Waves high school question -- Book answers are totally rubbish

[Barclay]

Homework Statement

Q Provide an explanation for the following statements.
[/B]
STATEMENT 1 The captain on an ocean ship is proceeding slowly into waves coming towards the ship. He notices that the waves change in two ways about 200 m ahead of where the ship is located. The waves get further apart and change direction quite noticeably.

My answer
The water 200m ahead is deeper and this explains why the wavelengths have increased (waves get further apart). The change of direction of the water waves is because the shallower ocean water (where the ship is presently located) has met the deeper water at an oblique angle and changed direction.

Book answer
The water gets shallower 200 m ahead = 1 mark

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.

My answer

In the middle of the river there may be a sudden change in depth of the water so that the waves change direction so the wood is now traveling right to left. Also there may be superposition of various waves sizes in the windy weather and when the depth changes so the wood is now traveling right to left.

Book answer

The current is going right to left = 1 mark

Homework Equations

Waves section in book

The Attempt at a Solution

I've put answers immediately after the question in this thread because it flows easier / is easier to read.

I think my answers are good though not certain about STATEMENT 2.
I think the book answers are rubbish and the book answer to STATEMENT 1 is wrong.

Please tell me what you think.Seasons greetings, happy New Year to you all.

 

[Mister T]

I think the questions have little value in teaching physics to students or to anyone else.

 

[Barclay]

I think the questions have little value in teaching physics to students or to anyone else.

I don't know why you say this. The questions are checking understanding that wave change direction when water depth changes.

 

[Mister T]

If they are addressing the issue of waves changing direction when the depth changes then they should reference that in the question. The questions are vague and it's not clear that that is the physical principle that's the focus of the questions.

You say the answers are rubbish. I say the questions are rubbish. That's my professional opinion.

 

[haruspex]

@Barclay , I agree with you on statement 1 except here:

The change of direction of the water waves is because the shallower ocean water (where the ship is presently located) has met the deeper water at an oblique angle and changed direction.

That makes it sound as though the waves are coming from behind the ship.

I agree with the book on statement 2. Surface waves do not propel objects along (unless they catch a wave and stay with it a while). The wind can blow floating objects along, but the wind is unlikely to be locally blowing in a different direction.
@Mister T , I feel the questions have value in getting students to apply physical principles to the real, messy, everyday world. In the river question, the student has to think outside the narrow confines of wave theory and consider what else might affect the movement.

 

[Barclay]

@Barclay , I agree with you on statement 1 except here:

That makes it sound as though the waves are coming from behind the ship.

Yes your're correct because the question clearly says "... ship is proceeding slowly into waves coming towards the ship". The question is just worded incorrectly (I think). I think the way the question has been worded means that there's no possible answer (at basic physics level).

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.

I forgot to include the whole book answer. The other parts of the book answer are shown now below:
Book answer

The current is going right to left = 1 mark
The waves make the piece of wood bob up and down= 1 mark
But they do not move it along = 1 mark

 

[Barclay]

I agree with you on statement 1 except here:
That makes it sound as though the waves are coming from behind the ship.

I've just re-read what you've said and may be I should have written: The water 200m ahead is deeper (and heading towards the ship) and this explains why the wavelengths are larger ("...waves get further apart"). The ship is presently sitting in shallow water. The deeper water has met the shallower water (at an oblique angle) and changed the direction of the water waves

 

[Ray Vickson]

I've just re-read what you've said and may be I should have written: The water 200m ahead is deeper (and heading towards the ship) and this explains why the wavelengths are larger ("...waves get further apart"). The ship is presently sitting in shallow water. The deeper water has met the shallower water (at an oblique angle) and changed the direction of the water waves

The water may not be heading towards the ship; it could be stationary, or heading toward or away from the ship (due to currents, etc.). The question said the waves are heading toward the ship, and waves can move quickly without the water moving horizontally very much at all.

 

[Barclay]

The water may not be heading towards the ship; it could be stationary, or heading toward or away from the ship (due to currents, etc.). The question said the waves are heading toward the ship, and waves can move quickly without the water moving horizontally very much at all.

