wiredfool

Looking at science for early elementary (home) school is depressing, and not just because of religious conflicts. This time, it’s just plain old “not knowing the equations”. And really to be fair to the curriculum that we’re using, the errors are all over the internets too, even in some otherwise reasonable references. (And to be fair, there are some that really get it right). What am I so annoyed at?

The speed of sound does not vary based on how close together atoms are. It depends on the stiffness of the material and the density. (the equation is of the form v= sqrt(k/m), where there are various versions of the stiffness, depending on if it’s a gas, liquid, or solid).

Counter example #1: Ice vs water.

Ice floats, so it’s less dense than water (atoms are farther apart), but the molecular bonds are stronger. If the speed of sound was faster when atoms were closer together, then the speed of sound in water would be higher than in ice. But the speed of sound in ice is roughly twice that in water, 3000m/s vs 1400m/s.

Counter example #2. Helium Balloons, inhaled

The molecules in a breath of helium are just as far apart as the molecules of ‘air’, as long as they’re at the same temperature and pressure. The helium is just far lighter and the molecules are going far faster. If you were predicting based on the proximity of the atoms, then you’d expect the speed of sound to remain the same. It’s not, it’s about 3x faster in helium, and so your voice shifts up.

Stiffness and density, vs “how far apart are the atoms”. I’d say that stiffness and density are actually two things that kids can relate to, or at least better than than the distance between something that they can’t see or visualize.

So, what are we to do? Explain it, and demonstrate something else. The speed of beans in sound.

What you’re not hearing here is the end of the 1812 overture, with digital cannons, as played through a subwoofer covered with beans. Click to hear…