Keith Melville for President!
“Melville says: I won’t get into pissing matches, so whatever!”
Good enough for me Keith!
Your Idiot Friend Finds the Line-up.
Note: If you object to something, please read the PS below.
You’re spinning a rock attached to the end of a string in a vertical circle (see figure 1.) You’re standing in one place, that is you’re not moving, just the rock around the circle. The rock will have the velocity at the position shown, in the direction indicated. You can work out what its velocity will be, at least its direction, at any other position by just remembering it will be in a direction tangent to the circle.
You then start to walk forward still spinning the rock, the velocity of the rock will have the velocity as indicated in figure 2. Remember now, it had a velocity from spinning around and now an addition forward velocity; its total velocity will be the vector sum of the two.
At some point you realize you’re moving towards your idiot friend. You idiot friend stands still; he’s an idiot.
Your friend ducks under your arm as you move past and the rock misses him, sadly however the rock connects on its upward path and hits your friend in the nose. As your friend’s head bobs back he utters ‘Shit!?’ Before his head has a chance to completely bob back to its initial position, the rock having come around again, hits him again. Why, because you’re still moving forward and have closed the distance even more. Again your friend’s head bobs back, and he utters ‘Shit!!!’ This time he steps off to the side. Luckily he’s an idiot and doesn’t proceed to beat the living crap out of you.
Your other friend, who is smarter than you, is also twirling stuff, in fact he’s twirling two rocks and walking directly towards your friend. Once again your idiot friend manages to escape the rocks at first, but gets clipped in succession by both rocks on their upswing. Your smarter friend continues to walk on. “Dude, what’s up with the rocks!” your idiot friend proclaims and turns and steps off to the side.
But he really is an idiot, for in stepping off to the side your idiot friend has placed himself in the path of your really smart friend who’s twirling more rocks than you could possible count (at least while they’re spinning.) Once again, at first your idiot friend manages to duck out of the way, but then like some bad scene out of a Three Stooges movie, your friend’s head is machine gunned back until he manages to turn away once more.
Your idiot friend, dazed by getting hit with all the rocks looks at you and proclaims, “DUDE?!” You rest your hand on his shoulder and say, “Dude, your head just went surfing!” Your friend says “How’s that? Surfing’s a gravity sport?”
Kevin
PS
If you’re interpretation of this is that I think the same water particle moves with the wave right to the beach, then just start a new rock moving with each cycle, leaving the old rock more or less in the same position in which it was started - same result.
If you’re interpretation is that none of this will work in outer space, you’re right, it helps to have gravity to make sure your friend doesn’t fly off after being hit by the first rock.
If you’re wondering how this relates to water particles in a wave, in particular how water particles might be held in their circular paths like rocks on a string, its all about the unique intermolecular forces that exists in a liquid, and the way disturbances are carried through liquids. For a greater and far more knowledgeable and detailed explanation, please see any (at least that would be my guess) introductory physical oceanography text regarding gravity waves. By the way, in the example, you’re not meant to be a wave; you’re just there to make sure the rock travels in a circle. (That’s not to say that you’re not a wave, go with whatever works for you.)
If you’re wondering how a lot of rocks coming one after the other constitutes a flow, like water particles in a wave, because ‘rocks don’t flow, they tumble, or spin or something like that’, then you’ve got me.
If you’re wondering how your idiot friend managed to miss getting hit with the rocks before ducking under your arm, it’s because he’s an idiot.
If you’re wondering how a rock on a string could somehow come up and around from below, as if it came up between his legs, and hit your friend in the nose and then continue on in its circular path; it’s because he’s an idiot.
And finally, this example neither proves nor disproves anything.
Disclaimer: Any resemblance of any of the participants in the above example to known individuals is purely unintentional.
Gah! Loads-a-posts!
Here’s how I see the propulsive forces at work from most important to least…
-
wave height, translating to potential energy that is tapped via gravity.
-
wave forward motion.
-
upflow.
-
rider input.
1 and 2 are close togethor, sure. But in my books the rest won’t happen without #1. So that represents the primary energy charge which starts the whole thing. And when you go back up the face you recharge using that again. I don’t remember ever seeing a good roundhouse cutback on a 9" wave, for example, regardless of it’s forward motion.
