In a bowling ball?
no the stored energy in the wave. Gravity doesn’t make ocean waves. It’s wind that puts the energy in the ocean, when it blows across its surface over a long enough fetch. Just like the wind can store energy by bending a tree (think of a bow and arrow, what makes the arrow fly? stored energy being released).
Yes - the original source of the energy is the sun, but as a wave rises up - you definitely have some gravitational energy stored for a fraction of a second…
Hi all,
The way i ‘resolve’ surfing in my head - its basically like skateboarding on a ramp (1/4 pipe say) who’s surface is a conveyor belt moving upwards (see wake in photo), and of course this ramp is also moving towards the beach.
The fact that the wave is moving towards the beach doesn’t really factor into it. you don’t feel yourself moving on a train. Remember we can’t feel velocity, only acceleration.
So gravity wants to pull you down and the water surface moving up the wave is pulling you up, or increasing relative velocity between the water and the board giving you a lift force keeping you above the water. And of courses there’s always a drag force proportional to velocity squared slowing you down.
It’s as simple as that (in my head anyway)
the shoreward component of the wave is what required Laird to have to tow-in to the wave in the photo you attached. And it’s what requires us to paddle into waves, which affects how our boards surf. If folks want to ignore it in the way they think about surfing, that’s ok with me. But I still believe that there’s more going on than just gravity. Surfing’s not skydiving, skateboarding or skiing. We’re not just going downhill. It’s as complicated as that (in my head anyway). YMMV, gotta go surf now, I hope our new swell has arrived.
Nice explanation Al, thats also the way I thought about it.
… However…
This picture of Rasta begs the question as to what is propelling him
the fact that waves move toward the shore is just a nuisance and required laird to increase his carbon foot print and us to waist calories trying to reduce the relative speed between us and the wave. (also think of the the flowrider)
If we reduce the relative velocity between us and the wave enough, gravity has a chance to accelerate us (overcoming drag) and then we’re ‘on the wave’.
once we’re on the wave it doesn’t matter how fast the wave is travelling toward the shore. Long period swell moves faster than the shorter stuff but I don;t imagine many of us feel it while we’re on the wave.
As for Rasta on the bodyboard - he’s a smart man and has used the force of gravity to accelerate him, he now has speed and momentum and is basically a ball of kenetic energy. How he uses that is energy up to him - but he’s certainly not wasting it
;o)
Its all about the equilibrium of forces and trimming along a wave means cutting that diagonal line, shifting weight forward to upset the balance and accelerating down the line until equilibrium is restored again.
Again, that’s my take on it.
Can of worms: what would surfing in lower/higher gravity be like?
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Hi all,
The way i ‘resolve’ surfing in my head - its basically like skateboarding on a ramp (1/4 pipe say) who’s surface is a conveyor belt moving upwards (see wake in photo), and of course this ramp is also moving towards the beach.
That is how I see it too. It is more complicated than that, but that is a good mental image.
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once we’re on the wave it doesn’t matter how fast the wave is travelling toward the shore. Long period swell moves faster than the shorter stuff but I don;t imagine many of us feel it while we’re on the wave.
This is where the wave is different than the ‘skateboard ramp with conveyor belt moving toward beach analogy’.
Different parts of the wave are moving toward the beach at different speeds, especially when you are close to the crest of the about to be breaking wave (in the curl). Relative to the trough the crest is moving toward the shore pretty fast.
It is easy to feel when you do a mellow 70’s style top turn.
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Exactly - you can’t surf a ripple, because there is not enough energy in it. My point is this… picture a bowling ball on the ground next to a U shaped ramp. Gravity exists, right? The ball is not flying up? Now, pick up the bowling ball and put it on the ramp. You’ve just added energy to the system, potential energy. Put it on the ramp and let it go, it will roll down the ramp and then fly up in the air. IMHO, it is just a semantic difference. What caused it to go back up in the air? As far as I know, physics would not say “gravity” but “stored energy”. Yes, gravity caused the ball to move down, but only because it had stored energy to start with.
Waves are similar, they are energy. What causes us to surf is not gravity, it is the energy in waves. That energy may interact with gravity, but without the wave energy, we wouldn’t be going anywhere. It’s a “but for” thing.
Keith,
As far as I know, the force that caused the ball to go down was gravity. Gravity is (in this case) the force that gives objects potential energy.
The difference is Rasta is inside the wave. I’m riding around on the surface.
sidenote: I did just learn how to harness the hydraulic happening down in the wave by sinking the board extra-deep on takeoff by grabbing very near the nose as the wave arrives, letting it come up to plane from within in the wave. The push picks up the tail better. No-paddle takeoffs, and way more waves made. YMMV. I watched a lot of guys paddle for takeoffs ahead of the wave, not get anywhere, and then they didn’t make the wave anyway. Even if they do, it looks and feels like a lot of work for no payoff if you don’t actually move until the crest bashes you forward. After the crest hits you, that’s one thing, but if you don’t move a foot until that happens and you’re paddling your ass off… I took on no-paddle takeoffs as a project.
I know empirically that the act of surfing is on and about renewing grade.
But for theoretical discussion I think this holds up:
"A flexible sheet of galvanized steel, textured slightly for traction, 500 yards wide and 1 mile long sits on a field of ball bearings. A fleet of tractors with special tires and 3 foot diameter rollers mounted on top is arrayed in a 500 yd.-wide phalanx underneath. A rider begins by lying prone on a 45 inch long skateboard of the standard equipage.
