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  #31  
Old 01-26-2011
Lawrence Lawrence is offline
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Quote:
Originally Posted by haschu33 View Post
The spearing arm always travels forward, and forward only. It travels with a faster speed than our body thus creating a high drag.
It never travels backwards, and it never pushes water backwards, it pushes some water down at the most.
If the spearing arm travelled forwards only it would move over the surface of the water parallel to the pool bottom. That isn't what happens.

It travels forwards and downwards simultaneously. Downwards, because (i) it is angled downwards as it moves forwards, and (ii) because it is attached to a shoulder which is dropping into the water due to body rotation.

Component (i) doesn't, I think, give you propulsion. Why? Because any force that the arm exerts on the water by virtue of (i) is exerted at the finger tips, so is negligible. The arm is cutting through the water rather than displacing it.

Item (ii) does all the propulsive work. The weight shift essentially pushes the angled arm directly against the water. The water pushes back up, in reaction. That upward force is exerted by the water at right angles to the arm, and therefore resolves into two perpendicular components, one of which is vertical lift and the other of which is forward push.
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  #32  
Old 01-26-2011
Richardsk Richardsk is offline
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I do not believe that it is true that the only to move forward in water is to push water back. It Is true that water has to be accelerated backwards but this can be done e.g. by sculling. It's an important part of the breaststroke arm action and one can move quite fast by just slicing the hand through the water in a forward diagonal motion. This is somewhat akin to the action of the propeller but of course much slower. A propeller doesn't push air back- it slices through the air at an angle and lift is created on the upper surface (or forward surface if you are behind the propeller), which accelerates a stream of air backwards and moves the aircraft forward. The airflow over the wing from the forward motion of the aircraft then creates lift in a mainly upward direction and the airplane rises off the ground. A completely symmetrical wing is capable of producing lift, although completely symmetrical wings are rare in full-sized aviation, if they exist at all - I must try to find out. I know that almost symmetrical wings are used on aerobatic planes. If lift on an airplane depended entirely on its airfoil shape then an airplane could never fly inverted, which of course many of them can do, by altering the angle of attack of the wing.

I remember reading in a children's encyclopedia when I was a boy (a long time ago) about a ship that was powered with cylindrical quasi sails so even if our arms were truly cylindrical they could under certain circumstances produce lift, although it is hard to imagine how we could move them in a rotary fashion like the cylinders on that ship.
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  #33  
Old 01-26-2011
Lawrence Lawrence is offline
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Quote:
Originally Posted by Richardsk View Post
I do not believe that it is true that the only to move forward in water is to push water back.
The universe is made that way. Otherwise one could swim in space.
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  #34  
Old 01-26-2011
Richardsk Richardsk is offline
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And yet one can travel through space and one can move water backwards without pushing it.
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  #35  
Old 01-26-2011
rcrawford2@verizon.net rcrawford2@verizon.net is offline
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It moves forward all the time.

Which doesn't mean that the correct angle will not give us a benefit. But the benefit is (neglecting the contribution to our overall movement cycle) that it creates the least drag possible.[/quote]

I like the propeller analogy. Remember, the body is already moving at a certain speed. If the hand, arm enters too far out, fighting the surface of the water, one is putting on the brakes. Too steep, also putting on the brakes. However, here is the key, "synced with the speed of your body", a perfect, angle of entry, and then straightening your arm out horizontally, will create the perfect stroke of least resistance. I think the steeper arm entry disturbs less water, a full 360 degrees around the arm. The propellor already has the advantage of being at the proper angle, both streamlined and propulsive.

http://www.courant.nyu.edu/~weinkauf...lver_c1200.jpg

Thought provoking.
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  #36  
Old 01-26-2011
Lawrence Lawrence is offline
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Quote:
Originally Posted by Richardsk View Post
And yet one can travel through space and one can move water backwards without pushing it.
One travels through space by throwing rocket exhaust in the opposite direction (the equivalent of pushing water backwards).

You can't move water backwards without pushing it.
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  #37  
Old 01-26-2011
jeetkevdo jeetkevdo is offline
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Interesting discussion... I think I actually understand this a little better now.

Is the action of the forearm dropping at the correct angle squeezing/propelling water backwards?

Kinda like stepping on a tube of toothpaste???

-Kevin
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  #38  
Old 01-26-2011
Lawrence Lawrence is offline
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Kevin, that is exactly my understanding.
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  #39  
Old 01-26-2011
KatieK KatieK is offline
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Quote:
Originally Posted by Lawrence View Post
To make sense of this I think we need to distinguish (a) vertical lift from (b) forward-directed propulsion.

Item (a) gets you nowhere fast although it may help you avoid sinking.

Item (b) is what is in point on this thread, if we're trying to explain why a steeper entry angle provides a seemingly effortless 'squirt' of propulsion.

The only way to be pushed forwards is to push backwards on the water. That's just Newton's third law of motion.
Until a few days ago, I would have been in 100% agreement with this statement. However, based on what I've learned thru this fascinating thread, I think it's more complex than that.

I read this article Dave Barra posted. http://www.coachesinfo.com/index.php...ing&Itemid=138 Based on my (limited) understanding of that article, I think the Bernoulli principle plays a part here. Because the arm is curved, the water is moving faster under the arm than above it. I believe this difference creates some forward propulsion. I don't understand it well enough to say anything more than that.

A few more things:
1.) Boken--awesome video!
2.) Richardsk--thanks for the really interesting insights
3.) As much as I LOVE the theory, I agree with Terry that I don't have to understand it to practice it.
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  #40  
Old 01-26-2011
terry terry is offline
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Boken
Your experiment, and documenting it on video, was inspired. I was particularly struck by the fact that the spatula wanted to 'ski' to the bottom an angle almost exactly like that of Mail Slot entry. When you placed it flat, it would wobble, until finding that angle, then shoot forward and down. And when you released it at that angle, its path was straight, true and sure.

I'm not certain exactly how to interpret this but I take encouragement that it reinforces the hydrodynamic merits of our Mail Slot technique. Which empirical experience had already confirmed for most of us.

I also find it most appealing to think of the arm itself skiing down a ramp composed of viscous water, which has been somewhat compressed by the arm's passage.

It doesn't help me swim better. It mainly entertains me, both while swimming and at other times.
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