Blog 14: Colours

Pictured: not my foot

So I went over to my friend's house the other day to do some movie watching. Some of our other friends were also due to come over, so we sat outside and waited for them. While we were out there, my friend had the good idea to bring out food substances to sustain us. Her food substance of choice? Watermelon.

Now I'm not a particular fan of watermelon, but I am a fan of spitting, so we engaged in a seed-spitting contest. One of the things I do like about watermelon (besides the seeds) is the colour. The red-pink is just so cheery.

Physics time: why does red look like that? Actually, we're not sure about that. We can explain why things appear red, though.

Visible light has three colors: red, green, and blue. These are different from the primary colors we learn in elementary school. The colors we use to make the other colors in fingerpainting are red, yellow, and blue. That's because using pigments like paint to make colors is a subtractive way to make colors, and the red/yellow/blue combo is pretty similar to the magenta/yellow/cyan (and black) scheme used in printing, but I digress.

White light is comprised of "all" colors, which can be made from red, green, and blue. The watermelon (remember that?) appears red(ish) because the RGB light from the sun hits it, but the green and blue wavelengths are absorbed, and only the red is reflected to our eyes. The rind appears green because red and blue are absorbed, leaving only the green to reflect. Only blue is absorbed by the yellow part of the rind, yellow being the marriage of red and green. Finally, the black seeds appear such because all red, green, and blue wavelengths are being absorbed, so there's nothing to reflect back to your eyes.

It's like spitting little bundles of negative light. (Not really.)

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 12: All's well that ends

I'm trying to decide it something that happened recently was lucky or unlucky. Here's the story:

My folks and I were on our way back home from school when we decided to stop off at the grocery store. Mum and I stayed in the car while Dad went in to pick up food preparation stuffs. He got back and tried to start the car and... nothing. The battery had died. The good news is that there was a mechanic/garage thingy right across the parking lot.

As the crow said, "Car! Car!" (A little Mainer humor for y'all)

<PHYSICS TIME> So a car need the battery to get it going, right? The batter starts the car's motor. A motor, of course, is a device that uses a magnetic field to convert electrical energy (current) to kinetic energy (movement). A wire coil is connected to a source of change in voltage (the battery), meaning it gets current flowing through it. If the wire coil is in a magnetic field, it will start to move. Neat! Now the car's wheels can start turning! 

BUT WAIT, THERE'S MORE! Anything that conducts current gets a magnetic field OF ITS VERY OWN. That means the spinning coil is now an electromagnet, and what's more, it's spinning! We all know that it's possible to induce voltage (and thus current) if there's a wire loop and a changing magnetic field, and with the spinning coil, there's a constantly changing one! That means that as the car (remember the car?) drives and the motor works, it can also recharge its own battery! Nice. </PHYSICS TIME>

Mum and I were listening to the radio with the motor off. Yes, that's right, we ran down the battery. It was still strange because we were only in the car for about twenty minutes. Our car battery was faulty or something. BUT if we had been running the air conditioner while we listened to the radio and rolled the windows up and down for an hour (or something), there wouldn't have been anything strange about the battery dying.

(PS: Okay, guys. What's the verdict? Unlucky that the battery died, or lucky that the car busted less than a street away from a mechanic?)

 

blog 11: ALL ABOUT MAGNETISM!!!!1!!!1111etc

My assignment sheet says I'm supposed to blog about magnetism, so here I go. (Good of you to specify, Mrs. Chen.) I was going to talk about motors and car batteries, but... eh. Let's just do this.

another picture because it is required

oh look hello. it is my refrigerator. as you can see, we have many items posted on it by means of magic magnets. such items include winter ball photographs from i think last year and an announcement of birth from also last year and a report from my eye doctor and also a schedule for the chorus. there is also a christmas wreath that we put up i think in january and still haven't taken down.

on its own the refrigerator is not magnetized. if i were to hold a steel pin to it and let go the pin would fall to the floor and i would have to look for it for like twenty minutes or something because our kitchen floor has this weird pattern that makes finding small things triple hard. the magnets are, well, magnetic and also magnetized. they are permanent magnets and when they are on the refrigerator the bit of the refrigerator they are on is also magnetized.

