Friday, April 13, 2012

You Do The Math - How Much Does a Discus Weigh?

I have been noticing for years that when throwing the discus, the kids can't hold onto it when they put all their effort into the spin. So, I made a spreadsheet using physics, neglected wind resistance completely, and found out that a 50 foot throw would make a 1 kilogram discus feel like 50 pounds, even though it really only weighs 2.2 pounds.

Here's a colorful diagram with the physics equations for the spin alone, not projectiles. Ever hear of "paint by numbers"? Well, this is "numbers by Paint", Microsoft Paint, that is.

So, today I made a couple videos of one of the kids throwing about 65 feet. With the camera, stepping through the video frame by frame, it appears that he required only 2.75 frames to do the last quarter turn before release at maximum speed. In his second toss it looks more like 2 frames. Here's that video.

With his radius of spin at 2.5 feet, from the center of spine to curled fingers, the slower spin makes the discus feel like 50 pounds, while the faster spin makes it feel like 95 pounds. It sounds like a lot, but I know that when I throw it 130 feet, it rips itself loose from my grip all by itself, I don't have to let go. And if I try to throw 140 feet, it releases prematurely, and it goes out of bounds. Sometimes it goes backwards.

The moral of the story is... make your kids do pull ups! If an 80 pound kids can yank himself or herself up at 1.5 g's, then he/she is lifting 2.5 times his/her weight. That's 200 pounds, so it puts 100 pounds in each hand, which is good for a 40 mph toss.

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