Its getting more and more complex. The book I'm sudying has picture like this:

http://4.bp.blogspot.com/_FAgA6-CHYr8/TR8ON92vf7I/AAAAAAAAAsw/HfuRdJPuHQk/s1600/defrac.bmp

so that's why my answers are as they are.

What do you think of my answers now, based on the pictures shown. I need them to be worthy of 9/10 marks. This really is not homework. Its personal study so no one can help me too much. Thanks

 

[Mister T]

I feel the questions have value in getting students to apply physical principles to the real, messy, everyday world. In the river question, the student has to think outside the narrow confines of wave theory and consider what else might affect the movement.

In the real, messy, everyday world of educating students these questions won't have that desired outcome.

 

[Barclay]

Hello, I need to refresh this thread please so I'm going to make it colorful & magnify words in an attempt to make it attractive and draw attention ... but not so bright that it stops looking like a physics forum. Please no one complain. Thanks

I still need help with the questions.

Q Provide an explanation for the following statements.

STATEMENT 1 The captain on an ocean ship is proceeding slowly into waves coming towards the ship. He notices that the waves change in two ways about 200 m ahead of where the ship is located. The waves get further apart and change direction quite noticeably.

My REVISED answer
The water 200 m ahead is deeper (and heading towards the ship) and this explains why the wavelengths are larger ("...waves get further apart"). The ship is presently sitting in shallow water. The deeper water has met the shallower water (at an oblique angle) and changed the direction of the water waves (and this is where the ship is sitting). How good is my answer ... out of 3 marks?

Book answer
The water gets shallower 200 m ahead = 1 mark ... is this this book answer wrong? Its meant to be a 3 mark question but the book gave itself 1 mark. The answer I'm pretty sure is wrong anyway . The water 200 m ahead is deeper NOT shallower.

*******************************************************************************************
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.

My answer
In the middle of the river there may be a sudden change in depth of the water so that the waves change direction so the wood is now traveling right to left. Also there may be superposition of various waves sizes in the windy weather and when the depth changes so the wood is now traveling right to left.

Book answer
The current is going right to left = 1 mark .... doesn't that mean the waves are traveling from right to left?
The waves make the piece of wood bob up and down= 1 mark ..... I say what's that got to do with the question
But they do not move it along = 1 mark ... if there is a current it must surely move the wood along

waves can move quickly without the water moving horizontally very much at all.

No idea what this means. How can waves move yet not move horizontally?

Ever so grateful for further assistance

Barclay

The book I'm sudying has picture like this:

http://4.bp.blogspot.com/_FAgA6-CHYr8/TR8ON92vf7I/AAAAAAAAAsw/HfuRdJPuHQk/s1600/defrac.bmp

 

[haruspex]

Hello, I need to refresh this thread please so I'm going to make it colorful & magnify words in an attempt to make it attractive and draw attention ... but not so bright that it stops looking like a physics forum. Please no one complain. Thanks

I still need help with the questions.

Q Provide an explanation for the following statements.

STATEMENT 1 The captain on an ocean ship is proceeding slowly into waves coming towards the ship. He notices that the waves change in two ways about 200 m ahead of where the ship is located. The waves get further apart and change direction quite noticeably.

My REVISED answer
The water 200 m ahead is deeper (and heading towards the ship) and this explains why the wavelengths are larger ("...waves get further apart"). The ship is presently sitting in shallow water. The deeper water has met the shallower water (at an oblique angle) and changed the direction of the water waves (and this is where the ship is sitting). How good is my answer ... out of 3 marks?

Book answer
The water gets shallower 200 m ahead = 1 mark ... is this this book answer wrong? Its meant to be a 3 mark question but the book gave itself 1 mark. The answer I'm pretty sure is wrong anyway . The water 200 m ahead is deeper NOT shallower.

*******************************************************************************************
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.

My answer
In the middle of the river there may be a sudden change in depth of the water so that the waves change direction so the wood is now traveling right to left. Also there may be superposition of various waves sizes in the windy weather and when the depth changes so the wood is now traveling right to left.

Book answer
The current is going right to left = 1 mark .... doesn't that mean the waves are traveling from right to left?
The waves make the piece of wood bob up and down= 1 mark ..... I say what's that got to do with the question
But they do not move it along = 1 mark ... if there is a current it must surely move the wood along



No idea what this means. How can waves move yet not move horizontally?

