Homework Help: Waves high school question -- Book answers are totally rubbish

1. Dec 25, 2015

Barclay

1. The problem statement, all variables and given/known data

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.

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.

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.

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 travelling 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 travelling right to left.

The current is going right to left = 1 mark

2. Relevant equations
Waves section in book

3. 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.

2. Dec 25, 2015

Mister T

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

3. Dec 25, 2015

Barclay

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

4. Dec 25, 2015

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.

5. Dec 25, 2015

haruspex

@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 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.

6. Dec 26, 2015

Barclay

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).

7. Dec 26, 2015

Barclay

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

8. Dec 26, 2015

Ray Vickson

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.

9. Dec 26, 2015

Barclay

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 thats 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

10. Dec 26, 2015

Mister T

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

11. Dec 29, 2015

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.

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?

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.

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 travelling 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 travelling right to left.

The current is going right to left = 1 mark ................. doesn't that mean the waves are travelling from right to left?
The waves make the piece of wood bob up and down= 1 mark .............. I say whats 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

12. Dec 29, 2015

haruspex

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.

13. Dec 29, 2015

Ray Vickson

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 travelling 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.

14. Dec 29, 2015

Barclay

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

15. Dec 29, 2015

Barclay

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

16. Dec 29, 2015

Mister T

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

17. Dec 29, 2015

haruspex

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.

18. Dec 30, 2015

Barclay

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:

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

BUT you've written .......
....... 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 .......
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

Last edited: Dec 30, 2015
19. Dec 30, 2015

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.

20. Dec 30, 2015

Mister T

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

21. Dec 30, 2015

Barclay

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

22. Dec 30, 2015

haruspex

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.

23. Dec 30, 2015

Mister T

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!]

24. Dec 30, 2015

haruspex

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.

25. Dec 31, 2015

Barclay

Happy New Year everyone.