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  #101  
Old 02-01-2011
FrankJ FrankJ is offline
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Quote:
Originally Posted by Lawrence View Post
I'm not sure I follow. When the spearing arm spears into the water, isn't the swimmer switching from having a lead arm extended on one track to having the other lead arm extended on the other track? Aren't these positions equally streamlined?
Yes, but during the time between switches, there will be a moment when both arms are not in an extended position, which should offer more resistance; to be permanently streamlined, one would have to switch only when the recovery arm also becomes fully extended, which would be sort of an extreme catch-up drill.

The way I see it, before you start extending, your lead arm is contracted. As you extend, you are getting into a more hydrodynamic position. As you are adding propulsion with your other arm, this results in an acceleration, or squirt, forward. I think it is plausible, but there may be other factors into play as well.

Regarding how rotation aids propulsion, I don't believe there must be a direct contribution, as the force is perpendicular to the direction of travel. On the other hand, we are not rigid bodies, so we may not displace water evenly as we rotate. If the front part of the body 'sinks' in advance of the legs, we may have displacement backwards (as if the body was a giant fin).

On way to test this, would be to push off the wall, and then rotate side to side. If water displacement is uneven, we might propel forward.

I also think the problem is probably quite complex, as rotation, extension etc. will also create turbulence, and I don't think the effect of that will be easy to abstract into our discussion.
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  #102  
Old 02-01-2011
Lawrence Lawrence is offline
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Some responses below.

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Originally Posted by fjconti72 View Post
Yes, but during the time between switches, there will be a moment when both arms are not in an extended position, which should offer more resistance; to be permanently streamlined, one would have to switch only when the recovery arm also becomes fully extended, which would be sort of an extreme catch-up drill.

The way I see it, before you start extending, your lead arm is contracted. As you extend, you are getting into a more hydrodynamic position. As you are adding propulsion with your other arm, this results in an acceleration, or squirt, forward. I think it is plausible, but there may be other factors into play as well.

Got it. Yes, that makes sense.

Regarding how rotation aids propulsion, I don't believe there must be a direct contribution, as the force is perpendicular to the direction of travel.

Glad someone agrees.

On the other hand, we are not rigid bodies, so we may not displace water evenly as we rotate. If the front part of the body 'sinks' in advance of the legs, we may have displacement backwards (as if the body was a giant fin).

Interesting idea. Not unrelated to what I propose regarding how weight shift causes propulsion on the other thread, namely that it comes down to causing diagonally positioned body parts to press against the water with gravity's help, albeit in your example the whole body is a 'part'.

On way to test this, would be to push off the wall, and then rotate side to side. If water displacement is uneven, we might propel forward.

I also think the problem is probably quite complex, as rotation, extension etc. will also create turbulence, and I don't think the effect of that will be easy to abstract into our discussion.
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  #103  
Old 02-01-2011
Lawrence Lawrence is offline
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Originally Posted by mjm View Post
Attempting to pull too hard or "move water" is analogous to stripping the threads on the screw. As soon as you do, the rotational forces are no longer being transmitted to the water and are therefore wasted."
That is very well put.
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  #104  
Old 02-01-2011
CoachEricDeSanto CoachEricDeSanto is offline
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Lawerence,
I have a couple more thoughts.
First, I want to confirm your position. You obviously want to have an explanation of how the spear increases propulsion using physics laws. I have seen you say that what feels right probably is right. Does this mean that you agree it does work and you just want an explanation or do you hold the possibility that the feeling of propulsion during spearing may actually be a false sensation?

Second, I am still uneasy about your 3rd law explanation of the bowling ball example and therefore my momentum explanation. I agree with your explanation that the movement of the bowling ball is that the body pushes against the floor causing the ball to move forward. The part that confuses me is this. While the ball is moving and before the string is stretched, there are no forces acting on the body in the forward direction. Then, when the string is stretched, the body feels a forward pull.

So here is what I remember. Momentum is related to mass and speed. I don't remember the exact equation, you will. So if I have a 150lb person and a 15 lb bowling ball. You push on the ground with a force required to move the 15 lbs ball 10 mph and you slide forward at 3 mph until friction with the floor stops you. Then when the ball - which experiences far less friction -stretches the string it again pulls you with the same momentum at some speed faster than the body but slower than the ball.

