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08 January 2011

Mail Time: Bernoulli experiment troubles.

Michael Gray, of Georgia, writes:

I have been preparing the Tennis Ball can experiment, but the water doesn't go quite as far as the math predicts. With a hole to top height of 18.5cm and a hole to floor height of 93.5cm I get a distance of 83cm. However, the stream is only reaching the high 70's of cm. Also, the water breaks up from a solid stream before it gets there making the exact landing point difficult to determine.

The stream breaking up is the problem, I think. In class, I've always hit the prediction dead on. But on Monday at a workshop in Alabama, I missed. See, I had quickly jury-rigged a gatorade bottle for this demo. I used my cheapo pocket knife to make a jagged hole. The stream was not particularly clean, but was breaking up. Sure enough, I missed -- I predicted 65 cm, but the stream only went 50 cm.

I suspect -- though I'll have to play around a bit to be sure -- that this demo depends on getting a "clean" stream out of the hole. After all, Bernoulli's equation explicitly is for "inviscid" flow, meaning no viscous drag.

Let me know if you try again, and I'll do the same.

GCJ

1 comment:

1. I just did this demo in class and was successful. A couple of things I noticed: The back of the beaker should be placed at the measured horizontal distance; measure from hole to ground for vertical height. I had preformed this preclass by myself and was unsucessful at first. I let the students do it themselves and they were right on the money. Hope this helps.