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