So, While swimming, at the end of the pull the body is moving at is fastest and you have a 150lb body and a 15lb arm moving forward together. Because the arm moves through the relatively frictionless air, it slows less than the body. The body always slows during the glide (although we minimize this with good streamlining). So if we have an arm moving 10mph and a body moving 3mph after the push, the body slows slightly during the glide due to water resistance, let's say it moves at 2.5 mph at the beginning of the spear. Since the arm has not slowed in the air and since it has far less resistance when it hits the water, the total mass of 165 lbs should move forward at some speed greater than 2.5mph but less than 10mph (I know there is an equation to determine exactly what that speed would be.)

Add to this the hugely complex matter of hydrodynamics. There have been studies that show bodies moving through water can use the eddies and vorticies coming off the body to actually push the body forward so the expected resistance to forward movement is far less than expected. So the body and arm would slow less than expected so more speed is conserved.

How does this sound? As I have said, I don't need the exact physics explanation, but I love to explore them because many students do what this level of thought.
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  #105  
Old 02-01-2011
Lawrence Lawrence is offline
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Eric, responses below in bold.

Quote:
Originally Posted by CoachEricD View Post
First, I want to confirm your position. You obviously want to have an explanation of how the spear increases propulsion using physics laws. I have seen you say that what feels right probably is right. Does this mean that you agree it does work and you just want an explanation or do you hold the possibility that the feeling of propulsion during spearing may actually be a false sensation?

I assumed steep-to-flattened spearing lends a 'squirt' forwards (i.e. some propulsion) and wanted to know why. I could be wrong: perhaps it doesn't.

Second, I am still uneasy about your 3rd law explanation of the bowling ball example and therefore my momentum explanation. I agree with your explanation that the movement of the bowling ball is that the body pushes against the floor causing the ball to move forward. The part that confuses me is this. While the ball is moving and before the string is stretched, there are no forces acting on the body in the forward direction.

Agreed (assuming the ball rolls across a horizontal surface), which means the ball is decelerating due to air resistance and drag from the surface over which it is rolling.

Then, when the string is stretched, the body feels a forward pull.

You mean the person holding the string feels a tug? The string exerts a backwards force on the ball to stop it rolling. The size of this force depends on how fast the ball is rolling and its mass (since force is proportional to the rate of change of momentum: Newton's second law). The string is attached to the person. Assuming, for simplicity, that the string is rigid rather than slightly elastic, the string is really just an extension of the person's body. So the person's body is applying the force that stops the ball. And by Newton's third law, such force is matched by an equal and opposite force that the person experiences as a tug when the string reaches its full extension.

So here is what I remember. Momentum is related to mass and speed. I don't remember the exact equation, you will.

It is mass x velocity.

So if I have a 150lb person and a 15 lb bowling ball. You push on the ground with a force required to move the 15 lbs ball 10 mph and you slide forward at 3 mph until friction with the floor stops you. Then when the ball - which experiences far less friction -stretches the string it again pulls you with the same momentum at some speed faster than the body but slower than the ball.

The ball has slowed down a bit since being released, so the force required to stop it is less than the initial force required to accelerate it to 10mph. Yes.

So, While swimming, at the end of the pull the body is moving at is fastest and you have a 150lb body and a 15lb arm moving forward together. Because the arm moves through the relatively frictionless air, it slows less than the body. The body always slows during the glide (although we minimize this with good streamlining). So if we have an arm moving 10mph and a body moving 3mph after the push, the body slows slightly during the glide due to water resistance, let's say it moves at 2.5 mph at the beginning of the spear. Since the arm has not slowed in the air and since it has far less resistance when it hits the water, the total mass of 165 lbs should move forward at some speed greater than 2.5mph but less than 10mph (I know there is an equation to determine exactly what that speed would be.)

You seem to be trying to invent a perpetual motion machine. I don't deny the appeal but I don't think you'll succeed. Remember that if the arm enters the water quickly that's because it has been accelerated relative to the body: what pushes the arm forwards must push the rest of the body backwards (unless you treat the arm and body as a single rigid object, in which case what pushes the arm forwards must push water backwards - but that isn't what you are suggesting).

Add to this the hugely complex matter of hydrodynamics. There have been studies that show bodies moving through water can use the eddies and vorticies coming off the body to actually push the body forward so the expected resistance to forward movement is far less than expected. So the body and arm would slow less than expected so more speed is conserved.

