# Wave forces

Discussion in 'Physics & Math' started by Xmo1, Nov 24, 2015.

1. ### Xmo1Registered Senior Member

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If there is an (possibly spherical) object moving laterally in the trough of a wave because of some property of the wave, saying that the trough has its own set of surface angles and vibrations, is there a particular name for those properties and forces that cause the object to move laterally through the trough?

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Surfing?

5. ### exchemistValued Senior Member

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If you mean an object floating in water waves, it will trace out a more or less circular path, returning to where it started as each wave cycle passes. I am not sure what you mean by "lateral motion" exactly but I suppose that what you have, when you have circular motion projected onto one dimension, is Simple Harmonic Motion.

(I'am flailing a bit because I am not sure what you are really getting at. Perhaps a follow up from you will make things clearer.)

7. ### Aqueous Idflat Earth skepticValued Senior Member

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The only reason the object would move "horizontally" (other than a means of locomotion) is that there is a net force incident on the object in "horizontal" direction. We apply Newton's second law, F=ma. In an ideal (unachievable) scenario, the first encounter of the ball with a wavefront would set it in motion and it would ride along, as a leaf does in a stream. But in reality the ball encounters wind, variations (turbulence), and it also has drag, so it rolls.

Trying to deduce that horizontal net force - for a real world application - takes some work. The issue here is that we have to find the total forces acting on the object. There will be the force due to gravity, in the -z direction, the forces due to buoyancy (and reaction of the water to that buoyancy), in the +z direction, and - here's the kicker - the aggregate of forces due collisions of water molecules against the surface of the object. Exchemist arrives at the likely scenario - something like a beach ball bobbing in the water, by accounting for these in general. And since it's a spherical surface, we imagine a continuum of angles at which water molecules strike the object's surface, which makes describing the physics accurately a little bit tricky.

In order to get a beach ball to remain "in the trough" you are going to need to alter it - the point of Russ's comment (make it surf). Some of the things you have to take into consideration (again, assuming this is a buoyant sphere in a wave of water) are the "drag" on the object due to the water, any drag due to wind, gravity, buoyancy, and I guess hydraulics in general . To stay in the trough, there needs to be a way to prevent the sphere from simply rolling and riding the crests as per exchemist's remark. So you will probably have to add some kind of rudder and some kind of foil, but there will always be limitations which will prevent the object from "surfing" 100% of the time, for all kinds of waves. (Hence even well designed boats sometimes capsize.)

Not sure if that was where you were headed (solving a hypothetical problem) or whether you were just looking for words to describe something physical. Feel free to say more. If you had an electromagnetic wave in mind, scratch what I said and let's start over.

8. ### brucepValued Senior Member

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Something moving laterally over the face of a wave is going over the falls. Whether it's spherical or not. Believe I'm an expert.

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10. ### sweetpeaValued Senior Member

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I'm Just wondering or maybe guessing...
I'm not sure of what Xmo1 means by "lateral motion". But reading your reply I wondered if a current would move the wavefront so to speak. In other words...if you throw a stone into a pond to make waves, you get the circular motion of a ball in the waves like you described.
But, a ball in sea waves with current, would you get the circular motion plus the water molecules of the wave moving with the current and so moving a ball "laterally"? This is a big guess

Last edited by a moderator: Nov 26, 2015
11. ### Xmo1Registered Senior Member

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The solutions depends on the particles and forces at work. But, there is no general name for the actions of things involved in transversal geometric movements, or for things moving through a field at some angle that is not in the direction of the wave. I'm guessing these things would be isolated into their own functions if their behavior could not be explained in terms of the original waves. Surfing - almost, but not on the wave, rather in the trough due to (easily) positional variations of the plane that is hosting the medium that the wave is traveling in or on. Don't follow me. I need a better understanding of things before going further. I'll get it.

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13. ### brucepValued Senior Member

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There's a good reason you don't see any spherical surfboards. A key component for a horizontal path over the wave face is the wave side surfboard rail. A key component for a stable path is the skeg [Fin]. When you're moving horizontally over the wave face the inside rail and skeg are the only things contacting the wave face. Let any part of your body interfere with the rails path and you will slow down, stall, with a big mistake you'll lose enough speed to get sucked vertically up the face and possibly sucked over the falls. Very painful in big surf. Step back or forward to change speed. Shift the weight distribution.

14. ### Aqueous Idflat Earth skepticValued Senior Member

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That being said, I might be willing to take a ride inside such of a sphere, provided it could be done reasonably safely. The barrel over Niagara falls might be compared against the laws that came up in reaction to the notorious Dr Kevorkian. In other words, at some point, I might be willing to give it a go. (in the event of my demise, I leave all my science and math bookmarks to brucep).

15. ### SpellboundBannedValued Senior Member

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Death impatiently awaits you Aq.

16. ### sweetpeaValued Senior Member

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Death is a person?