I believe that the primary propulsive energy we tap has a direct relationship to the amount of energy in the wave THAT WE ARE ABLE TO HARNESS. This can manifest in a number of subtle ways, but when it comes down to it wave height and period (plus steepness) are the tell-tale signs.
I also agree that everything everyone is saying (so far) does have a place - yes, including concaves.
“The only water particles that matter to surfing are on the surface.”
I disagree! The rising water particles inside the wave give the wave height, which translates to potential energy. So I assume you are talking about upflow in terms of how it directly interacts with the craft, rather than how it affects the wave.
“ALL molecules have energy”
Yes - but aside from potential energy this is almost impossible to tap.
“I’m referring to water molecules, not the wave. We only surf in water as deep as our fins.”
So how does the wave get it’s height? It certainly isn’t as big/steep in the open ocean, even tho the energy is great in open ocean waves. When the wave approaches a break and the H2O molecules start to get pushed up (sucking water away from the front) we get wave height. So the whole wave really does matter. even tho we feel as tho we only interact with the surface 1’ or so.
“Are you talking about wind or something like a boat bumping into him? Lets try to remove variables, not add them.”
No, the mass of the surface+craft and the weight of air.
“It’s just gravity.”
No! Emphatically. It isn’t. Gravity simply allows us to harness wave energy. Physics is quite clear on this. If gravity and gravity alone was at work you would have a really hard time travelling faster than something like 20km/h. Yet surfers commonly exceed this. it’s about using gravity to tap into the energy band inside the wave. And that can and does happen via at least three paths I can see (there may be more).
Keith Melville is pretty close to the mark IMO…
KCasey, Silly, janklow, kenz… I suggest we start to find points we agree on so that we can approach our differences of opinion from a clearer, more adult perspective.
Hell, I don’ even care any more–Keith Melville has it right–what we had before this bunch of ah, posts, was good enough, nobody ever noticed the other thing, and nobody ever minded
I suggest we actually go on with our lives
I just find it hard to let things like this bunch of, ah, theory go unchecked, esp. with the political provocation
But whatever
You transposed #3 and #4 by the way
See ya elsewhere
; )
Doug,
I’m not sure that’s the way it works, finding common ground that is.
There is nothing I’ve written that I believe requires retraction or is incorrect. Though that doesn’t mean something won’t be put forward in the future and change everything. ‘Falling and not falling’ to proposing a yet to be discovered way wave energy is transformed into surfboard motion are reasoned arguments, they’re just not reasonable ones, (in any remotely mechanical way.)
I believe what I’ve been saying is actually very clear. Force is required to move a surfboard, and I’ve pointed out what I believe that force is, where it comes from and how it makes surfboards go. And I’ve not evoked mysterious transfers of energy nor felt the need to create a fog by spouting a lot of hydrodynamic terminology or use oxymorons to explain things. Energy itself doesn’t do anything. You need a force to make things go, or at least to start them on their way.
You may not like what I have written, nor the way I’ve written it, but that’s not an argument against it.
I’m not looking for a consensus. I just wanted to finish something I started on Swaylock’s awhile back.
Kevin
There may be something to what you are saying, but it amounts for a small amount of the reason a surfboard surfs compared to the number of words it takes you to describe it!
Surely most of how it happens can be described thusly: wave energy moves towards shore, bottom gets shallower, water rises, surfboard rises with water, surfboard falls down to bottom of wave, uses momentum to carry back up to top of wave, falls down again…repeat as necessary.
It would be nice if you’d explain, in one simple sentence, why you think there must be more to it than “gravity”.
janklow,
“You transposed #3 and #4 by the way”
Maybe, maybe not. You are headed off so I guess it doesn’t matter 
I dont think the amount of force from either is that significant. Now the control the rider exercises against the board is, of course, critical. but that’s the rider manipulating existing forces to a great extent (critical though it may be).
KCasey,
If you aren’t looking for any kind of agreement at all why bother posting it on here? And if you are looking for the truth why are you eager to simply discard what I have to say?
I said a while ago that I was listening and pondering. And that I’d get back to you when I had formed a solid opinion AND share the facts and conjecture I had. you seemed happy with that. Now you start talking about whether I “like” what I have written? Like doesn’t come into it. it’s the search for truth.
