Why do some objects float and others sink?

[Skipperchrldr]

Hey guys. I have several questions (big test tomorrow!) so here they are: Does anyone know what "Viscous fluid flow" and what "nonviscous flow" is? Could anyone explain to me generally what a barometer is? I don't get why a helium balloon floats but a regular air baloon sinks. I actually just don't get why a helium baloon floats at all. Ok, my next question may be a bit hard to understand- You have a piece of clay in a lump. You put it in water and it sinks. Then you take that same piece of clay and shape it like a boat and it floats. Why? Ok, that was the last one. Any help would on either one of these be greatly appreciated!

 

[HallsofIvy]

"viscous" is basically referring to internal friction in a fluid.

A true "non-viscous" fluid would flow along a solid wall without any slowing down because of friction.

A viscous fluid (think molasses in January!) has a lot of friction- even parts of the fluid itself, flowing at different rates, will have friction between them. If a viscous fluid were flowing past a wall, the friction at the wall would be transmitted inward. The fluid right at the wall would not be flowing at all, as you moved away from the wall, the fluid would be flowing faster.

A barometer is any mechanism for measuring air pressure.

A helium balloon floats and a "regular air balloon" sinks for the same reason wood floats and rocks don't. Helium is lighter than the air surrounding it (just as wood is lighter than water). The air pushes it up. A regular air balloon is very close to being the same density as air (of course!) it's just the weight of the balloon itself that gives it the excess weight and causes it to sink.

It's not just weight that causes something to sink in water (or air) it is "density"- weight divided by the volume. If you have clay formed into a small ball, you have all the weight concentrated into a small volume. If you shape it into a thin "bowl" then (since water doesn't know it isn't solid!) you get to count all of the volume, including that filled with air, below the surface of the water as volume.

 

[Skipperchrldr]

Thank You

Wow! Thanx so much! That was a lot of help!