1

Physics, Astronomy & Cosmology / Why do round rocks make higher splashes than flat rocks?

« on: 24/10/2009 21:46:00 »

Thanks leeE and everyone else,
Your answers have been very useful. We had already tried your one suggestion of dropping a cone, and, as you suspected, it made a tiny splash (I've been measuring the central splash). To test the idea of flow being limited around flat objects, we also tried dropping two objects of the same shape and mass into the water only one bigger than the other. We tried this with round and flat shapes. With the spheres, size made hardly any difference at all, whereas with the flat shapes, the bigger one had a much lower splash than the small one. This completely fits with your hypothesis that the flat ones will break the flow of the water, therefore producing a lower splash. It seems that the molecules don't have as far to travel to make a splash with the small flat shapes, but with the big flat ones, they have much farther to go, and have less energy left to make a splash. With the round ones, as you suggest, the molecules are likely flowing around the shape.
Once again, Thanks so much for the interesting discussion.

2

Physics, Astronomy & Cosmology / Why do round rocks make higher splashes than flat rocks?

« on: 21/10/2009 19:09:11 »

I noticed that when I drop a round rock in the lake it splashes higher than a flat rock (I was surprised--I thought it would be the other way around). I decided to make this into a home school science project. I made spheres and flat shapes the same mass out of modeling clay and took videos of my mom dropping them at the same time. The sphere splash was higher (I did 10 replicates). I also tried hemispheres--again the round side down made a higher splash. I tried looking up fluid dynamics on the web but the math was too difficult. It must have something to do with the way the water moves around the shape, but I'm not sure how--is it easier to move around the sphere and so it loses less energy? Thank you for your help.