Ever so grateful for further assistance

Barclay

The book I'm sudying has picture like this:

http://4.bp.blogspot.com/_FAgA6-CHYr8/TR8ON92vf7I/AAAAAAAAAsw/HfuRdJPuHQk/s1600/defrac.bmp

I think I just figured out where the confusion arises in Q1. It's the word "becomes". From whose perspective? We all took it to be from the captain's (or ship's) persepctive, i.e. as he looks beyond the transition he sees the waves further apart. But if from the perspective of the waves, it would mean they become further apart as they cross the transition on the way to the ship.

On question 2, as I indicated in post #5, I agree with the book.
Waves are not created by the current directly, they are created by the difference in velocity between the current and the wind. In the present case, you are told the wind is left to right. The waves go left to right because the wind does. This tells you little about the current; that could be going either way, or still. (It does tell you that the current is not going right to left faster than the speed of the waves relative to the water.)
If you create waves in still water (by a paddle, say, so no wind here) a floating object will stay in place overall, just bobbing up and down. If you could make visible a parcel of water at the surface you would see that also not moving on average. It would describe a circle in a vertical plane. At the top of the circle it moves with the wave, at the bottom against it. The bobbing object does the same. See e.g.
If the waves are created by wind, there will be a tendency for the wind to drag the floating object in the same direction. But here we are told the floating object is going the other way. The current is the most likely explanation.

 

[Ray Vickson]

Hello, I need to refresh this thread please so I'm going to make it colorful & magnify words in an attempt to make it attractive and draw attention ... but not so bright that it stops looking like a physics forum. Please no one complain. Thanks

I still need help with the questions.

Q Provide an explanation for the following statements.

STATEMENT 1 The captain on an ocean ship is proceeding slowly into waves coming towards the ship. He notices that the waves change in two ways about 200 m ahead of where the ship is located. The waves get further apart and change direction quite noticeably.

My REVISED answer
The water 200 m ahead is deeper (and heading towards the ship) and this explains why the wavelengths are larger ("...waves get further apart"). The ship is presently sitting in shallow water. The deeper water has met the shallower water (at an oblique angle) and changed the direction of the water waves (and this is where the ship is sitting). How good is my answer ... out of 3 marks?

Book answer
The water gets shallower 200 m ahead = 1 mark ... is this this book answer wrong? Its meant to be a 3 mark question but the book gave itself 1 mark. The answer I'm pretty sure is wrong anyway . The water 200 m ahead is deeper NOT shallower.

*******************************************************************************************
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.

My answer
In the middle of the river there may be a sudden change in depth of the water so that the waves change direction so the wood is now traveling right to left. Also there may be superposition of various waves sizes in the windy weather and when the depth changes so the wood is now traveling right to left.

Book answer
The current is going right to left = 1 mark .... doesn't that mean the waves are traveling from right to left?
The waves make the piece of wood bob up and down= 1 mark ..... I say what's that got to do with the question
But they do not move it along = 1 mark ... if there is a current it must surely move the wood along



No idea what this means. How can waves move yet not move horizontally?

Ever so grateful for further assistance

Barclay

The book I'm sudying has picture like this:

http://4.bp.blogspot.com/_FAgA6-CHYr8/TR8ON92vf7I/AAAAAAAAAsw/HfuRdJPuHQk/s1600/defrac.bmp

You say "No idea what this means. How can waves move yet not move horizontally?"

Well, if you will mis-read what I said then it is no wonder you are confused. I said: the WAVES move (say from West to East), but the water hardly moves at all, except up and down. When waves move, "energy" is traveling through the water but the water itself is not going anywhere very much. (Actually, out in deep water individual water molecules move in a more-or-less circular orbit, so they come back to their starting point after the wave has passed.) Of course, this breaks down when waves wash ashore, as you can plainly see when you view videos of the Boxing Day Tsunami in Thailand, and the Japanese Tsunami of more recent times. This is due to the fact that the water starts off elevated in altitude, so has to actually flow downhill--- that being on shore in the tsunami case.

