[Becca]

this was a question on my A level physics exam and has puzzled me for years since then. can anybody help me with this?

[jackson33]

Becca said:

this was a question on my A level physics exam and has puzzled me for years since then. can anybody help me with this?

in short waves follow the gravitational pull of the moon. some are created by under sea earth quakes and a few by moving objects on the oceans. they hit the shores only because the shores are in the way.

[HydrogenBond]

What is sort of strange is that even with the tide going out, the waves continue to come in. One explanation is that the moon has a greater tug on the water at the surface, with the deeper water trying to restore sea level.

[jackson33]

HydrogenBond said:

What is sort of strange is that even with the tide going out, the waves continue to come in. One explanation is that the moon has a greater tug on the water at the surface, with the deeper water trying to restore sea level.

many places tide surge goes, when lost so is the water. inlets and places where excess waters can go. winds, currents also increase size of what the moons effects are. the waves (weather related) ride the surge (moons gravity). either can influence the other and often seen to extremes in hurricanes or typhoons. a low pressure area off the New England coast can effect waves hitting the UK. however this is not reversed since weather pattern in the Northern Hemisphere normally run west to east or northeast. if there were no weather, the moons surge would not be seen, just a steady build up in level. no waves in or out while the effect is there.

[CraigD]

Becca said:

why do waves always seem to go towards the shore?

This is an excellent question to begin answering by examining your assumptions, namely: do waves always appear to go toward the shore?

Before doing that, it’s useful to take a detailed look at what we mean by a wave moving in a direction. If we measure the height of a point on the surface of a body of water against some fixed reference – for example, with a measuring stick stuck in the sand or mud of a lake or ocean, we’ll discover the height increases and decreases in a fairly regular pattern. If we make this measurement of several point equal distances from our original, we’ll discover that, usually, just after the measurement at our original point reaches its greatest value and begins to decrease, the measurement at one of the other points also reaches its maximum, while for the others, the height heights are decreasing. The direction drawn between our original point and the one reaching its maximum is the direction of the wave. More simply put, we tell the direction of a wave by “following its crest”.

Note that not all water waves have a direction. If you perform the above experiment in a basin, such a bath tub, you’ll often observe a “standing wave” where the water height at various point rise and fall regularly, but with no distinct direction.

Tough not directly important to the question, it’s important to note that water, and objects floating on or under the water usually remain pretty stationary, even if the wave crest is traveling swiftly in some direction. It’s also important to keep clear the distinction between tides, a rising and falling of water height that takes roughly 12 hours, and waves, a rising and falling that happen much more quickly. The question is only concerned with waves, not tides.

Armed with some basic definitions, you can now observe nature. If you’re lucky enough to live on a coast, a bit of boating or binocular/telescoping will show you that waves often move parallel to or away from the shore, except where they actually meet the shore. A little more observation will show you that waves tend to move in the same direction as the wind, and may be seen going different directions between observations days or just hours apart. A critical bit of additional observation will reveal that the waves going toward a shore tend to be smaller – though often have stronger “breaks” – when the wind is blowing directly opposite them (in an offshore direction).

So the real mystery to be unraveled (with the help of observations and some informal application of Physics) in this question is why do waves near the shore always go toward the shore, while waves far from the shore go many different directions, often the same direction as the wind?

In keeping with hypography tradition, since this is a question from Becca’s Physics class, I won’t give my answer until she’s had a chance to come up with her own. (in the same tradition, I’m also moving it to homework and projects) It’s a fun problem

[Becca]

i did have a go at the question but my answer involved the waves sort of defracting when they hit the shallower water and just bending towards the obstruction, rather like light entering a glass block. thaks for helping me think about the problem more widely guys, interesting stuff

[Qfwfq]

If the shore is good enough, it will reflect the waves back outward pretty well. This would give a standing wave, but doesn't normally happen because the shore dissipates much of the energy so you prevalently see the incoming waves. Of course if there were no waves coming in from far and the shore moved the right way, you'd see just waves going out. It's quite simple really.

jackson33 said:

in short waves follow the gravitational pull of the moon. some are created by under sea earth quakes and a few by moving objects on the oceans. they hit the shores only because the shores are in the way.

