My tuner (No floating things this time, I promise.)

Here is my beautiful combination tuner metronome, which I use to attempt to stay in tune when I play my flute. The little pointer shows you if you are flat or sharp. There is also a little calibration thing in the top left corner that I never took much notice of before this. If you look VERY carefully, it says 440 Hz, the frequency of the sound waves. This means that the tuner thinks that A is 440 hz, or 440 cycles per second. The little calibration buttons allow me to change this (if I really wanted to tune myself to some weird key, I could.) Also, since the period is the reciprocal of the frequency, the period is 0.0023 seconds. The sound coming out of my flute is actually a compression and uncompression of air that travels in waves through the air. So although I'm blowing across the headjoint of my flute to make vibration and sound, this traveling air doesn't actually have anything to do with the sound that echoes around the room. At average room temperature, these waves would be traveling at 331 m/s, but the density of the silver in the flute is much greater than that of the air, so technically the sound travels through the flute itself much quickly than it travels through the air. This means that I could feel the vibration of the flute before I hear the sound itself... if I could hear a difference of milliseconds. Yay! Last blog before Christmas break!

A floating sponge

Yes, yet another blog on floating things. :P Of course, this doesn't mean that I haven't done 2 days worth of physics hw due to 21 hours of robotics competition, 4ish hours of band concert, almost 3 hours of an activity at the zoo I went to with my family, 3ish hours of dinner/partying after the robotics competition, and 20 hours of sleep in 3 days in my attempt to catch up on sleep. Which leaves about 11 hours for hw and eating... >< Missing school = pain...

Anyway, while washing dishes, I realized that the floating sponge meant that the buoyant force of the sponge was equal to the weight of the sponge itself. If I pretend that the sponge mass is 0.25 kg (I hope it weighs significantly less than that, but I'm too lazy to do the math with a stranger number. And I'm not about to weigh the sponge.), then it's weight in newtons is 2.45 newtons downward. This means that the buoyant force is also 2.45 newtons. If the buoyant force is equal to density times the acceleration of gravity times the volume of fluid displaced, and the density of water is 1000 kg/m^3, then you can find the volume of the sponge. This volume would be 2.5e-4 m^3. (Amazingly, this is reasonable sized for a sponge.) The density of the sponge could also be found, since D = m/v. The density of the sponge is 10,000kg/m^3. This is a rather dense sponge. (Please ignore the plethora of bubbles in the picture...)

Floating FISH! (Alliteration!)

I feel like I have a lot of either one word titles or all-caps titles... but that's ok. :)

So when I was doing my physics homework, I saw the section about how fish use their swim bladders to go up and down. Immediately, my two fish (one goldfish, and one betta) came to mind.

Both fish expand or compress their swim bladders to deeper or shallower in the water. They fill these bladders with gas that their bodies produce, and when they expand their bladder, their density decreases while their volume increases. This makes them go up in the water, since when their volume is greater they displace a greater volume of water which causes a greater upward force. Likewise when they want to go down, they compress their bladders, so that their density increases as their volume decreases. This makes less of a displacement in the water, so the upward force (F = pvg) is less.

Occasionally, fish will get swim bladder infections or be born with a "broken" swim bladder. This means that they can't easily go up and down in the tank, and will often float sideways or upside down. Sometimes they won't be able to move in the tank much. :( And then the fish die. (My poor deceased other goldfish, who died a few weeks ago... )

This is a video of my goldfish swimming around in his tank! (The betta decided to hide from the camera.)

Balancing a pencil

Originally, I meant to prove my (nonexistent) skills at balance by skillfully balancing the pencil on the eraser. However, due to the odd shape of the eraser, the unequal mass distribution and shape of the pencil, and the slight tilt of the surface I was trying to balance it all on, the thing refused to obey and fell over. And fell over again. And again. Finally, I discovered something amazing... It is IMPOSSIBLE TO GET A PENCIL TO BALANCE ON A LUMPY ERASER. I'm sure that this is due to some special property of unstable equilibrium, since it won't stay in equilibrium with even the light force of the air flowing past pushing it, but it just proved how stupid my idea was in the first place. I've surrendered to the fact that I would write a blog post about how it WON'T balance at all I mean, easily.

The support, the eraser, exerts a normal force upward on the pencil equal to the weight of the pencil. The weight of the pencil is a force at the middle of the pencil in theory, if you pretend that its mass is equally distributed, and it is a perfect cylinder, not a weird composite of graphite, wood, rubber, and some sort of metal. Since in my head, pencils are perfect cylinders that are easy to understand using physics concepts, the weight will come from exactly the middle of the pencil. This means there is a torque force on the pencil of magnitude r x f, or the distance from the center of the pencil to the axis of rotation multiplied by the weight. Since the net torque is clearly pushing the pencil clockwise, it doesn't balance, and tilts to the right.