So let's restore use of my shift key and talk a little more srsly about magnets, magnetism, and magnetizivitytyy. (I only said "a little more srsly.") As we're all aware, matter is made up of atoms (lit: "thingies"). Obviously I'm cutting corners here, but who cares? We got some subatomic particles, but the ones I'm interested in are electrons (lit: "negatively charged subatomic particles"). They are negatively charged! They also have spin (well, actually, it's more complicated than that, but let's just stay in kiddie land understanding. Deal? Good.) The spins of electrons mean they have their own little magnetic fields. In most materials, though, they come in pairs that cancel each other out. In magnetic materials (and here I am thinking mostly of ferromagnetic materials like my steel pins) the electron spins do not cancel, and we get domains. If a material doesn't have domains, then it isn't magnetic and it won't be magnetized no matter how you stroke it. Things like iron, nickel, and cobalt have domains. This doesn't mean that they are magnetized, though. If the domains are all willy-nilly, there's still no net magnetic field. However, it's possible to get the domains all lined up the same way (by, say for example off the top of my head, taking a magnet and stroking it in one direction). Once you do that, there is a net magnetic field. The temporary magnetic field and the permanent magnetic field attract each other, and the Winter Ball photographs stay up.

blog 10: a blast from the past

Anyone remember momentum? No? Me neither. Allow me to briefly check my notes.

musical interlude

Oh, right, now I remember. momentum= mass* velocity

What fun can we have with this? Well, I weigh... let's see... 43.54 kg, and a man I will refer to only as "my dad" (not his real name!) weighs 75.45 kg. If I take off running at 5 m/s (according to this comic, that's faster than a jog but slower than a sprint), my momentum will be 217.70 (kg*m)/s. My dad would only have to ambulate at 2.89 m/s (somewhere between walking and jogging) to attain the same momentum.

That's well and good, but... eh. Boring. What else could we do with this information? Oh, right.

He seems to think I'm up to something.

We own a cat! After a good deal of squirming and piteous mewling (on his part, not mine), I have ascertained that his weight is about 5.90 kg (he is a chubby cat). For him to match our momentum, he'd have to run at 36.90 m/s. That's faster than the speed you drive at on an interstate! Nice. I may need to devise some sort of cannon. 

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 8: a frisbee ring and a tree

Jump and grab the other end. Yeah, that'll work.

So for the last day of the year 2010, my friends and I decided to go to the park and have a picnic! Indeed, there was feasting and fun for all. In accordance with tradition, one of our number chucked my frisbee ring (like a frisbee, but with a big hole in it) and it ended up in a tree. (This, by the way, was the same tree it ended up in last time we were in the park.)

After the traditional cuss words, hasty excuses to leave without helping get the blasted thing down, and general abuse, my remaining friend and I set about retrieving the ring. I tied a monkey's fist in our 20 foot rope, and we set about swinging the rope around and around. Yes, I'm sure you can see it, too: centripetal force! But I just did a post about centripetal force. Hm...

But wait, there's more! We tried to throw the rope over the branch the frisbee was caught on so we could shake it down. Using out unerring instinct for being kind of foolish, my friend and I first threw the rope over the branch near the trunk of the tree. This, of course was a bad idea, because we wanted a lot of torque acting on the branch for not too much work-- I mean, without having to apply too much force. (We're teenagers, of course.)

torque= force* distance from axis to force

We're applying the force perpendicular to the line from the axis of rotation to where the force is applied, which is convenient. Treat the base of the branch as the axis. Say the force my friend and I will be applying is going to be the same no matter where we are. See? See that? That's why we want the rope farther away from the tree trunk. Because we can have a bigger torque without having to increase our force! Hooray!

When we finally wised up, we went ahead and threw the rope over the branch farther away from the trunk. We were able to retrieve the frisbee after only one hour! (That's a vast improvement on the five hours it took last time.) Thanks, physics!