Explaining concrete while getting buried in it

I ranted last week about the lack of 5-10 minute YouTube videos, so I won't rehash that grumble pile this week.

Instead I'll mention that there's an urban myth (frequently disproven) that construction workers on the Hoover Dam either fell into an couldn't get out of or were purposefully dumped into the un-set concrete used to create the dam itself.

"No, they weren't," is the short answer. The long answer has to do with the inhumanity of such a practice, the long set time of concrete, and - as this video above shows - the high density of wet concrete.

Today's video sees Dr Derek in a wetsuit trying to fill a sphere with himself and a batch of wet concrete while cutting back and forth to various explanations as to the science of concrete, the most widely used material we have and the underlying backbone of our modern world.

And at about 9:30 we get the clear explanation as to why bodies were never sunken in concrete: bodies float in concrete. 

Yes, the rest of the video is well done, explaining the history of concrete - primarily around the Roman discovery, the science of concrete setting, slump testing, ingredients in different concrete batches, clunkers, the environmental effects of concrete production, cement v concrete, and much more.

It's a great, great video.

Even the densest metal doesn't exceed USPS shipping weight limit

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So many words there...

That's 48 pounds of tungsten in a post office mailing box right there that people are trying to pick up. 

Apparently it's really tough to pick up 48 pounds of tungsten - which makes sense because tungsten is really dense...but not as dense as osmium.

Apparently filling the same box with osmium would give you 61.5 pounds (source) - thankfully still below the post office's maximum weight limit of 70 pounds. To quote...

Osmium’s density is 22.6 grams per cubic centimeter. OP measured the inside dimensions of the small flat rate box and multiplied them to get the volume, 75.3 cubic inches. This is equal to 1,234.5 cubic centimeters. So all we have to do is multiply 22.6 x 1234.5. This gives us 27,899.7 grams, which is 61.5 pounds.

So, unless you go hunting neutron star matter or dark matter or something exotic like that, you're good to put just about anything inside one of those post office mailers.

Mystery blue & white beads

The blue and white bead bottle up there might look familiar to a lot of science teachers, especially the ones who have taken our ASM summer camps. The bottle can be bought from Educational Innovations or from Flinn Scientific (though Flinn's version is green and white).

The bottle is a spectacular demonstration of density and of solubility, both of which are explained by Steve Mould in the above video.

We do a similar activity in a lab in our material science course at Princeton (one we certainly didn't develop but have tweeked to our needs) using preforms and polymer pellets. An extension we particularly like is related to the food coloring demo that Mould mentions in passing near the end. I especially recommend green food coloring. The effects are far more dramatic than the red that Mould shows.

Another extension involves shining ultraviolet light at the bottle. It turns out that - as one of our Utah campers pointed out to me after I'd had the bottle for a decade or so - that the white beads are actually the UV beads that Ed Inn sells. Apparently having a classroom with no windows blocked me from seeing that happen until she pointed it out to me.

I did also find a video showing how you can make one of your own - without the UV beads, however. The YouTuber's full instructions with quantities can be found in the video description.

I'll copy the video description, however, in case the video experiences link rot...

"Sorry for the spelling mistake in one sentence!

Water and Isopropyl alcohol are SOLUBLE with each other. (solvable is wrong word)

Poly Density Bottle

Take any size bottle, divide number of ounces of the bottle into half. Half number of ounces distilled water and half number of ounces 91% Isopropyl alcohol.

I used 50oz bottle.

So I added 23oz water and 23oz Isopropyl alcohol.

Salt 1tsp for 1oz of water.

So I mixed 23tsps of Salt in water.

Not filling the bottle completely and leaving some space for air at the top is a good idea.

Easy to shake and mix liquids.

Beads 260 of each kind.

You can add or minus number of beads according to the size of the bottle."