Explaining the Mundane II

I started my undergraduate career set on getting a business degree from a local college and working as a branch manager somewhere. Then, I changed paths and decided I wanted to enter medical school and become a doctor. Like most 18-year-olds, I had no idea what I really wanted. As I started taking more science courses, I soon realized I had discovered my calling.

One appealing aspect of science and engineering was how seemingly mundane occurrences could be explained by scientific concepts that apply to a wide range of phenomena. I could wonder about the tiniest details of everyday life and try to use my knowledge of physics to explain how stuff works. I learned a lot this way and enjoyed testing my physical intuition. I’ve gathered a few of my favorite mundane events that are explained by seemingly-esoteric (but ultimately intuitive) scientific concepts.

Is it possible to ice skate on anything besides water?

Physically, ice skating is actually really fascinating. It’s not initially obvious, but the blades of your skate are rarely coming into contact with ice as you’re skating. Instead, you are actually gliding on a thin layer of water. This is because of the unique properties of water.

Unlike most solids, water can have a lower density in its solid form than in its liquid form. Most people don’t look at ice floating in water and think it’s strange since we see it every day. It is weird though; a solid block of iron would not float in liquid iron. Because of this weird property, liquid water must expand to change into a solid. The opposite is true then, too. To form a liquid, solid water must be forced into a smaller volume.

Ice skates have an extremely thin blade of metal that contacts the rink. Pressure is defined as a force exerted over an area. So, pushing really hard with a large sheet of metal produces a smaller pressure than pushing with the same force on a tiny sliver of metal. Due to gravity, the human body exerts a downward force on the ground. Snowshoes spread this force over a wide area so that the pressure doesn’t collapse the snow. Conversely, ice skates concentrate this force over the small blade, which produces a large pressure on the ice.

As it turns out, this pressure is large enough to melt a thin layer of ice. The pressure is forcing ice into a smaller volume, which, thanks to the special properties of water, transforms the ice into a liquid. If the properties of water were any different, we’d live in a very different universe (or maybe not be living at all!). Not only would you not be able to ice skate, but fish would not survive through the winter. (If you’ve ever wondered where the ducks go for the winter, only to be cut short by someone worried about the fish, you can reassure him that the fish are okay. Ice freezes on the top of a pond but insulates the rest of the water and keeps the fish swimming at a happy temperature beneath the ice.)

Technically, there aren’t any common materials that behave similarly to water. However, there is synthetic ice, which is typically made of polymers (like those used for plastic bags). You can skate on synthetic ice, but it requires monthly application of a gliding fluid and often requires more effort to skate.

Why do large nuts “float” to the top?       

If you’ve ever had a can of mixed nuts or a box of cereal filled with dry fruit, you may have observed that Brazil nuts and the dried fruit appear to float to the top without fail. If you shake the container enough, you don’t end up with a random assortment; instead, you get a pretty good size distribution with the larger components rising to the top. This might seem strange. It wouldn’t be expected that heavier components should rise to the top, but there is a good explanation for this behavior.

This can be explained by a concept called granular convection. This phenomenon predicts that a mixture composed of different-sized particles will behave like a fluid when shaken up. When a container filled with nuts is shaken up, the cashews, almonds, peanuts, pecans, almonds, and Brazil nuts bounce around inside. If a Brazil nut starts at the bottom and a peanut slips underneath it, the Brazil nut can’t return to its original position at the bottom. More time passes, more shaking occurs, more peanuts get stuck under the Brazil nuts.

As its name indicates, the process of granular convection is similar to convection. Heat convection, which occurs in a pot of boiling water, describes a process where hot, low density water floats to the surface where it is cooled (and decreases in density) and then floats back to the bottom. The container walls in a can of mixed nuts favor granular convection. Friction against the walls forces nuts toward the bottom, while the shaking thrusts nuts up the center completing a convection cycle.  Once at the top, larger nuts have no room to descend back to the bottom.

Granular convection has been employed in engineering avalanche safety equipment, among other scientific fields. Engineers have designed portable, wearable airbags. Skiers, mountaineers, and snowboarders caught in an avalanche can quickly deploy their air bag and easily float to the top by granular convection.

Questions of your own? Comments? Corrections? Post them below!

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