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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.

Any advice?

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.

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.