I’ve done the maths on a significant portion of what I believe AND discussed it with other people who agree with what I am saying. Don’t take any of this as me attacking you or defending myself. i don’t care about any of that. I care about honest and open discussion… Which leads me to…
You say energy and potential energy don’t matter. OK, tell be what causes water molecules to move?
If you take a rock and carry it up a 100 meter cliff and drop it over the edge whay does it move? That isn’t gravity on it’s own (otherwise a rock sitting on the ground will have the same force as one in motion).
It’s potential energy (Ug=mgh) being released via gravity. Simple. The Terminal Velocity (Vt = sqrt ( (2 * m * g) / (Cd * r * A) ), Momentum (p=mv) and Drag (D = Cd * A * .5 * r * V^2) equations come into this too. These are mathematically verifiable and I have put in some effort to work through those calculations. Naturally there are other more complex principles and equations involved but these macroscopic aspects of physics come back to Newtonian and Einsteinian principles, and those principles state that energy is everything.
These calculations easily and relatively accurately account for the travel speed of a given board on a given wave.
Now, what widely accepted physics have you referenced to support your position? One person. Some diagrams. And an unverified (AFAIK) formula. THIS IS NOT DISAGREEMENT, it’s a statement of my opinion and the evidence I have to support it. I believe that there is some merit in what you have been saying. But my OPINION is that you have overestimated the impact of the upflow component somewhat.
Lets put aside ego and discuss this maturely… Scientifically.
MaraboutSlim,
I’m interested to. In my opinion it’s something like “when triming across a clean waveface the upflow allows the board to climb without direct rider input (ok, aside from setting the rail), from where the board may once again slide down the wave face”. I can certainly see that in longboarding… Noseriding in particular.
Peace all.
can any of you gravity heads tell me where it comes in to play ( other than the confines of our planet)
if your riding on your gut on a 1/2 foot wave
on a body board
in front of the whitewater
your not up the top of anything! (except water and the body board)
you are at the bloody bottom of the wave out in front!
but your still moving
wtf is going here ?
Kevin :D:D
love the rocks!
just get in to man
you go!
its coolies
very entertaining
and seems as reasonable explanation as any other ive heard
i like it!
if someone doesnt like it, they can find stimulation elsewhere
Quote:
can any of you gravity heads tell me where it comes in to play ( other than the confines of our planet)if your riding on your gut on a 1/2 foot wave
on a body board
in front of the whitewater
your not up the top of anything! (except water and the body board)
you are at the bloody bottom of the wave out in front!
but your still moving
wtf is going here ?
Easy. The wave energy is continually lifting up the rear of your body or mat or surfboard as it reaches you. You remain in balance - the same amount of lifting as gravity is pulling you back down and since the wave (energy) is moving towards shore, it moves you towards shore as you stay in that balance. But it’s not really pushing you forward so much as it is pushing you upwards and you are falling downwards and therefore keeping up with it.
Lean back so that more of your body is underwater too much and the orientation of your craft/body is more vertical and there won’t be enough surface for the wave to lift up on, and the wave will pass you right by. Just like all the beginners failing to catch waves from way back on their longboards - they sink the tail and therefore allow wave energy to pass over the board instead of allowing it to lift up the tail as it would if they pushed the nose down more and kept the tail up.
Do I need to draw a picture?
you could draw a picture if you like
but it wont be of what is acutally happening
i could be wrong but this is how i see it
if your body surfing in the white water
most of your body is submerged in the foam.
sometimes you can put your hands out in front to plane a bit
so you can keep going further in and keeping your body rigid
i cant see the foam lifting you other than through bouyancy.
i dont buy it.
its the water pushing you!
to get that minimum speeds on land on snowboard you need a fair incline to get up and running
you cant just say it just gravity mate
shure it plays its part
but basically your saying
other than lifting you through bouyancy
and having a surface to plane down with gravity as the attractant force that moves a surfboard
thats the only role the waves plays!!
that is pretty narrow minded
just put a rubber duck in the water
or a balloon
and watchit get washed in by a wave
it aint gravity!
its the moving water moving it along
a wave
its water that moves
Marabout
i suggest you stand in a heavy shorebreak and wait for a wave so you can experience water moving
and how much forward velocity it has .
i gurantee it will knock you off your feet
and gravity will assist your decent!
unless you brace yourself
and reduce the surface area you are presenting to the wave(ie. dive through it)
keith
I agree
doug i agree(and yes i meant mass of the rider and air pressure)
kevin I see your point
MS is somewhat right.
