I see a little of my "essay" is redundant :D But I'll post it anyway. Sorry it's delayed, but I wanted to give this the time it deserves.
RoyStewart wrote:Surfboards are driven by gravity, and only indirectly by the wave. The amount of force driving the surfboard and rider is directly proportional to the mass of the surfboard and rider
Energy physics tells us that gravity is simply the mechanism that allows us to release the stored potential energy of the wave. Without both a surfboard cannot work. Without a wave your board won't go anywhere. Without gravity the wave is useless. One of the basic laws of physics is "Energy cannot be created or detroyed, outside of a thermonuclear reaction, only moved".
Another way to think about it. Stand on your board on a flat ocean. Where do you go? Nowhere - you need a water wave for the energy transfer to occur.
Stand on a skateboard at the top of a slope and the board will go down the slope. But at some stage something had to get you and that skateboard to the top of that slope, so going down the slope is just releasing that potential energy.
My point was that surfboards do not have "drive" in the purist physics sense. That is they are not able to produce acceleration in and of themselves, just able to store and release external potential energy. So to me words like thrust and drive used by the surfing fraternity are mostly incorrect and inappropriate. That's not a biggie... But the confusion they can lead to is.
RoyStewart wrote:Speed and velocity are the same thing sorry, it isn't possible to measure speed without a vector qauntity . . . . the only difference between the two terms (as used by yourself) is the scale. . . . speed is being treated as distance divided by time using smaller time periods whereas velocity is distance divided by time over a larger time period. . . . . this is only a matter of the scale of the measurement. . . it's like saying that millimetres don't measure the same thing as metres, because they are smaller. Velocity is just as easily used to measure directional changes as speed is, because they measure exactly the same thing.
Sorry, but from a purist physics perspective they aren't the same at all. Like IdRatherBeSurfing quotes there are accepted meanings. And when we step outside of these confusion often edges in. I think a lot of the arguing on Swaylocks (I'm not just referring to you, it happens in plenty of threads you don't post in) is a result of that.
What I am saying is that HOW you measure velocity impacts the result. If you measure it as the time it takes to get from point A to point B where the two are 100 meters apart you will get one number. If you "chop" the path travelled into ten even slices you may find that you have travelled more than 100 meters. Maybe much more.
We need to use words with agreed meanings to correctly transmit ideas without misunderstanding.
RoyStewart wrote:I do understand your point that by travelling further beween two points in the same amount of time due to directional changes en route one is travelling faster, (and with greater velocity!)
Well... Not me! But established physics. Again, it depends on how you measure things.
Something else I find interesting - these small, light boards can't retain/conserve energy. They need to expend it. And if they do that by running out in a straight line away from the pocket indefinately they'll expend all their energy potential quickly, stop planing, then sink under the rider's weight. The only way they can work is by using that energy they are hemorraging to get back to the pocket and recharge. So they HAVE to freneticaly whiz around the wave surface and constantly go back to the pocket. Bigger boards don't charge up as quickly or easily, but they don't need to because of the massive amount of energy available in a wave. And because they conserve energy better and need less energy to plane and travel they can start further out, then plane through flatter and fatter sections. That elongates the distance between the start and end of the ride.
And yes, I have made some basic assumptions about board design ;)
RoyStewart wrote:Perhaps I didn't make myself clear.
What I meant was that if a glider is scaled up it will go faster. .. . . scaled up means exactly the same shape and density, this is because the gravitational potential energy of the glider increases with the cube as it is scaled up, but the drag it produces (a function of surface area) increases with the square. . . . thus the larger scaled object will always have a better thrust/drag ratio. This applies to surfboards as well, because surfboards are falling objects just like gliders.
Gravity cannot provide potential energy because it is a constant. Poetntial energy needs to come from an active source. Like a thermal or an air vehicle tow. In a sense large wing area allows more external energy to be stored as potential energy. Also, there are limits to scaling. For example a glider cannot be perfectly scaled up past a certain point, because the wings will fall off. The design of larger craft experiences more stress and has to be altered to compensate. Aeronatics theory and application is complicated for good reason :)
Scale has interesting physics all on it's own. A bee actually swims in the air (sort of). A cat can fall ten stories and suffer no serious injury. A small child will take a fall that will do us serious damage if scaled up, but they just walk away with a bruise, crying. The forces and pressures at work change with scale. That's why a scale balsa model of a DC10 works, whereas a real DC10 made of balsa would fall apart the first time you try to us it.
