Ask Dr. LJ: Floating objects

[saxikath]

Okay, this has been driving us around the bend at work for weeks.

What is an object, preferably spherical or close to it, that would float below the surface in the water? Fresh or salt, either's fine. Something recognizable to kids and preferably visually interesting.

Comments

[rikchik.livejournal.com]

If it doesn't need to be inanimate, a jellyfish. Or maybe a very round submarine?

[fizzixrat.livejournal.com]

"A very round submarine"...i.e., a bathysphere.

[sylvanstargazer.livejournal.com]

I believe we we did measuring the speed of a stream in middle school we used an orange.

[meranthi.livejournal.com]

Well, ice sorta does (http://www.pbs.org/wgbh/nova/shackleton/classroom/w2icebergs.html).

Or you could try freezing salt water. It sinks after it freezes.

Or coat raisins in sugar and watch it bob up and down.

Ummm, pumice floats for a while, but then sinks as the holes fill up.

I know this isn't helpful....

[qarylla.livejournal.com]

If they don't need the specifics, you could try bubbles, like the ones in carbonated soda. They mostly float (or more likely cling) on the sides of the container, but they are often below the surface of the water.

[sylvanstargazer.livejournal.com]

Oooh. The other thing you could do is get a balloon and fill it with water. You could even use different balloons with different suspensions of salt in them.

This was how our physics teacher in elementary school demonstrated how much air weighed.

[bookishfellow.livejournal.com]

That was going to be my suggestion. Or a balloon filled with some other liquid with a specific gravity close to water: creosote has a SG of 1.066, ethylene glycol 1.097, sea water 1.022. These are from http://www.simetric.co.uk/si_liquids.htm; at http://www.simetric.co.uk/si_materials.htm they have densities of solids—most of which are not helpful. (Dry clay is a good candidate at 1072 kg/m³, except for the "dry" part. Crystal magnesium sulphite has a density of 1121 kg/m³, and so does wet peat.)

From another page, I find that milk has a density of 1.03 g/cm³, so that's another candidate for filling a balloon.

[metagenie.livejournal.com]

While not spherical, I float just below the surface in water. I've always wanted a place in a textbook. :)

[ringrose.livejournal.com]

I seem to recall that if you very carefully fill a glass half-full of salt water, and half-full of unsalted water, you can float an egg between the layers. I don't know if the egg has to be hard boiled.


Hm. How about a water balloon (clean water) in salt water?

[tinhorn2.livejournal.com]

A styrofoam ball with a string attached to a light weight, perhaps? As the weight hits the bottom, the whole system becomes floaty, and then as it floats up, the weight will start to tug it down again. Sort of like how a helium balloon on a (heavy) string starts "walking" around after a few days when helium has leaked out (or air has leaked in) making the balloon just slightly buoyant. (Have no idea if this will work in water, but might be worth trying.)

[mamagotcha.livejournal.com]

A tennis ball split and filled with water?

Are you looking for a specific size?

[crossfire.livejournal.com]

Is this a joke? Are these Polish objects? ;-D

[chmrr.livejournal.com]

Well, it's probably not recognizable to many kids, but it's spherical, floats below the surface of the water, is is biologically rather cool -- Pectinatella magnifica, which is a kind of bryozoan. I first ran into them on the Concord river, and my mother, being a biologist and science teacher, promptly pounced on them and claimed a sample to take home and peer at under the microscope.

I am highly amused to now discover that there is an International Bryozoology Association.

[fizzixrat.livejournal.com]

Well, the first thing to came to mind was a spherical piece of ice, which would be mostly floating just beneath the surface of the water, but as we both know, a little bit (the "tip" of the "iceberg") would be sticking up. But I take it you want something completely submerged but not sinking too far down. I guess what you need is something that is a solid, but with a density of a smidgen more than 1.00 g/cc?

According to CRC, amber appears to have a density between 1.06 and 1.11. I've not tried submerging a piece of amber to see what it would do though. Kids *might* know what that is, from Jurassic Park.

Hmm. Milk has a density of 1.028 - 1.035 g/cc. I wonder what the density changes to if you freeze it (make a milk ice cube)? That might also do the trick. Perhaps you should try that one out experimentally to see if it does the trick?


BTW, a chem teacher I worked with used as a demonstration a can of cola and a can of diet cola (both unopened), placed inside a water-filed pretzel barrel (one of those Costco-sized containers of pretzels). The diet cola floats about 1/3 the way down, while the cola sinks all the way down.

Why? You need less mass of artificial sweetener to produce the same sweetness as sugar (or high fructose corn syrup), so the diet cola can (and the diet cola liquid) has a lower density than that of the cola can (and the cola liquid).

If one did this experiment a couple of years ago when Coke released "Diet Coke with Splenda", one could have tried the sugary Coke, the one with Nutrasweet, and the one with Splenda, and the one with Splenda should have ridden highest (because you need less Splenda to produce the same sweetness as Nutrasweet, which you need less of than sugar high fructose corn syrup).



Oh, wait...what if you used one of those plastic bottles of milk as your submerged device?