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rotational and translational into translational

Posted by: ray porco (rporco@verizon.net) on Sun Jul 3 18:57:22 2005

The following article is extremely interesting (for me anyway) and can (my opinion) shed light on recent discussion.

Conversion of rotational velocity to translational velocity.
“Blocking” (tripping effect) and momentum transfer.


Throwing Muses
Why throwers throw the way they throw.
By Nick Schulz

The throwing events—shot put, discus, hammer, javelin—are old. Really, really old. In this year's games, athletes threw the shot in Olympia, on the same site where athletes tested their strength thousands of years ago.* But even though this generation's throwers have much in common with their ancient counterparts, the "heavy" athletics have changed significantly since the olden days. For one, the drugs are better. And those ancient Olympians didn't spin around in a circle before they heaved a stone.
Before the 20th-century spinning craze, shot putters and hammer throwers stood still, either behind a wooden block or within a wooden square. Discus throwers faced a far more difficult challenge: In the first modern Olympics in 1896, they threw a wooden disc with a metal rim from atop a 60 centimeter by 70 centimeter pedestal. Again, the best technique was to stand still—if a thrower got a little too creative with his body movements, he might fall from his perch and hit the ground.
It's not easy to generate power when you're not moving. If you want to toss a heavy object a long distance, it helps if you generate momentum behind it. A pitcher creates the momentum necessary to throw a baseball by simply moving his arm like a windmill. But a 16-pound sphere is way too hefty to just toss like a baseball. In order to create enough momentum to propel the shot or the discus, throwers have to turn their whole bodies, not just their arms, into windmills.
As modern athletes figured out that spinning fast meant longer throws, all the throwing disciplines, with the exception of javelin, moved to spin-friendly circles. By the 1950s, concrete surfaces and smooth-bottomed shoes grew universal—it's tough to spin with spikes on—allowing for more and faster twirling.
But even though the shot, discus, and hammer have all moved toward circles, spins, and concrete, their techniques won't ever be precisely the same. The different ways the objects are held—the shot must be in the crook of the neck at all times, while the hammer and discus can be swung freely—and the mass of the objects are the biggest influences on technique. As a thrower spins, his body acts as an axis of rotation. The farther the object is from the axis of rotation, the larger its "moment of inertia," and the more resistant the weight will be to rotation. (The moment of inertia is equal to the object's distance from the rotational axis squared times the mass of the object. Translation: If an object is heavy or far away from the rotational axis, it's harder to get it moving than if it's light or close to the rotational axis.)
Because the shot is held close to the body at all times, it's always in extremely close proximity to the thrower's rotational axis. That means throwers need just a short, quick burst to reach maximum rotation speed. Additional turns won't generate more power and will just risk a loss of control. Since the discus is much lighter than the shot and hammer—4.4 pounds versus 16 pounds—throwers can hold it farther away from the body and still get it moving as quickly as the shot. The hammer, which is swung around on a wire, is farther from the athlete's axis of rotation than the shot or discus and requires more spins to reach top speed. Indeed, hammer throwers would take more turns if the throwing circle were bigger or their feet were smaller—some female throwers with smallish feet spin five times, compared to the typical three or four.
It is possible to throw the shot without acting like a dervish. For a long time, the most popular shot technique was the "O'Brien Glide," invented in the early 1950s by American Parry O'Brien. Instead of just pointing his body in the direction he wanted to throw, O'Brien faced the back of the throwing circle and, by shuffling his feet and turning 180 degrees, threw his body weight from the back to the front of the circle. Using this technique, O'Brien broke the world record 16 times. But in the 1970s, converted discus thrower Brian Oldfield adopted the rotational, or discus-style, spin throw and crushed the glide world record. Today, most men are spinners, including all of the U.S. team, but there are both glide and spin throwers competing in the Olympics. Decathletes in particular favor the glide because it's more consistent and easier to learn than the spin.
The evolution of javelin technique has been a bit more haphazard and controversial. While the spears have pretty much always been chucked overhand, in the 1950s some converted discus throwers started tossing the javelin after completing a 360-degree turn. These renegade spinners put up impressive distances, but didn't necessarily have great control over direction—sometimes the javelin would come out sideways, menacing spectators. In the 1950s, the International Association of Athletics Federations decided not to tempt fate and banned the erratic spin throw from future competitions. In this year's Olympics, you'll only see the javelin thrown by athletes who run straight ahead down a long runway. We're probably safer for it, but who are we to fight the laws of physics?


For additional info on javelin spinner throwers (if you care to) see:
“All about the ‘Spanish-style’ javelin throw technique.”



So what’s the point? The point is, that translational velocity is the goal, but rotational velocity is the best way to achieve maximum translational velocity.

There is another point. We may sacrifice control for achieving maximum rotational velocity.

Cheez! Sounds like we can draw parallels from Olympic competition to another sport I know.

What is the physical goal of a batter in a baseball game? To transfer as much momentum (whichever way we can) into the sweet spot of the bat, so as to hit the baseball ON A LINE. Our goal is a linear goal. The discus, hammer, shot, and javelin throwers all try to throw their respective implements in as straight (and long) a line as possible. They use max rotational velocity to achieve max translational velocity, but they have a huge advantage over the baseball batter, - they know WHEN to release.
For a batter to be accurate he must keep the sweet spot (or as close to the sweet spot) of the bat in the hitting impact area as long as possible. And since the ball is coming at the batter in a line (more or less), the more straight in line with the ball the sweet spot is, the more accurate. So there is a trade off and it’s up to the batter to decide what he wishes to achieve.
Don’t you think it would be wise to keep at least some linear movements involved in a baseball swing? Do you think the hands really go in a circular path? Or curviLINEAR?
Is the linear stride solely a timing mechanism, or does it provide a “blocking” or “tripping” effect? Does the body stop it’s linear movement at “toe touch” or later?

Rotation is king, but don’t discount linear movement. For examples of how rotational motion, linear movement and “blocking effect”, ALL contribute see:


Timmerman – shot – O’Brien Glide linear movement, blocking, and push.

Barnes – shot – no glide, instead rotate (but in linear line). Rotation on left leg, then spin on right leg. Then left leg blocks.

Riedel standing – discus – no stride. Rotation around left leg and left leg blocks.

Riedel – discus – rotation around left leg, spin on right leg, block with left. Linear line.

Litvinov – hammer – rotation around left leg with blocking action at finish. Movement in linear line from rear of circle to front.

Zelezny – javelin – running in linear line with blocking.


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