other question I have, is this possible only because the water waves, in addition to a transverse component, also have a longitudinal component?

 

[haruspex]

The water waves at the bank of the river are traveling against the current, being propagated propagate left to right by the wind (a localized area of wind that has enough energy to create the waves against the current).

I'll say it once more: There is no need to consider any special areas of the river surface.
As long as the wind is in the opposite direction to the current it can generate waves in the opposite direction to the current (as viewed from the shore). (Maybe if the wind is very weak compared to the current the waves can appear to move with the current - not sure.)
Which way the block moves depends on which of wind and water generates the greater drag on it. If the block floats very high in the water, or the wind is much faster than the current, the wind might win, but more commonly the water will. Since we only need to explain the movement of the block, it is reasonable to suppose that the water wins here.

 

[haruspex]

So, I wonder if this can be done only with waves that are about to break. In other words, could it be done out in the deep water where the waves show no sign of breaking any time soon? And the other question I have, is this possible only because the water waves, in addition to a transverse component, also have a longitudinal component?

Good question.
The link I posted seems to imply unbroken waves can be caught more generally than that, but that would mean the surfboard can behave differently from a parcel of surface water. I suppose that is true, or it would not be possible to ride breaking waves, right? I can't find any explanation for the difference in behaviour.

 

[Barclay]

Firstly ... I feel that I must apologize for taking so much of your time ... and am very grateful for all the help. I sit here feeling embarrassed as I write my understanding of physics (every post). Its quite stressful to share my poor understanding even though its in a cyber world to cyber people.

I'll say it once more: There is no need to consider any special areas of the river surface.

I know you had said that before but I don't understand.

That block in the middle of the river is being carried right to left by the current in the river because the river that is running right to left. Okay I accept that.

When I started this thread I thought current was the waves and that the waves were moving the block. I've since learned that surface waves do not move L to R or R to L but simply oscillate up and down. So what is this current? In electrical wires I understand current to be the flow of charge so in water the current must be the flow of water molecules? BUT you've said that waves (that are made of these water molecules) do not move R to L or L to R but simply up or down. So what is carrying the block along... is the block simply sliding across the water, pushed by the wind? Is the current water molecules under the surface of the waves?

The water waves at the bank of the river are traveling against the current, being propagated propagate left to right by the wind. How can this be if not by a localized area of wind that has enough energy to create the waves against the current? Anything else sounds like science fiction Star Trek, paradox, vortex, black hole stuff. Some of the water is flowing down hill and the waves are moving up hill.

I will re-read your post on this Haruspex and try to make sense of it

 

[Barclay]

Oh I think I see it ... I'm having visions of the block of wood and the waves ...its all very fragile ... going to write it down quick before I lose it:

Its irrelevant that the block of wood is in the middle of the river. It could be anywhere in the river. I just thought that the question wanted me to think that something (like a depth change) was happening in the middle of the river.

So the WHOLE river is flowing down hill from RIGHT to LEFT (because it originates from the top of a hill that is somewhere on the left and flowing down the hill due to gravity). So the current (water molecules) are flowing RIGHT to LEFT.

The wind is flowing across the WHOLE river from LEFT to RIGHT and disturbing the water, producing waves that propagate LEFT to RIGHT too. But the current is the thing that is carrying the block (not the waves because they simply move up and down) and the current (water molecules) is flowing RIGHT to LEFT so the block travels RIGHT to LEFT.
There is no localized wind anywhere.


Eureka ? I think I've got it. Please advise. Very excited. Can't wait to for your opinions.If my answer is correct is there a better word for THING in the sentence above? "...current is the thing that is carrying the block".

 

[haruspex]

The wind is flowing across the WHOLE river from LEFT to RIGHT and disturbing the water, producing waves that propagate LEFT to RIGHT too. But the current is the thing that is carrying the block (not the waves because they simply move up and down) and the current (water molecules) is flowing RIGHT to LEFT so the block travels RIGHT to LEFT.
There is no localized wind anywhere.

Definitely getting there.
But you should not say the waves only bob up and down. A wave is an entity ("thing") distinct from the medium it travels in. The waves definitely move along. The surface water both bobs up and down and shuffles back and forth. I.e. the wave is a combination of transverse and longitudinal. This up-down, back-forth motion results in a circle in a vertical plane.

 

[haruspex]

Good question.
The link I posted seems to imply unbroken waves can be caught more generally than that, but that would mean the surfboard can behave differently from a parcel of surface water. I suppose that is true, or it would not be possible to ride breaking waves, right? I can't find any explanation for the difference in behaviour.

Ok, I think I see how the surfboard differs from a parcel of surface water.
When a surfer paddles on the board, they float, so weight equals weight of water displaced. But when riding the front of a wave, not enough board is immersed to provide full buoyancy. The surfer and board would 'sink' down the wave, but are held up by drag from the rising water. This allows the surfer to stay on the front of the wave and be pushed along by it.

 

[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..