But IMHO the forward motion of the wave does impart some energy.
It goes something like this in my rough calculations so far:
60% potential energy
30% forward energy
5% rider energy
3% upflow
2% other sources
The lower percentages are necessarily guesses. And all of them are rough estimates based on real calculations (although some of them were simplified somewhat). For one thing Cd is nearly impossible to calculate 
Anyone want to point out where I am wrong? Or need to see the figures themselves? I am listening because I don’t claim that I am going to be right. You SHOULD question it and work it out yourself.
Silly - are you saying that a craft cannot move faster than the wave? Or that the drop imparts nothing to the ride?
Cheers!
Quote:
i suggest you stand in a heavy shorebreak and wait for a wave so you can experience water movingand how much forward velocity it has .
i gurantee it will knock you off your feet
and gravity will assist your decent!
unless you brace yourself
and reduce the surface area you are presenting to the wave(ie. dive through it)
Shorebreak is a little different situation, sure. But it’s not so much the forward movement of the wave that knocks us off our feet. It’s the downward movement of the broken waves water as it falls. If I wade out into your imaginary shorebreak just a few more feet so that the wave is still a swell when it hits me, it’ll basically just lift me up and set me back down again, perhaps pushing me very minorly towards shore. Right? And the more horizontal I am on the surface of the water, the less it’s going to push me towards shore. It’s only when the wave is strong/steep enough to lift me up enough and give me a sloped platform to slide down that I begin to move towards shore with any sort of speed.
I like that list Doug. Here’s mine:
what moves a surfboard:
95% stuff that can be described in one paragraph a kid could understand
5% stuff that took page after page of technobabble
lmao:D
i think its moves you closer to the shore say about 1/2 meter maybe if its in close
but yeah not much.
but if your rigid and its comming to meet you,
then thats different!
like being in a moving car and the car stops but you keep going to some extent unless your part of it(starapped in).
yes gravity makes the lipfall down
but at the point where you can stand on the shore and the wave is a wall and hits and then it breaks!
When a surfboard is neither rising nor falling no work is being done against or by gravity.
Doug,
Energy is an accounting principle it doesn’t make things go.
I suggest you read an elementary physics text, say The Feynman Lectures On Physics (ISBN 0-201-02016-1-p).
As for agreement, I’m not running for political office. It may be a shock to read, but people agreeing to something, doesn’t make that something true, or correct. ’What makes surfboards go’ isn’t Global Warming, a consensus isn’t required.
And by the way, forces make things change the way they move, not energy. Whatever you believe, in the end, if it doesn’t produce a force, then nothing will change the way it’s moving –i.e. change its state of motion (see Newton’s First Law, it’s also discussed in the above mentioned reference.)
Kevin
Roy,
You might want to recheck your discovery.
Pressure = Force/Area = “newton/square-meter”=“(kilogram meter/square-second)/square-meter”
Density = Mass/Volume = “kilogram/cubic-meter”
Velocity = Length/Time = “meter/second”
velocity times velocity = “square-meter/square-second”
density times (velocity times velocity) = “(kilogram/cubic-meter)(square-meter/square-second)” = “(kilogram meter/square-second)/square-meter)”
= Force/Area
If you’re having trouble with exponents, I suggest reviewing an elementary Arithmetic text.
In the meantime, given that you’ve already discovered the edit feature on Swaylock’s you might want to create another ‘post without posting’.
Kevin
PS
Before you make demands of me you might want to try and understand the terms you’re using to describe whatever it is you seem to think is happening. Energy doesn’t move anything; force is required to change motion. I suggest reviewing an elementary text in physics (see above post to Doug.)
Get any emails back yet?
I can’t speak for anyone else, but I know, I want to know.
Thanks,
Kevin
I suggest you consult an elementary text in manners.
Nope.
what’s the optimum trim angle and maximum lateral speed on a (isolated) wave-perpendicular upflow of 4 miles per hour? Like if you had a longboard and a perfectly straight member (with a perfect rope and a rolling cross pulley, okay) erected across a 4mph river …
nose (right) 30 deg? 45? 3 mph? 4?
I have to go now.
This whole thing is the very definition of a tar baby.