On Density

Density is one of those topics that science classes frequently include – especially in the physical sciences. Yes, it’s the ratio of mass to volume – or as I like to think – how much stuff is contained in a given space.

Like any formula as D=M/V, given any two variables, it’s possible to calculate the unknown. Density is more than just working formulas – after all – it is an important concept to understand – but most teachers focus on density as it’s covered in a textbook or as their designated drills to pass a state-mandated test.

To me, it’s the application of density into our everyday world that gives the topic relevance. For instance, wood is more than just wood. Product information for a new fireplace or wood-burning stove may include information about softwood and hardwood.

Given 2 logs of the same size, the hardwood log (oak) will have more mass (think of it as heavier when you pick it up) than the softwood log (pine). There’s more wood substance packed into the given space as the same-sized log of softwood. Bottom line being that the hardwood log will burn longer and release more heat.

When density is applied to populations in biology, Hong Kong is very dense – just like hardwoods – well, more like ebony, one of the most dense hardwoods.

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Which is heavier, five pounds (kilos) or one pound (kilo) of water? That’s a no brainer – the oil is heavier, so will five pounds (kilos) float on one pound (kilo) of water? Sure it will because oil is less dense than water (Note: we could include a discussion about solubility, but will stick to density). Yep – that’s why we shake that bottle of Italian dressing before we use it.

Hot air doesn’t rise – (it never has and it never will) – but it is displaced upward by the colder air that is also more dense. (Here’s a past post that addresses that misconception). The same idea can be applied to any fluid (liquid and gases), so now density helps explain currents in the atmosphere and in bodies of water. https://afrankangle.wordpress.com/2012/08/08/on-hot-air/

You may remember the story of Archimedes (Greek mathematician, physicist, and inventor) whom the king called upon to determine if the crown was real gold or not. Legend has it that the explanation came to Archimedes while in a bathtub – “Eureka!” Of course, his points about density and displacement eventually led to how boats and ships float.

While at a party, you want a soda – which is found in a large metal tub. All the ice has melted, but the cold water is still keeping the cans cold. You notice some of the cans are floating and others lie on the bottom. The sign says Diet Soda and Regular Soda. You want a Diet soda, and density is telling you which one to pick.

Readers are wondering why I wrote this post – or at least what sparked the idea. After all, long-time readers here know I have reasons for what I do. I like Chex cereals – and earlier this year I bought a box of each of my favorites in the Chex family. (The written number represent ounces and grams.) Personally, I like the more dense one better – and it’s more filling – which should not be a surprise.

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On Hot Air

Although we don’t mean to make generalizations inaccurate, they are not the full story. After all, these statements are, by definition, generalizations. Sometimes people generalize the generalization, but failing to remember that continual generalization moves one further from the truth – thus eventually turning the generalization into misinformation that leads to misunderstandings.

Repeat after me – Hot air does not rise.

Just to make sure, let’s try again – Hot air does not rise.

Regardless of the links you can provide to the contrary, or ever provide a reference for a reputable textbook, hot air does not rise – it never has and I very much doubt ever will. Yet, how many times have we heard or said, Hot air rises? (Yes, I cringe when I hear TV weather forecasters use it.)

Nah baby nah … hot air does not levitate. Hot air does not move upward on its own – It is pushed upward by more dense cold air that displaces it – Just has bathtub water level moves upward on the sides when one takes their seat in a bathtub. However, there is no need for you to run down the street naked like Archimedes yelling, Eureka, eureka, I found it!

Here’s the story. During my science teaching days, we were preparing a lesson for the next day. Because my colleague had not seen this activity, he took one set of the materials to the location of the ice machine – the boys’ locker room.

My colleague returned saying it worked great, but shared an interesting story. One of the physical education (PE) teachers watched without knowing any details, and then said, “This means hot air doesn’t rise.” Bingo! Meanwhile, this gave me an opportunity to have some fun with the good-natured PE teacher.

During the eventually discussion with the students after the activity – and knowing students would quickly deliver my comment – I stated, “If a PE teacher like Mr. X can figure this out on his own, surely this is easy to understand.”

We had fun with it through the years, for as students are so predictable. But the point remains the same – Hot air doesn’t rise because it is pushed upward when it is displaced by more dense, colder air.

By the way, here’s a similar activity – but ours was better. Yo, Starla, show Navar.