 

[Barclay]

We all took it to be from the captain's (or ship's) persepctive, i.e. as he looks beyond the transition he sees the waves further apart. But if from the perspective of the waves, it would mean they become further apart as they cross the transition on the way to the ship.

So are you saying I'm correct : the waves are further apart (greater wavelength) 200 m away therefore they must be in deep water (not shallower water as the book said) ?

Thanks for finding the video and other comments but its getting beyond my book level ... no circular wave motion is discussed. Only shallow water = slow waves with shorter wavelengths and deep water = fast waves with long wavelengths

 

[Barclay]

Well, if you will mis-read what I said then it is no wonder you are confused. I said: the WAVES move (say from West to East), but the water hardly moves at all, except up and down. When waves move, "energy" is traveling through the water but the water itself is not going anywhere very much.

Thanks for helping out again. I'm going to look at another textbook because I feel I'm missing information so that's why I'm not understanding. I don't mean to waste your time.

 

[Mister T]

I'm going to look at another textbook because I feel I'm missing information so that's why I'm not understanding.

Stay away from USA high school physics textbooks. College level are much much better.

 

[haruspex]

So are you saying I'm correct : the waves are further apart (greater wavelength) 200 m away therefore they must be in deep water (not shallower water as the book said) ?

no, I'm saying the question as given is ambiguous. We all read it one way, but the intended meaning was the other. The waves become further apart as they approach the ship. That is, the waves are further apart where the ship is now than they are further away.

 

[Barclay]

no, I'm saying the question as given is ambiguous. We all read it one way, but the intended meaning was the other. The waves become further apart as they approach the ship. That is, the waves are further apart where the ship is now than they are further away.

Hello again, thanks for helping me out. I've got to get this right but I can't make sense of it.

The book answer is that the "the water gets shallower 200 m ahead". This means what you've written above is correct:

... the waves are further apart where the ship is now than they are further away.

because they must be in deeper water now for the waves to be further apart.

BUT you've written ...

The waves become further apart as they approach the ship.

... but they are still 200m away from the ship. For the Captain to interpret the waves 200m away to be further apart must mean he's sitting on water where are waves are not so far apart i.e his ship is sitting on shallow water (slow water).

Please Haruspex ... I think my problem may be that I'm not reading the question properly. As you say ...

We all read it one way, but the intended meaning was the other.

So would you be so kind to re-write the question as it should be written . Then I may see what you're seeing.

That'll be go good for me because right now I'm losing confidence on this whole wave thing. Will help me know that I'm at least thinking the right way. Will be soo helpful

 

[Barclay]

Part 2 of the question about the wood floating around ... circular waves have been mentioned ... that's just too far into high level physics ... grateful though I am for your explanations I really can't consider circular wave motion. Its nowhere in my text. In my books waves are just in SHALLOW WATER (at slow speed and small wavelengths) or DEEP WATER WAVES (fast speed, large wavelengths).

Can someone help with part 2 of the question without talking about circular waves.

 

[Mister T]

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.

The river flows from right to left, carrying the small piece of wood with it.

 

[Barclay]

The river flows from right to left, carrying the small piece of wood with it.

Your wit is beyond measure. Or are you just being sarcastic?

Anyway I get your point.

 

[haruspex]

Part 2 of the question about the wood floating around ... circular waves have been mentioned ... that's just too far into high level physics ...

The waves are not circular, they are perfectly normal surface waves in water. But it really does help to understand how it is that the waves move even if the water, overall, does not. If you follow what happens to a small bit of the water at the surface, it goes through a circular motion. At the top of a passing wave it is moving with the wave. As the wave passes, it starts to descend and the horizontal motion slows, so that it finds itself on the back of the wave. Naturally, it slides down the back of the wave, so that by the time it is in the trough it is moving with a velocity equal and opposite to the velocity it had at the peak. Next, it finds itself on the front edge of a wave, so it is lifted up and propelled forwards again.
Did you watch the video?
Once you have understood that, you can see how it is that floating objects are not pushed forwards by waves (unless they manage to surf a wave, i.e. stay on its front face). Hence the drift of floating objects is mostly down to wind and current.
In the present problem, we are told the wind is in the same direction as the waves, so the remaining explanation is the current.

