blog 13: the (un)lucky blog

http://www.springfieldhistory.org/CarolinnSkylarPlayingGlassArmonica.jpg

Since we're allowed to use photos from the series of tubes this week, I thought I'd talk about something for which I wouldn't be able to obtain my own picture: Franklin's Glass Armonica.

This is a nifty instrument our friend Ben Franklin contrived. Franklin (according to wikipedia) got the idea for the way the glasses are arrayed after seeing water-filled glasses being played. Instead of rotating the fingers along the glasses' edges, the glasses of the armonica are mounted on a main shaft-spindle thingy which turns by means of a foot pedal. Thus, sound can be produced by lightly placing moistened fingers (presumably the player's own) on the glasses. 

See how the glass bowls go from big to small? That's so different notes can be played. The big ones produce lower notes, and the little ones produce higher notes. This works because natural frequency varies with several properties including material, shape, and size. As the bowls get bigger, the natural frequency gets lower, and the note also gets lower. Too bad the glass armonica is not a very common instrument because of those pesky rumors that those who play it suffer bad luck and-slash-or madness.

If you'd like to hear about what it sounds like, I suggest getting out any expensive, delicate wineglasses you have stashed away in your house. (Delicate ones work better for me than glasses with thicker sides.) Wet a finger and run it along the edge of the glass (getting it to produce sound may take a few tries). Once you've got that, try adding water to the glass for different notes. It's fun for the whole family! If you've got impressionable young children with poor motor skills in the house, definitely encourage them to try, too. Don't mention my name.

Blog 9: Another xkcd-based blog

So I've been following xkcd for a while, and when we got to our section on angular momentum, I finally understood one of the comics!
 

It's got all the nerdiness of angular momentum combined with all the romance of throwing up due to nausea! Nice.

Actually, the physics here is (are?) sound enough. Angular momentum of a system (in this case, the girl and the Earth) is  constant. That means if the girl spins in the counterclockwise direction, the Earth will spin more in the clockwise direction. Time travel ahoy!

Hardly. If that worked, any time we needed more time to cram for tests, the school would be filled with kids spinning. Why doesn't it work?

Angular momentum= moment of inertia* angular velocity

Moment of inertia= mass* lever arm distance* lever arm distance. (Actually, the equation for moment of inertia varies depending on the distribution of mass, but go with me here.) The girl's mass is going to be around 60 kg, but the Earth's mass? More around 5.9 followed by twenty-three zeroes. Pretty big difference.

Who needs skin?

The Earth's huge mass means the change in angular velocity caused by the girl would be negligible. She could probably spin so quickly that her skin ripped off her body without having a discernible impact on Earth's rotation.

Clearly this girl didn't  hear, or maybe she doesn't care. Either way, she should put her arms out to increase her moment of inertia. Some bowling pins would not be amiss, either.

Blog 5: Coefficient of friction and a graph

Anyone in my lovely audience of four read xkcd? Well no worries if you don't. This was the comic for Friday, November 5:

As the CoF approaches zero, productivity goes negative as OTHER people get pulled into chair spinning contests.

Do you see what I see? Physics~

As we're all aware, the coefficient of friction is the ratio of friction to normal force.  (F/N)

Coefficient of friction comes in two flavors: coefficient of static friction and coefficient of kinetic friction. The former is for when the two surfaces are, well, static. That'd be, for example, when you're pushing your couch but it isn't moving yet. When it does start to slide, we're looking at coefficient of kinetic friction.

The smaller the CoF, the more easily the two materials slide past each other. A larger CoF means that more force is needed to get the desk chair spinning. If you have a very stiff desk chair, you'll have already noticed that there's a max amount of force the static friction can counteract. After you pass this max static friction, well, I think you can guess what happens next.