And not that anyone cares, but I've managed to sneak Beren into another blog entry... completely by accident this time. It's not my fault that most of the regular pencils in the house are labeled with his name... (Believe me, mechanical ones are even harder to balance. I tried.)

Happy Early Thanksgiving, Everyone!

Spinning toy

So as I was on my weekly search for something to write a blog about (I swear I'm going to completely blank someday and be completely unable to think of anything), my gaze fell on a toy in my brother's toy box. With, his permission, I stole it, and here it is. (please ignore the random conversation of my dad and brother in the background.)




Clearly, it demonstrates circular motion, as the motor inside provides the force needed to keep it spinning. (This force must exist, because of Newton's second law). It accelerates at first, then reaches a constant speed. There is centripetal force pointed inward, with constant acceleration once it reaches the final speed, since the velocity is constantly changing direction. The axis of rotation would be the center of the toy.
If the toy spins about 5 times a second (a random number) then the period (the amount of time it takes to spin once) is about 0.2 sec. If the radius is 0.05 cm, and the period equals T = (2)(pi)(r) / velocity, then the velocity of the outside of the toy is about 1.57 m/s.

Erasable pen? With friction?

Ok, so slightly random post... I was playing with my erasable pen, when Teresa pointed out to me that the pen was called a "Frixion pen". Hmm, I thought. Physics blog? So today, I went and googled the pen.
According to Amazon, the pen features "thermo-sensitive gel ink that disappears with simple erasing friction". You rub the back of the pen against the paper, and the ink disappears. Supposedly, the heat created by the friction of the pen rubbing against the paper makes the ink change to clear. It works... mostly.

This means that the kinetic energy of the pen's movement is being converted to heat, and so kinetic energy is being lost. This also means that the friction constant must be pretty high to subtract a significant amount of heat. Sure enough, the back of the pen feels sort of sticky and rubbery, and after rubbing it for a while it gets a little warm.
Video is strangely fuzzy, and has odd hissing and high pitched noises in the background...

BALLOOOOOON!

So, while trick-or-treating at Ala Moana Mall this Saturday, my little brother got a balloon. And sadly, this made me think of physics. I then asked him if I could steal his balloon (of course he told me no) and finally settled for borrowing it for one picture.

Naturally, after one random haunted house in some people's garage (pretty impressive, yeah?)

and a couple of ours of trick-or-treating, I forgot to take a picture of it. when I did try to take a picture this morning... well, let's just say that regular balloons FAIL. We can pretend that it's still floating, right? :P It sort of is... with help.

Now I'm sort of tempted to see how many photos I can get my brother into somehow... and I just realized that he's wearing the shirt that says "If you think I'm scary, you should see my sister". Ummmmm....

Ok, so for the real physics.... The balloon is pulling up with a force since the helium in the flimsy rubber is lighter than the air. This force is the tension in the string. The tension is also equal to the force that my brother has to pull down with to keep the balloon from floating away plus the force of mg (mass of the balloon and string times gravity), since the balloon isn't exactly going anywhere. (At least you hope so.)

Another physics concept that I noticed about the balloon was the tension as my brother ran away and the balloon streamed behind him. He pulled on the string, causing tension in the string, and this tension caused the balloon to have x velocity.

Hope everyone had a fun Halloween!

~Kira

Dropping Textbooks

Today, when I dropped my textbook in the back of the car, it made a loud thumping noise. Later, I dropped my book on my bed, and it barely made any noise at all. Why was this, I wondered?

Physics, of course.


My bed is much more soft and squishy than the floor of the car, so when the textbook falls onto it the book sinks in. This makes the time of the collision much longer than when it falls on the hard floor. Since J (impulse) = change in time x force. If you say that each time the book was dropped from about the same height, the initial velocity before the book impacted was about the same. The final velocity will always be zero, after it hits the surface. This means that the impulse was the same in both collisions, the collision with my bed and with the car floor. J/t = F, so the greater the time, the less the force. The time in the collision with my bed is much longer than the collision with the car floor, so the force in the first collision is less. The greater the force, the more noise the collision makes.

Rolling balls that collide!

So yesterday sitting in robotics, as I was trying to think of something to write my physics blog about, I caught sight of the little green balls that we have to use in the VEX game. Basically, we have to build a robot to get them to the other side of the wall as the opposing robot is dumping the balls on our side of the wall.

Naturally, the first thing I thought of (since I had just been working on Physics homework -_-) was momentum. Let's pretend that there aren't any friction forces, and that the balls are moving in 2 dimensions... despite my efforts I couldn't get the balls to collide and move perfectly in opposite directions.
The moving ball, the one that comes from the left, has momentum that equals it's velocity times it's mass. We'lll assume that the second ball, the one that starts in the middle of the screen, starts from rest (yes, I do realize that it appears to be rolling slightly, but we can pretend it's not for the sake of making this easier) since it has zero velocity.
After the balls collide, there is a change in momentum - the momentum of the ball from the left decreases, and the other ball's momentum increases. In other words, both balls have an impulse value, and the energy is transferred between the balls. According to conservation of momentum, the total momentums of the balls at the beginning and end of the video will be the same. This means that all the momentum that the first moving ball lost was transferred to the other ball.
Since we are assuming that this is a frictionless environment, we can assume that this is an elastic collision - hence, the total kinetic energy stays constant.