People keep using that word, hydrodynamics. Doubtless it's complex but saying that doesn't shed any light. And no one here has explained how vortices coming off the body push it forward. Maybe they do, but if they do, it's because the body or some part of it is pushing back against them. There are no freebies.
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  #106  
Old 02-01-2011
CoachEricDeSanto CoachEricDeSanto is offline
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Now the nerd in me is really beginning to enjoy this.

First, the last point. In my mind it seems that the arm recovering is moving through the air so it can accelerate against the air without really affecting the movement of the body in water. This could lead to the question,"do you feel the same boost during underswitch?" In that drill you lose much of the gravity and the arm feels more resistance during the recovery. That is why it is slower. But if we can still feel the boost in that drill, then we may have a whole new can of worms to open.

The idea of vortex propulsion is basically this. When anything disturbs water, it creates eddies or areas where the water spins in a circle. This then creates an area near the body where the water is actually moving in the same direction as the swimmer. Therefore the water pushes forward slightly. This of course does not eliminate all of the water's resistance, but it adds enough force that we can move forward more easily. I know it has been shown in fish, and the vorticees have been observed in swimmers. It is by extension, as far as I can tell, that swimmers actually get force from this.
So if I am pushing water at 5 lbs of force and the water resists forward motion 4.5 lbs the net 0.5 is what accellerates me forward. the vortex might add 0.05 lbs of force which helps a little.

I know enough to know perpetual motion does not occur. My thought is that, like my "math" for the vortex, the momentum of the recovering arm adds a small fraction of the force back. Not equal to the resistance, but enough to feel. Our arms are feeling greater pressure from the water during the pull than our bodies or we wouldn't be able to move at all. Right?
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  #107  
Old 02-01-2011
Lawrence Lawrence is offline
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More replies below on what I think are the material points. The trouble is, I think, that your analysis appeals to intuitions that run counter to physics. What I find odd about this thread is that there appear to be people here who think intuition is a better guide to how things work than science. It isn't, which is why science was invented.

This isn't to say that you can't be a great swimmer without understanding, in scientific terms, how you do it. But no one is saying that.

Quote:
Originally Posted by CoachEricD View Post
Now the nerd in me is really beginning to enjoy this.

First, the last point. In my mind it seems that the arm recovering is moving through the air so it can accelerate against the air without really affecting the movement of the body in water.

You need to read a book on high school physics. The arm can't accelerate without being subjected to a force. The air isn't pushing the arm forwards. So something else is. The options are: the swimmer's body or the water.

This could lead to the question,"do you feel the same boost during underswitch?" In that drill you lose much of the gravity and the arm feels more resistance during the recovery. That is why it is slower. But if we can still feel the boost in that drill, then we may have a whole new can of worms to open.

I never do drills and given the massive confusion on this thread already about basic physical principles I'd suggest we stick with the issues already outlined. They haven't been explained, notice.

The idea of vortex propulsion is basically this. When anything disturbs water, it creates eddies or areas where the water spins in a circle. This then creates an area near the body where the water is actually moving in the same direction as the swimmer. Therefore the water pushes forward slightly.

Do you really believe a swimmer moves forward because tiny eddies are pushing him in that direction? That is absurd, frankly. It's also an incomplete explanation. If you want to argue that eddies push forward when swirling in one direction, you should acknowledge that they suck the swimmer backwards when they twirl in the other direction. Nil yards gained.