For Q1, a better wording would be
The waves approach the advancing ship from the front. At a line 200m ahead, the captain can see a change in the waves. As they cross that line from beyond, the wave crests become further apart. That is, they are further apart near the ship than they are beyond the line 200m ahead.

 

[Mister T]

Your wit is beyond measure. Or are you just being sarcastic?

Anyway I get your point.

I'm being completely serious. There is nothing whatever wrong with that answer, it could very well be the answer intended by the question's author, or it could be all that's left of it after one committee got done messaging it into a form that the publisher could sell to another committee.

[I only just now went back and looked at the first post of this thread, and I swear, that only then did I see that my answer in Post #20 matches it!]

 

[haruspex]

[I only just now went back and looked at the first post of this thread, and I swear, that only then did I see that my answer in Post #20 matches it!]

Just guessing, but that could account for Barclay's reaction. It appeared to be purely a restatement of the book answer, whereas Barclay was looking for explanations.

 

[Barclay]

Happy New Year everyone.

 

[Barclay]

I'm being completely serious.

... Oops I didn't realize. I've been thinking about complex physics High School level) but you gave me a seemingly simplistic answer ... I thought you were mocking me because you've not liked this question from the very beginning:

I think the questions have little value in teaching physics to students or to anyone else.

I say the questions are rubbish. That's my professional opinion.

... these questions won't have that desired outcome.

So I thought you were against this post from the beginning ... I took it to heart and it hurt ... then you gave me a basic answer that was very similar to the book answer ... and I thought book answers were so ridiculous ...

The current is going right to left.
The waves make the piece of wood bob up and down
But they do not move it along

Now I realize that you were being serious. So I apologise for my reaction …. though no offence was intended.

 

[Barclay]

For Q1, a better wording would be
The waves approach the advancing ship from the front. At a line 200m ahead, the captain can see a change in the waves. As they cross that line from beyond, the wave crests become further apart. That is, they are further apart near the ship than they are beyond the line 200m ahead.

Fantastic. Now I know I've understood high school waves concepts.

Now I see that the waves become further apart once they cross the line and are approaching the ship. Therefore the ship is sitting on these waves that are "... further apart". Therefore the ship is sitting in DEEP WATER with fast moving waves.

The water beyond 200m is shallower water with slower waves of smaller wavelengths ... such as in a port.

Thanks for all the help.

NOW onto part 2 and the bit of floating wood. Will get back to you all soon after reading what you've all written again and watching the video)

 

[Mister T]

So I thought you were against this post from the beginning ...

Not the post. I simply thought, and I still do think, that the questions are of little pedagogical value. Unfortunately, high school physics books in the US are riddled with questions and other passages of this type of nonsense. The fault is with the process. A lot has been written about it, going way back to Feynman's famous account.

http://www.textbookleague.org/103feyn.htm

 

[Barclay]

@Barclay
I agree with the book on statement 2. Surface waves do not propel objects along (unless they catch a wave and stay with it a while). The wind can blow floating objects along, but the wind is unlikely to be locally blowing in a different direction.

Did you watch the video? YES I DID WATCH THE VIDEO WHEN YOU FIRST SENT IT. Watched it again and another video in the same link
Once you have understood that, you can see how it is that floating objects are not pushed forwards by waves (unless they manage to surf a wave, i.e. stay on its front face). Hence the drift of floating objects is mostly down to wind and current.
In the present problem, we are told the wind is in the same direction as the waves, so the remaining explanation is the current.

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.

SOME THEORY THAT I'VE LEARNT:
The wind is creating the current and moving the waves left to right.
A transverse wave may appear to be traveling from left to right but the particles on the surface wave do not move from left to right, they simply oscillate up and down. Matter is not transferred left to right but energy is transferred left to right.

MY NEW ANSWER:
So the piece of wood simply bobs up and down (oscillates) and it is the local wind in that region of air that pushes it from right to left. So the wind is creating a current in that region that moves the wood right to left.
Hello, please can you check my thinking. I've think I've finally figured out the 2nd part of this question

 

[Mister T]

The wind is creating the current and moving the waves left to right.

Gravity is creating the current, not the wind. The river flows downhill, but it's on a day when the wind happens to be blowing uphill.

 

[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.

 

[Mister T]

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.