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Easy physics question

  1. Sep 17, 2003 #1
    I am taking physics for the first time this year and i am stuck on a problem
    I will admit that this is a bonus question for my class but the teacher allowed up to get answers from the internet and since i can not find it anywhere i thought i would try here
    the question is,

    "You go to the beach and stand in the water. Why is every sixth (or seventh)wave bigger than the previous waves?"

    Thanks to anyone who helps with this.
    Any help would be great.
  2. jcsd
  3. Sep 17, 2003 #2


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    This is an example of "wave tranes". Not sure if you want the answer or would like to figure it out yourself (allways more satisfying), so I'll give you a hint. The waves approaching the beach do not all belong to one wave pattern.
  4. Sep 17, 2003 #3
    that is exactly what i wanted.
    I should have said that before but really i just wanted a hint so thanks.
    I will look into it tomorrow and see what i find then if i am still having trouble i will post again.
    Thanks again
  5. Sep 18, 2003 #4
    This is not my subject but there was a very interesting programme on TV recently on Freak Waves. It appears that years ago someone produced a new wave theory that was not accepted. It now turns out to accurately predict the occurence of so-called 'freak waves'. I am sure someone could give you a reference if you are interested. My point being that (water) Wave Theory is about to change radically.
  6. Sep 18, 2003 #5


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    I thought it was due to the waves reflecting off the beach, and interfering with the next waves.

    elas: The theory isn't really new. Non-linear wave theories have been around for ages. But people have used what they thought were good approximations because they are HARD to solve.
  7. Sep 18, 2003 #6


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    Yeah, I think alot of people think that. But this wave pattern exists even out in the deep sea. Usually every sixth or seventh wave is a bigun'.
  8. Sep 19, 2003 #7
    I did say the theory was not new. It is not correct to say that every seventh wave is a high one. Having spent 12 years on my home built catamaran in the Caribbean, I have had both time and opportunity to count waves. Following waves catching up with each other have a totally different pattern to cross waves from two different weather systems, but rum punch is not conducive to in depth wave studies.
    Fortunately freak waves or large breaking waves in a storm rarely have any great length so with a little concentration at the helm one quickly gets the hang of dodging the big breaking sections. Big breaking sections can occur on consecutive waves, but not in the same line of advance, in fact the distance along the wave front between two big breakers (well offshore) is so great it is difficult to decide which wave the breaking section is on. Inshore the pattern is more confused due to reflected waves.
    For those who have not had the experience the most frghtening aspect is the unbelievable noise, best described as the noise made by a large high speed steam engine passing through a built up station when you are standing on the platform edge. The noise comes at you from all directions with such intensity that you feel it inside you.
    Last edited by a moderator: Sep 19, 2003
  9. Sep 21, 2003 #8
    So has anyone actually answered the first question here because I can't find it?
  10. Sep 21, 2003 #9
    I looked for most of the weekend and still didnt find the answer so if someone were to help me out that would be great.
  11. Sep 21, 2003 #10


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    There may be three or four different sets of waves present, all with different wavelengths. Try to imagine what would happen as these wavetrains all travell across the same body of water. At any point along that surface, there will be times when two wave peaks arrive at the same time. There might even be times when all three or four arrive riding on top of each other. These events will recur regularly.
  12. Sep 21, 2003 #11
    Hey thanks a lot
    I will make sure i understand it before i write it out in my own words.
    Thanks again
  13. Sep 22, 2003 #12

    jimmy p

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    I personally would have thought it would be to do with superpositioning and interference of waves. If the interference between the waves is constructive (eg, both wavesd fit perfectly on eachother eg sine + sine), then the size of the wave would double, however if it were destructive interference (sine + cosine wave) then they would cancel each other and a wave would not be produced. That also explains freakishly huge waves
  14. Jul 24, 2004 #13
    seventh wave

    try and contact the proudman observatory web site it is the worlds greatest authority on tidal predictions and they are sure to know of the reason. i myself have known of this phenomena and i believe it is caused by several waves building into one at the same time. the reason i think is unclear. however the number seven is critical and ive experimented at my local beaches on numerous occasions. its almost certainly caused by astral interference in some way. hope ive been of help yours tony fisher tonfshr@aol.com
  15. Aug 1, 2004 #14
    why is it the sixth or seventh, not the fourth, fifth or ......., why does the question just refer to the 2 possibilities ? or it actually really doesn't matter for it is just some random numbers to indicate that interference does happen.
  16. Aug 2, 2004 #15
    Different weather positions, locations etc. would mean waves travel a different distance up the shore and would receed at different speeds. This would alter the resistance caused against the new waves.

    Perhaps it could be classified as a fairly primitive cycle. The large wave would consist of a lot of water and thus provide a large resistance when it went back into the sea, prompting smaller and smaller waves. However, as the waves become smaller the resistance would also decrease and thus you would get waves of different sizes, such as a large one reguarly.

    That's my primitive and unresearched idea anyway. Think of it what you will.
  17. Aug 6, 2004 #16
    well I just went over wave pattern is physics 20 and what they talk about is that the reason every 6th or 7th wave is a big one is because of intreferance of the waves. they say that the big ones ar formed by have 2 crests of the wave combine (constructive) and they form one big wave, also you would notice that there would be a super low between waves, thats because you have 2 troughs combining and creating a deep dip (also constructive. and then when you have a crest and a trough combining you creat a node, or a nothing, (called destructive)

    so I believe jimmy p was right in his thinking
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