Last edited by Lawrence : 02-01-2011 at 08:24 PM.
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  #108  
Old 02-01-2011
cynthiam cynthiam is offline
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Quote:
Originally Posted by mjm View Post
This is how Emmet Hines in "Fitness Swimming" p. 39, describes how rotation aids propulsion:
"In swimming, when the hips begin to rotate, the trunk muscles stretch and twist around the spine. When these muscles contract, the combined large forces of hip and trunk rotation are transmitted through the shoulder and upper arm to the lower arm and hand. The job of the forearm and hand is, primarily, to find and hold onto a spot in the water, allowing the hip and and trunk rotational forces to be applied linearly to the water, thus propelling the rotating shaft of the body through the great resistance of the water. The hands and arms serve roughly the same function in the water as the screw threads do in the piece of wood--to grip and hold onto one spot while the shaft rotates past that spots. Attempting to pull too hard or "move water" is analogous to stripping the threads on the screw. As soon as you do, the rotational forces are no longer being transmitted to the water and are therefore wasted."
This explanation totally captures it for me...I can feel it in my body. I know that it's been said in varying ways around here, but this particular paragraph really lit up my neuromuscular system. Thanks for posting it, mjm.
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  #109  
Old 02-02-2011
CoachEricDeSanto CoachEricDeSanto is offline
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Alright Lawerence, Now you are just being offensive. I know high school physics. I know that the arm can't accelerate forward with out a force. I never said it did. I have said that the shoulder does accelerate the arm forward. I have said that it takes much less force than most people think if the stroke finishes in a way that allows the allows the arm to flow in a circle. That way the ligaments of the shoulder can produce the force to turn the arm around and the shoulder only has to accelerate the arm slightly. I liken this to having a ball on a string and one end of the string fixed so the ball can only move in a circle. If you throw the ball tangentially to the circle, the string will provide the centripetal force needed to change the ball's direction. This allows the muscular effort you used creating force on the water during your pull to create the arm speed needed to recover the arm without speeding it up. So then you would say that the water is pushing on the hand equally. The arm is moving faster than the body, so the system now matches the bowling ball example and the body feels a forward tug. In this case, the tug does accelerate the body. As you say, the whole system (arm and body) will not accelerate, but since the arm slows to the speed of the body at the end of the spear, the body accelerates some at that point. (I can acknowledge that the arm does not need to speed up during recovery. And I can see that if the arm does accelerate during recovery it may slow the body slightly. This may explain why Terry advises to recover the arm slowly. I personally like to accelerate the arm during recovery, but this may only shorten the duration of the glide and get me to my next switch faster.)


It is also clear that the finish of the stroke can be used to accelerate the arm separate from the body. See Shinji's "underwater finish" for this concept. This could also accelerate the arm in relation to the body without affecting the force moving the body forward.

To my credit, part of my graduate work was done in an anatomical physics lab looking at the physics of shoulder movement. i do understand the concepts present here. I may get the details and equations off, that work was 15 years ago, but the concepts have been demonstrated in peer reviewed scientific research.

Second, I know the vorticees are not solely responsible for forward motion. As I said, the hypothesis says it adds a small fraction to that forward force. We know that any eddies spinning the opposite direction resist the body's movement. That is part of the resistance the water provides. The idea is that we can observe those eddies and learn to produce more that are beneficial than those that are not. This came from observations of elite swimmers and competitive swimmers that have not reached the elite level. Those at the elite level produce more eddies that benefit them than the swimmers at the sub-elite level. As I understand, and I could be wrong about this, swim scientists are just beginning to explore this possibility to see what components of this technique are trainable. Also, this hypothesis is not absurd because it has been published in peer reviewed science journals in fish models. Hydrodynamic engineers were trying to figure out how trout can hold their place in a moving stream with far less metabolic energy (producing the forces) that are required to hold a static fish model in the same stream. They found that the fish instinctively bend their body around the eddies produced by rocks and sticks in a way that pushes them upstream. When they moved their model fish in a similar fashion, the force on a string used to hold the model in place was reduced. If the strings force was reduced, then there must be another force coming from something else. The only independent variable was the movement around the eddies. So that was the only observable possibility for the source of the force. Of course we cannot match the efficiency of a fish, but we might be able to take advantage of this behavior.

And I dropped this earlier, but I have evidence that counters your jumping analysis as well. A study done at SF State, which has a very strong kinesiology and physical therapy program, looked at volleyball players jumping ability with 4 jumping methods - standing jump with the arms at the sides, standing jump with the arms starting at the shoulders and pressing up during the jump, a wind up prep with a jump with the hands held at the sides during the jump and a full wind up and jump with the arms swinging overhead. In every case, the jump with the arms finishing overhead was higher than the similar jump with the hands at the sides. So the arms finishing overhead does have some effect. Again, this was master's thesis level work reviewed by several scientists working in the field.

Last edited by CoachEricDeSanto : 02-02-2011 at 07:17 AM. Reason: More time to think about my understanding
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  #110  
Old 02-02-2011
Lawrence Lawrence is offline
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I will reply more fully later. I'm not trying to be offensive, as should be evident from the patient, clear and concise nature of my replies. I'm surprised you take offence, actually, given that your posts are based on something other than Newton's laws and, in my view, tend to get over-complicated as a result. I personally don't mind people telling me I'm wrong or should 'look at it this way' although I appreciate that isn't fashionable these days.
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