How does a water wave propogate outward?

Question

I know that the individual molecules just bob up and down. What makes the wave as a whole move outward, and not say inward?

I mean, since the edge of a wave is a wavefront, what is preferred about the outward direction, for the wave to propagate. How does a water molecule bobbing 10 feet away from the stone know from which direction it got the energy to bob up and down?

Since all the water particles are moving perpendicularly to the surface, the force it would exert in the outward direction would be 0 (Cos 90 = 0).

So how can the bobbing particles 'push' other particles outward?

I'm having difficulty getting my question across. Suppose I'm a water molecule in a lake. I'm sitting quietly. Now someone throws a stone into the lake behind me. Who tells me to start moving up and down? Suppose the fellow behind me is moving up an down, how does he make me start moving up and down? He's behind me not under me. He can't 'push' me out because cos 90 = 0, and eve if he did, I would not move up and down, but sideways, but then it's not a transverse wave any more.

Tim F: Thanks. I didn't know that. I was always taught that the molecules move up and down. But then it's not a transverse wave is it? What sort of wave is it then?

Answer 1

The particles don't just bob up and down, they move in a circle. The direction of motion at the top of the circle depends on (or causes) the direction the wave propagates in. (That's how adjacent particles "know" about the wave direction.) See the ref. (the "water waves" example) for an animation that clarifies this effect.
Addressing your add-on: from the web page I cited, it seems there is both longitudinal and "transverse" (in this case meaning vertical) motion. It's also a type of surface wave.
Also, back at your original question, if a wave that starts at a point (like when a stone is tossed into the water) propagates at all, it must be outward. But why does this initial displacement propagate? Consider the alternatives.
1) The displacement generates a local rise and fall in the water level. This rise wants to stay local, a small bit of the surface oscillating vertically, piston-like. But gravity (and to a lesser extent surface tension) makes the moving mass spread when it's above sea level and fill when below, both effects generating radial motion.
2) It expands radially, sliding along the existing surface until it levels out, like what would happen on a solid surface. The rest of the water is unaffected. (This one is even less realistic than 1 because we know this would depress the water mass.) But over water, this sliding layer "engages" the water below through turbulent mixing and resulting momentum exchange in the horizontal, radial direction.
So the initial vertical displacement inevitably results in outward-moving horizontal and vertical motion.

Answer 2

First of all, I suspect you might be misunderstanding the nature of a wave. A wave is "is a disturbance that propagates through space or spacetime, often transferring energy." That is, the wave CAUSES the individual molecules to "bob up and down".

From then on, the question you are asking is, basically, "if I set a ball rolling left, why does it roll left? Because at any one instance, the ball is still - what causes it to move left and not right?" The answer to that would be its kinetic energy, whose direction is left.

A wave is a means of transferring energy. In a medium (such as water), there is no overall change in position of the water molecules (they bob up and down, but afterward, are still in the same place).

A water molecule bobbing 10 feet away from the stone DOESN'T know from which direction it got the energy because it only moves up and down - it does Not move in the same direction as the wave (obviously). The wikipedia entry I listed gives a nice graphical demonstration of this.

In conclusion, the "bobbing" particles are mere symptoms - they are not cause, but effect. It is the wave motion (perpendicular to the "bobbing"), which causes the particles to bob.

Answer 3

waves on the water surface are generated whenever it is agetated or disturbed. Only this diturbance is propagated in the forward direction and not the particles. For example if a pebble is dropped into a still pond then the disturbance so produced has to travel in the forward direction having no other way for its propagation. The disturbance is transfered from particle to particle, as the particles are interlinked with each other because of intermolecular forces between them. This fact can be well understood by taking the example of similar waves on a string.

Answer 4

Water does NOT simply bob up and down. The individual bits of water move more or less in circles, moving slightly forward at the top and slightly back at the bottom. So the water doesn't have a net motion forward or back, but the the particles 10 ft away DO "know" from which direction the wave came.

Answer 5

Ultrasonic testing is based on time-varying deformations or vibrations in materials, which is generally referred to as acoustics. All material substances are comprised of atoms, which may be forced into vibrational motion about their equilibrium positions. Many different patterns of vibrational motion exist at the atomic level, however, most are irrelevant to acoustics and ultrasonic testing. Acoustics is focused on particles that contain many atoms that move in unison to produce a mechanical wave. When a material is not stressed in tension or compression beyond its elastic limit, its individual particles perform elastic oscillations. When the particles of a medium are displaced from their equilibrium positions, internal (electrostatic) restoration forces arise. It is these elastic restoring forces between particles, combined with inertia of the particles, that leads to the oscillatory motions of the medium.

In solids, sound waves can propagate in four principle modes that are based on the way the particles oscillate. Sound can propagate as longitudinal waves, shear waves, surface waves, and in thin materials as plate waves. Longitudinal and shear waves are the two modes of propagation most widely used in ultrasonic testing.
FOR MORE CLICK:
http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/wavepropagation.htm
http://www.worldscibooks.com/engineering/1241.html