Most waves are generated by wind and have nothing to do with the moon. The tide is a very long wave that you don't really see as a wave (as far as you can see, all the water rises or lowers, over the course of hours).

[CraigD]

Becca said:

i did have a go at the question but my answer involved the waves sort of defracting when they hit the shallower water and just bending towards the obstruction, rather like light entering a glass block.

Waves do behave this way – if you observe waves crests traveling from deep to shallow water, such as over a sand bar, or in one of the special “water table” apparatuses found in many science classrooms, you’ll see them change direction as you describe – and this is part of the answer. However, this explanation alone can’t fully explain why waves near the shore usually move toward the shore.

Here’s a hint. Begin with the assumption that air blown over the surface of a smooth body of water will cause waves that travel in the direction of the blown air (you can experimentally test this with a fan and a basin of water). Note that, once started, waves continue after the wind stops, or even if it blow briefly in the opposite direction. Now, consider what happens when a wind blows over a small area of the surface of the water, while another wind blows at the same speed in the opposite direction over a larger area. Finally, assuming ocean winds blow in random, changing directions. What is the likelihood that such conditions will produce waves moving away from the shore, vs. the likelihood that they’ll cause waves moving toward it? (especially accounting for waves traveling more parallel to than toward the shore changing direction due to refraction)

[hallenrm]

Here's my wild take on the question!

We see waves coming towards us while standing on a sea shore, and not moving away from us because these waves are a result of oscillations set in deeper waters because of various perturbations. The origin of these perturbations is deep waters. So, we can not observe that all waves travel towards the land only from a position in deep waters.

The perception that all waves travel towards the land only, is a misconception:)

[Boerseun]

Air tends to rise over land as the land is generally hotter than the ocean. Air tends to drop over the ocean as it cools down again. So there is a permanent cycle of cool air blowing in from the sea, to be heated over land, to rise, to circulate back over the sea again.

Waves are formed through wind action. The ocean wind generally blows from the cold ocean to the warm land, forming waves which looks as if they're always aiming for the land.

It all depends on the specific local weather system. You can even find waves running parallel to the shore if the wind conditions are such. What I wrote above was merely a generalisation.

[jungjedi]

where is the center of the ocean and why dont waves go there?-pablo neruda

[arkain101]

The shortest possible answer I could say is that.

Waves travel in all directions. Energy in the ocean is all over the places. They are more visibile when the originate from deep water and become amplified in shallow water.

So I would say, waves do not always travel toward land, but when energy reaches land it becomes visible.

[freeztar]

Boerseun said:

Air tends to rise over land as the land is generally hotter than the ocean. Air tends to drop over the ocean as it cools down again. So there is a permanent cycle of cool air blowing in from the sea, to be heated over land, to rise, to circulate back over the sea again.

Waves are formed through wind action. The ocean wind generally blows from the cold ocean to the warm land, forming waves which looks as if they're always aiming for the land.

It all depends on the specific local weather system. You can even find waves running parallel to the shore if the wind conditions are such. What I wrote above was merely a generalisation.

I think you nailed it Boerseun!

I liked the different ideas that came up from the original question, but this seems to be the simplest explanation, occam's razor.

[Becca]

thanks a lot guys, i think i understand now. sorry its taken a while for me to look at this again, not really had time for a lot of extra-curicular pondering recently

[The Artificial Kid]

Last year in India I saw an ocean beach that acted as quite a good reflector for small waves, bouncing them back and creating interesting interference with patterns, although the quickly got swamped by the prevailing shoreward wave action. This makes me think that the main reason waves seem to move shorewards is that the shore usually neither generates nor reflects waves. The ocean has a lot of wave potential and the shore is a wave sink.