Feb 27

Newtons laws of Motion

Sir Issac Newton is hearlded as one of the most brillant minds ever and if he were alive today, most likely he’d love wakesurfing, because it exemplifies his three laws of motion. We want to explain one phenomenon associated with the bucket board and then also apply it to a strange situation for tall but light weight folks.

Newtons third law goes something like this: When a body exerts a force on another body, that second body simultanesouly exerts a force on the first body equal and oppoiste.

Yikes! What does that mean! Ok, you push against a wall and nothing happens, hopefully. The wall is actually pushing back with an equal force and in the opposite direction as your push. It’s possible to push the wall down, but it requires a net force that is LARGER than the wall can push back with.

Now we know that gravity and our weight have a downward force. We won’t have THAT discussion, but trust us on the whole gravity thing. We are able to balance that downward force against the upward force of our wakes and we call that wakesurfing! What we use as the intermediary is a wakesurf board that collects lifting forces on one side and matches those with the downward forces from our weight and gravity pushing down. In effect our wakesurf board is a collection device on one side and a distribution device on the other side. The net balance of those two allows the wakesurf board to sort of skim across the surface. We used hydroplaning in the previous post for want of a better term, but what it’s doing is collecting lifting forces from underneath and spreading our mass across that collection system to balance the two.

Let’s define a balanced system of wake forces UPWARD, wakesurf board collection system and then a rider with weight and gravity pushing downward. The rider is in trim following the boat and having a nice tie.

Here is a marvelous hand shaped rendering of that concept.

diagram 1

If you look at the wakesurf board, such as it’s depicted, what does it do? It captures the flow of water UP the wake allowing the rider to balance his weight against the lifting forces. So our wakesurf boards are collection devices, in a sense.

What happens when we change out the rider? Let’s make him weight 400 pounds. Ok save from the likelihood of a stroke, our collection device is going to need to capture MORE of the lifting forces, to the tune of a total of 400 pounds before that new rider can wakesurf. It can do that with length and/or width. It’s the whole wetted surface concept that is talked about. You get the idea, a larger board or more of the existing board must be utilized. The typical response is to go longer, but really wider does the trick also we just need to capture more of the lifting forces when we introduce more mass on the top side of that equation, so to speak. Most likely if the wakesurf board/collection device was appropriately sized for the first rider, it won’t work for the 4-hundred’er. Effectively it would need to capture twice as much of the lifting forces and most likely there won’t be enough wetted surface area or effective shape to capture the flow.

Now let’s go the other way, let’s cut the rider weight in half. So now all we need is 100 pounds of lifting forces. We know from experience that a smaller/lighter rider can almost always rider a bigger wakesurf board and now you know why! As a collection device the wakesurf board sized for the 200 pound rider is under-utilized when there is a 100 pound rider on top.

Here is what that concept looks like in hand-shaped goodness.

diagram 2

You’ll notice that we red X’ed out 50% of the lifting foces. Wouldn’t that be cool if we could just do that? Insert or delete lifting foces with a crayon???!!! How does the Rider actually achieve that? That aren’t balancing on the same amount of lifting forces, becuase we know from Newtons 3rd law, they are equal, if they aren’t equal the rider and collection system get push up and off the top of the wake. You gotta love physics! So what does the rider do? Most likely he/she moves backwards to a point where the collection device (wakesurf board) is ONLY collecting 100 pounds of lifting forces.

We’ve all experienced this where smaller folks are all the way at the back of the board, riding it like a wakeboard almost. So Let’s say that both the 200 pounder and the 100 pounder can ride the board, would the stance be the same between those two riders? Not in trim, because to the lighter weight rider, according to Newtons Third law, will need less balancing forces and will ride in a position which captures less of the forces. Now some of that will happen simply because there is less weight lifting the board but also, the rider will wind up riding further back on the board to balance the change in the center of the lifting forces.

What happens when those riders go to lean forward? Put more weight on the nose of the board to make it turn down the face of the wake? The heavier rider is going to have an easier go of it, isn’t he/she? His/her weight is more centered over the board, probably his center of gravity is closer to the apex of the wakesurf board and so he/she will be able to engage that quicker.

Can you guess what happens with the lighter weight rider, if it’s the same board as the 200 pound rider? If the lighter weight rider is capturing less of the water flow along the rails and really the whole bottom surface of the board, he/she is going to have to shift further forward on the board to engage more of the rail closer to the apex, right? That riders center of gravity is further back and so MORE of it will be reguired to shift forward to offset the increase in lifting forces engaged by the board.

What we think happens for TALLER yet LIGHTER riders is that their stance is probably the same as the heavier rider, but their body position isn’t. BUT also the rail line that is engaging the wake is less than the rider that weighs more. It’s not really sitting deeper in the water, that’s a misnomer, but there is more of the wake lifting the bottom of the board. It’s probably both longer and wider that the bottom of the board is engaged with the wake.

As the lighter weight rider goes to engage the rail line, he/she has to really move much further forward than the heavier rider before that happens.

What we see with that issue, is that if the board is identical, lets say it’s the same board AND the riders are the same height and have the same stace and one isn’t a super buff body builder, just the same basic muscle strength, the lighter weight rider will have to be further forward on that same board in order to place enough weight on it to counter act the increased collection of the wakesurf board. Longer board, right?

There are a number of ways to effect changes here. Lighter weight riders can rider narrower boards than heavier riders of the same height and stance and physical ability. They can also ride boards with more rocker or even with an apex further back.

BUT if the board is exactly the same, and the wake forces are exactly the same what happens? The lighter weight rider has to shift his weight forward or sideways or something MORE than the heavier rider in order to effect the same amount of net force, because well he/she is lighter.

Here are some more hand-crafted drawings to represent the concept. The heavier person

diagram 3

The lighter person, note the repostioning to effect the same net downward force.

diagram 4

Now there are a LOT of “if everything is the same” in there. So we can’t compare aplles to oranges. If you stick a 40 pound kid on that board with a teeny-tiny stance it’s not the same thing. Our theory here is that everything is the same except the weightof the riders. Imagine the same rider/borad/wake combination but in the first instance that rider has a 100 pound backpack on and then in the second instance he’s chucked the backpack.

Would they ride in the same position to start with? Nope, we know that answer already and we also know that leaning forward while wearing the 100 pound backpack will be more effective than without it! 🙂

So that’s our Physics discussion for today as we observed from wakesurfing the bucket board and it’s has interesting implications for wakesurf board design that we mostly don’t see employed.  Thanks for sticking around and reading it all, we were working had to describe our conclusions.  Most likley you’ll have different conclusions as this is certainly not an exact science!

Thanks so much for following along, we appreciate it!


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