I watched the video on Schoeman's start swimswam.com/.../ and it raised a question I have had for a long time;why jump straight out from the start? Schoeman noted another swimmer who dove slightly up at the start and "stalled out"..In a previous thread Rich Abrahams said a coach told him the same thing about stalling out.The physics of this statement make no sense to me.Horizontal velocity is going to remain fairly constant,vertical velocity will decrease as one goes up and then increase again past the apex. I emailed Brent Rushall and he said to jump straight out or slightly down,but the article he referenced said
" Enter the water steeper rather than flatter (this should reduce the amount of splash (irrelevant water movement)).
Practice diving out as far as possible (maximal horizontal velocity produced primarily by leg drive off the block) before entering the water.
Dive deep so that resistance is reduced and more effective double-leg kicks are executed before surfacing."
To maximize distance(diving as far out as possible) one should angle up about 35-40 degrees(if the top of the block was even with surface of the water it would be 45 degrees(Rob Copeland said 32 degrees in another thread but: en.wikipedia.org/.../Ballistic_trajectory )
No one still does that,but some really good starters used to 1984 Olympic Men's 100m Breaststroke final - Steve Lundquist - YouTube .
When I ask coaches why the start should be straight out instead of angled up I never get an answer other than it has been found to be faster.In researching "found to be faster" I have found very little real confirmation.The best study I found(which I can no longer find the reference for) stated that the most important variable in speed to 15M was clean entry and that the greatest correlation with clean entry was experience.This also means that studies that just compare speed to 15M of different starts need to take experience with the start into account.
When I try the straight out start I have variable success with my entry(as would be expected with a new start.)I am willing to practice to get more consistent if I can get an explanation of why it is faster that makes "physics sense". I have seen too many trends in swimming change to think something is right just because everyone does it.(The first lesson I learned about starting was "ït is not a good start if it doesn't give you a red chest". I have been variously taught to swim freestyle without rolling and to kick out on BR kick so I know common wisdom isn't always wise.)
I found this discussion about the physics of a broad jump or standing long jump. It don't completely agree with their reasons for a reduced velocity due to a larger jumping angle but they did some interesting testing with real subjects and a high speed camera:
Takeoff angle vs. Speed
They fit the data to a parabolic line. I'd argue that it should be fit to a sinusoidal line. Having actual velocities gives us something to use for working out actual trajectories. From the chart, lets say the swimmer has an initial velocity of 3.8 m/s for a 0 degree start and a velocity of 3.5 m/s for a flat start. I'll also throw in the case where velocity is roughly 30% larger for a 0 degree start at 4.55 m/s (my prediction from the previous post). Let's also say the center of mass for the swimmer is half the swimmer's height or 1 meter.
For a 0 degree start, the starting height is .75 meters and the starting distance is 1 meter from the wall.
For a 30 degree start, the starting height is 1.25 meters (.75 m + 1 * sin(30)) and the starting distance is .866 meters (1 * cos(30)) from the wall.
To reduce complexity, I'll just calculate until the center of mass is touching the water and not the fingertips. I'm also using a java based projectile simulator rather than figuring out the equations on my own.
For the 0 degree start, 3.8 m/s initial velocity:
The swimmer would travel 1.49m as a projectile hitting the water at 2.49m from the wall. The entry velocity would be 5.95 m/s (Vx = 3.80 m/s, Vy = -3.84 m/s) at an inclination of 45.3 degrees. Total hang time would be .391s.
For the 0 degree start, 4.55 m/s initial velocity:
The swimmer would travel 1.78m as a projectile hitting the water at 2.78m from the wall. The entry velocity would be 5.43 m/s (Vx = 4.55 m/s, Vy = -3.84 m/s) at an inclination of 40.1 degrees. Total hang time would be .391s.
For the 30 degree start, 3.5 m/s initial velocity:
The swimmer would travel 2.16m as a projectile hitting the water 3.02m from the wall. The entry velocity would be 6.06 m/s (Vx = 3.03 m/s, Vy = -5.25 m/s) at an inclination of 60 degrees. Total hang time would be .714s.
So, the higher angled dive nets a couple of advantages:
* Longer distance travelled
* Higher entry velocity
but also a couple disadvantages:
* Longer hang time (more time spent moving vertically instead of horizontally)
* Steeper entry angle (less velocity in the horizontal direction)
Let's assume that the flat divers lose 25% of their speed in the water for the extra .323 seconds that the 30 degree diver is still in the air. The "weaker" 3.8 m/s swimmer would travel and extra 0.92m in that time while the "stronger" 4.55 m/s swimmer would travel and extra 1.10m.