RoyStewart wrote:Possibly, although I am not so sure. .. . there are a few notable swaylockians with physics and engineering degrees who are still under the mistaken impression that any given wave will drive two surfboards of differing mass with the same force. . . . hard to respect that kind of mistake. .. .I find that qualifications are no substitute for clear thinking when it comes to physics, and i must say that as far as i am concerned, you are making more sense than some of the swaylockians who claim to be correct while spouting erroneous formulae, misquoting Newton, and constantly referring to their qualifications. Personally, i have learnt a lot from MTB and Blakestah, but as for some of the other 'experts'. . .. . .
Just because someone says they have a degree doesn't make it so :) Just because someone has a 50 year old degree doesn't make them right. I was speaking of people on there who have a good grasp of physics AND it's practical application. Degree or layperson - makes no difference to me. Definately agree re MTB and Blakestah.
Wave force is wave force is wave force. There are only really a few variables at work - how quickly a craft can store energy, how well it can conserve energy and how it releases stored energy. All the other things people talk about (drag, release, rocker, rail shapes, etc) boil down to those factors.
So a given wave has the same amount of energy regardless of board. A smaller, lighter board will pick up more of that energy... And haemorrage it almost as quickly. A bigger board won't pick up quite as much that quickly... These lighter boards are more critical about takeoff, they need to "take the drop" because they need to store a lot more energy to start working. But they cant hold onto that energy. Bigger boards need less energy because they conserve it better and need to release less energy to work -- so they are more energy efficient.
RoyStewart wrote:Cheers Doug here's one from last week, with gps speeds posted after trolling through many pages of data . . .. not a fast surf by any means , and my first time in the water for a couple of months (it shows)
Looks fine to me! I like it :)
IdRatherBeSurfing wrote:so speed and velocity are different Wink . SIMILAR - but different
RoyStewart wrote:Well it is really a matter of definition. .. . I will go with yours if you like (that speed is velocity without direction)
It's not really ours, tho. It's the terminology widely accepted from entry level physics onwards. I just think that's the safest to use for the sake of clear communication and discussion.
RoyStewart wrote:An interesting consequence of this definition is that a racing car doing laps has no velocity if it returns to its original position.
Yes, depending on how you measure! This is where I previously said that there are sensible limits to how we should measure.
RoyStewart wrote:Also it seems that another consequence is that an object has zero speed if we quote the direction of net movement, rather strange if you ask me, but if it helps to promote the Huntington Hop then it's all good.
Oh perish the thought! I hate that wretched, ugly hop! *shudder*
RoyStewart wrote:Oh, and another point is that the gps unit can be set at a short interval (say half a second) and this enables it to record the speed during radical direction changes without being handicapped by the difference between the fastest moment and the overall direction of the ride., also the unit I am using takes into account VERTICAL movement as well, so there's really no way that shortboarders can complain about gps speed readings.
Intersting... Since civilian access GPS satellites have a well documented granularity of 10 meters. Perhaps your unit has some additional circuitry, that'd also account for it being able to measure vertical movement at better than 10 meters.
RoyStewart wrote:By the way, movement ALWAYS has a direction, therefore speed if defined as movement without direction cannot exist. . . . . . if speed is defined as movement without reference to direction, then the number in metres per second will be the same as the velocity over the same track.
What about a treadmill? There's speed. There's direction. But there's no velocity.
RoyStewart wrote:Speed cannot exist without direction, because movement cannot exist without direction, therefore speed cannot exist without velocity.
Speed is the act of motion. A merry go round has speed, as does a child's spinning top. But but little direction or velocity.
RoyStewart wrote:All this new distinction between speed and velocity does is attempt to show that the net velocity of a track is not necessarily the same as the peak velocity over a portion of the same track.
It isn't new, mate. It's a well established fundamental of physics. And it deals with the separation of motion (speed) and travel (velocity). What it tells us is how we measure is important... And that words are important.
RoyStewart wrote:The only difference between 'speed' and 'velocity' is that we are now supposed to assume that speed does not specify direction whereas velocity does.... . this is very misleading because all we are really doing then is ignoring the direction of movement. ... . . while the direction still exists !
Physics can be like that - sometimes counterintuitive. Intuition tells us that two spheres identical in all respects except weight will fall at different rates - the heavier will fall faster. But that isn't the case.
RoyStewart wrote:Yes, but the point is that the gps unit can measure velocity over very short straight lines, thus it can measure the peak speed and velocity of an extremely zigzagged track just as easily as a slightly zigzagged or straight one. . . . thus the distinction between speed and velocity is completely irrelevant when measuring surfing speed via gps.
See comments about satellite GPS granularity above. Some frenetic shortboarder can do a big cutback or two in a ten meter cube... Maybe travelling 20 meters. Whilst I just cruise through it at ten meters. Takes us the same amount of time.