In unison, the tractors begin forward, and quickly reach their cruising speed of 15 miles per hour, the sheet of metal conforming over their rollers, causing a wave of sheet metal to process toward the rider’s feet. Shortly, they reach the point where the rider is aboard his rolling surfcraft.
The “wave” arrives. The rider rides the wave of undulating sheet metal laterally, trimming, running, sliding, carving, laybacks, cutbacks, etc, just as a skateboarder would on any other bank in the world, or as a surfer would with the advantage of an ocean wave’s forward motion.
The rider’s speed is a function of the “wave’s” forward speed in combination with his facility at harnessing the gravitational energy the renewing grade gives him. His acceleration “runs” are overwhelmingly a function of the acceleration component of gravity"
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The difference is Rasta is inside the wave. I’m riding around on the surface.
Your fins and rail are often inside the wave.
Ob… you are moving in on the ‘‘final answer’’, good job. Think bottom drag, and how it distorts the wave, and why. Question… is the reduced forward speed (energy) of the shoreward advancing wave ‘‘lost’’ or is it ‘‘stored’’ as increased energy in the increased wave height?
Surprisingly, this conversation remains civil.
is the reduced forward speed (energy) of the shoreward advancing wave ‘‘lost’’ or is it ‘‘stored’’ as increased energy in the increased wave height?
Energy can only be converted to one form or another. Some of the wave’s energy is converted to heat, some is converted to potential as the wave height increases.
I’ll leave it to some one else to further explain the implications to a surf boarder…that’s where things get a bit touchy…mostly because some of us picture ourselves only trimming in one spot on the face, others picture ourselves dropping quick to the bottom, return to the top and repeat. The former is in a state of equilibrium, the latter makes for fast changes of energy transfer.
You can delete the fin(s) and still move down the line though, although that surface area is useful lifting area. I would argue the finless dynamics in perfect trim and snowboarding down and around a perfectly conical mountain are exactly the same.
An average surfboard has fins, so drift or slide is reduced and drive increased. Drive is lift, and release is drift, correct?
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Different parts of the wave are moving toward the beach at different speeds, especially when you are close to the crest of the about to be breaking wave (in the curl). Relative to the trough the crest is moving toward the shore pretty fast.
of course you are right but I think perhaps you’re just trying to complicate what I have tried to simplfy.
either way I’m not sure this makes any difference to the “forces of surfing”, unless you want to jump on the lip and have it throw you forward relative to the wave.
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Question… is the reduced forward speed (energy) of the shoreward advancing wave ‘‘lost’’ or is it ‘‘stored’’ as increased energy in the increased wave height?
I’m not sure I understand what you’re getting at Bill and I think you may have got your answer.
energy is dissipated by a wave as is enters shallow water and interacts with the seabed, the energy goes into turbulence and moving sand etc.
The wave getting higher means that some of the kinetic energy is being converted into potential energy. Deep water waves have a perfect balance between the two.
What I don’t understand is how you think we are using the wave energy? It takes energy to pick us up to the crest (giving us potential energy) but once we drop I’m not sure we affect the wave that much (i.e. absorb some of its energy), we’re just riding along it.
Or I guess you could say the conveyor belt is constantly pulling us up… mmmmmmm
This is great stuff for keeping the brain warm in the line up
What I don’t understand is how you think we are using the wave energy? It takes energy to pick us up to the crest (giving us potential energy) but once we drop I’m not sure we affect the wave that much (i.e. absorb some of its energy), we’re just riding along it.
That is totally dependent on where you are on the wave, up high, mid way or at the bottom. Read my previous post about dynamic changes in energy transfer as it relates to surfing. Top/bottom/repeat type surfing involves fast changes in energy transfer OF THE SURFER and has almost no affect on energy changes of the wave. The energy changes of the wave happen whether the surfer is there or not. Two seperate things requiring two seperate analysis.
HOWEVER, if you know how to smack a powerfull lip and come down with it, you know the wave’s energy can affect the riders energy. A pitching wave’s lip is a high energy packed mass of moving water, moving faster than the wave itself. Good surfers have been harnessing this high energy zone for decades. Surfers who know how to smack a powerfull lip, particularly with a small lightweight board, realize this better than those who cannot.
perhaps i miss read you Bill, I think I saw anther post where someone talked about ‘energy’ and thought it was you.
so I must apologise.
so you got your answer about energy changes in shallow water waves, and i agree with what you’re saying. In fact i think we’re both saying the same thing.
back to my conveyor belt - and this is directed at non believers and not Bill.
if you flick one end of a hose, table cloth or carpet, you can make a wave travel away from you along the ‘medium’ that you are flicking.
the top of the wave is the crest and whatever the medium (water included), it must lift up to the height of the crest. So just in front of the crest the medium is moving upwards - like a conveyor belt.
the wave is a disturbance travelling through the medium, and it puts it back where it left it when its finished, other wise the water would pile up on the beach.
So i must conclude that water is moving up the face of a wave.
I have seen this on videos, but can’t find a good example now.
In fact i think we’re both saying the same thing.
but once we drop I’m not sure we affect the wave that much (i.e. absorb some of its energy), we’re just riding along it.
I dont think Bill ever said this.