Jumping Berens~ Conservation of Energy!

In my never-ending quest to flaunt the rules without actually breaking them and getting in trouble, I decided it'd be a good idea to post my blog at exactly 11:59 pm. Of course, this is all due to hours of planning, not because I almost forgot to do the blog at all and realized that at 11:30pm. No, of course I would never really be that forgetful.

Unfortunately, this plan was thwarted by the insistence of blogger to write the post times in some strange, non-Hawaii time zone. I'll have to be satisfied with the knowledge that I sneaked in- I mean, carefully planned to post- right before the deadline.

Today, as I watched my little brother Beren hyper-ly jump around the room while he looked as his birthday presents, naturally the first thing that came to mind was that image of the little kid on the trampoline in our physics book, where you're supposed to find the PE and KE.
For some strange reason, he isn't smiling in this picture, don't ask me why. To prove his normal levels of cuteness, I'll find another, better picture, where I'm not asking him for help on my Physics homework at 11pm at night... like this one.
Ok, never mind. In that one, he just looks evil. Like he's plotting, or something. Back to Physics.

In the first picture, he hasn't yet left the ground, and in the second one he is (theoretically) at the highest point he'll reach. This means that in the first picture, kinetic energy = .5mv^2, and PE = 0, while in the second picture, PE = mgh while KE = .5mv^2. According to the Law of Conservation of Energy, the inital PE + the initial KE = the final KE + the final PE. This means that if I knew either the inital velocity or the velocity at the highest point, I would be able to solve for the unknown velocity. Of course, this would also only work in a world where there was no friction or air resistance.

If there was no friction or air resistance, think how much higher and faster he would jump.... O_O

Now to wait 10 more minutes so I can post right at 11:59... *laughs evilly*

Pushing Pull Doors... = fail...

Last night, when my family ate at KuruKuru sushi (YUM, sushi...), I failed to open the restaurant door. Why, you may ask?

Because I have an crippling inability to read, which results in humiliation in front of large crowds of diners as I stubbornly continue to push on a door labeled "Pull". It's a regrettable talent.

Certainly, I felt like I was doing a considerable amount of work. But work is defined in our textbook as "the product of displacement and force", and that door clearly wasn't going anywhere. Since the door wasn't moving, there was ZERO displacement. And everyone knows that zero times anything will always equal zero. No matter HOW MUCH FORCE I used trying to get that stubborn door open, it just wouldn't budge.

Therefore, I was doing zero work, and still managing to humiliate myself. It almost leaves me wishing I had been doing some kind of work, because that would mean, well, that the door was opening...

Physics in a Ceiling Fan pull cord

So as I was staring at the ceiling in my room, trying to think of something to write a blog about, I caught sight of the pull cords hanging from my ceiling fan in my room. (Apparently the shutter speed of my camera is fast enough to catch the fan blades as still, even though they're moving really quickly, but maybe that's another blog. :P) Ignore the strange shadows that my camera flash caused on the ceiling.

Both the pulls, the round crystal-ey one and the dolphin one, are like the weights that we've been studying and doing problems about. They both have two forces working on them: the tension of the metal cord pulling them upwards, and their weight (mg) pulling them downwards(NOT SIDEWAYS, despite how I sometimes want to draw them in free body diagrams). The weight, mass x the acceleration of gravity, is equal to the mass of the plastic pull itself. The tension of the cord must be equal to the weight of the fan pull, since the pulls aren’t randomly floating upwards.

I've also just learned that if you try to paste to straight from Microsoft Word to Blogger, Blogger becomes unhappy. And the picture disappears. Okay, won't do that again.

First post of the year ~ 30 something more to go?

Surviving day by day... and fighting confusion.

It can't possibly have only been four weeks of school. It seems like it's been so much longer. So far, AP Physics B has lived up to my expectations as one of the hardest classes at 'Iolani. (I say "one of" because my Precalc-Honors teacher told me that PCH was the hardest class at 'Iolani). Sometimes I feel swamped with the homework and reading, and I'm finding it so much harder than Chemistry and Biology. Physics isn't just memorization, like Bio, or even plugging numbers into equations, like Chem. It actually requires thinking, which could be a problem. :D The labs help a lot, though, so I can kind of understand what the reading is trying to tell me. The little powerpoint explanations are helpful too. :) I feel like I understand about 90% of what we go over, but actually doing it on problem sets and tests is another matter altogether. I am worried about the pace of the class getting up to speed, though. It already seems pretty fast to me. :O