At entry for the angled dive: here are the stats:
0 deg, 4.55m/s start - 3.88m from the wall
0 deg, 3.8 m/s start - 3.41m from the wall
30 deg, 3.5 m/s start - 3.02m from the wall
I don't see anything here that conclusively says that the flat start is better or worse. It seems to be a trade-off between distance and entry velocity. My big concern for an angled dive would be the steeper entry angle. The key would be in someone's ability to translate all that entry velocity into velocity towards the far wall without too much loss.
That is exactly the kind of analysis I was looking for.While you made some assumptions that may or may not be accurate,the overall gestalt helps me.This seems to imply to me ,that if one cannot transfer the steeper dive into forward motion readily that the flatter start would be better.On the other hand if you can,especially if you have really good SDKs ,or perhaps a really good BR pullout,so that going deep was an advantage then maybe the steeper start would be better.
I found this discussion about the physics of a broad jump or standing long jump. It don't completely agree with their reasons for a reduced velocity due to a larger jumping angle but they did some interesting testing with real subjects and a high speed camera:
Takeoff angle vs. Speed
They fit the data to a parabolic line. I'd argue that it should be fit to a sinusoidal line. Having actual velocities gives us something to use for working out actual trajectories. From the chart, lets say the swimmer has an initial velocity of 3.8 m/s for a 0 degree start and a velocity of 3.5 m/s for a flat start. I'll also throw in the case where velocity is roughly 30% larger for a 0 degree start at 4.55 m/s (my prediction from the previous post). Let's also say the center of mass for the swimmer is half the swimmer's height or 1 meter.
For a 0 degree start, the starting height is .75 meters and the starting distance is 1 meter from the wall.
For a 30 degree start, the starting height is 1.25 meters (.75 m + 1 * sin(30)) and the starting distance is .866 meters (1 * cos(30)) from the wall.
To reduce complexity, I'll just calculate until the center of mass is touching the water and not the fingertips. I'm also using a java based projectile simulator rather than figuring out the equations on my own.
For the 0 degree start, 3.8 m/s initial velocity:
The swimmer would travel 1.49m as a projectile hitting the water at 2.49m from the wall. The entry velocity would be 5.95 m/s (Vx = 3.80 m/s, Vy = -3.84 m/s) at an inclination of 45.3 degrees. Total hang time would be .391s.
For the 0 degree start, 4.55 m/s initial velocity:
The swimmer would travel 1.78m as a projectile hitting the water at 2.78m from the wall. The entry velocity would be 5.43 m/s (Vx = 4.55 m/s, Vy = -3.84 m/s) at an inclination of 40.1 degrees. Total hang time would be .391s.
For the 30 degree start, 3.5 m/s initial velocity:
The swimmer would travel 2.16m as a projectile hitting the water 3.02m from the wall. The entry velocity would be 6.06 m/s (Vx = 3.03 m/s, Vy = -5.25 m/s) at an inclination of 60 degrees. Total hang time would be .714s.
So, the higher angled dive nets a couple of advantages:
* Longer distance travelled
* Higher entry velocity
but also a couple disadvantages:
* Longer hang time (more time spent moving vertically instead of horizontally)
* Steeper entry angle (less velocity in the horizontal direction)
Let's assume that the flat divers lose 25% of their speed in the water for the extra .323 seconds that the 30 degree diver is still in the air. The "weaker" 3.8 m/s swimmer would travel and extra 0.92m in that time while the "stronger" 4.55 m/s swimmer would travel and extra 1.10m.
At entry for the angled dive: here are the stats:
0 deg, 4.55m/s start - 3.88m from the wall
0 deg, 3.8 m/s start - 3.41m from the wall
30 deg, 3.5 m/s start - 3.02m from the wall
I don't see anything here that conclusively says that the flat start is better or worse. It seems to be a trade-off between distance and entry velocity. My big concern for an angled dive would be the steeper entry angle. The key would be in someone's ability to translate all that entry velocity into velocity towards the far wall without too much loss.
That is exactly the kind of analysis I was looking for.While you made some assumptions that may or may not be accurate,the overall gestalt helps me.This seems to imply to me ,that if one cannot transfer the steeper dive into forward motion readily that the flatter start would be better.On the other hand if you can,especially if you have really good SDKs ,or perhaps a really good BR pullout,so that going deep was an advantage then maybe the steeper start would be better.
