Name: 説明されています。5つの楽しい物理現象 (Explained: 5 Fun Physics Phenomena)
Uploaded: 2021-01-14T10:25:41.000Z
Duration: 6 min 11 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

- In my last video I showed you five fun physics phenomena and asked you how they work. You

responded with thousands of comments and some video responses. Well here are my explanations.

Let's start with the cereal because it seems the simplest but it turns out to be one of

the most surprising. The simple explanation goes like this. So, I showed you cereal is

magnetic. I have ground up all of this cereal into a very fine powder. Let us see what is

in it. I'm going to take these strong magnets and run them over the cereal powder and see

if anything is attracted to them. Look how the cereal actually sticks to the magnet.

And it's because there's pieces of iron in this cereal. They've been added because iron

is something we need, it's an essential nutrient that we need to survive. In fact, this cereal

has 60% of your recommended daily intake of iron. But there's more to it. I received a

video response from Maarten Baert showing non-magnetic objects like plastic and paper

also apparently being attracted to a magnet. So, how does this work? Well water is diamagnetic,

which means in the presence of a magnetic field it generates its own magnetic field

in the opposite direction. This means the water is very slightly repelled by the magnet

and this causes a depression in the surface of the water into which a floating object

will slide. You can even see this depression by looking at the reflections off the water.

So cereal is attracted to a magnet due to its iron content but when floating on the

surface of water there is an additional effect, the depression of the water's surface due

to its diamagnetism. I showed you that you can find the center of mass of a cane or another

stick-type object just by moving your fingers in towards the middle from the outside. But

how does this work, even when you start, in say, an asymmetric position? Well, one finger

is closer to the center of mass and therefore it carries more of the weight of the cane,

and so the friction force between your finger and the cane is greater until the point where

the other finger catches up at which point this finger slides in and eventually they

must meet in the middle. So this a way you can find the center of mass of any cane or

cane-like object. I showed you that if you trt to flip your phone end over end, there

is no way to do it without it also rotating around the short axis as well. Why is that?

The phone has three axes about which it can rotate. There is the long axis, which has

the maximum moment of inertia, meaning it requires the most torque to accelerate it

in that direction. Spinning about the short axis has the least moment of inertia. Then

there is the intermediate axis which has a moment of inertia in between the other two.

Now the intermediate axis theorem says that if you try to flip any object along its intermediate

axis it will not maintain simply that rotation, it will also get rotations in any of the other

directions. That is, if there is any slight deviation from a perfect rotation. So why

does this happen? Well, the mathematics is kinda complicated but it's similar to the

mathematics of a rigid pendulum. So, if you're flipping the phone along its long axis or

its short axis the phone acts a little bit like this pendulum in that any perturbation

will cause it simply to go back to where it was before. But, if you're flipping it along

its intermediate axis it's as though you're trying to balance the pendulum on its end,

in which case it's very unstable and any slight perturbation may cause it to exponentially

increase. So that is why you can't just flip your phone along its intermediate axis without

it also spinning along one of the other axes. I showed you that an electrically charged

object can deflect a stream of water. But it is not due to the common explanation, the

common reason which is given, which is that water is a polar molecule. So what really

is causing this water to be attracted towards the cup? Well, it is charges, but it is ions,

it is dissolved ions in the water. There will be some OH ions, some H+ ions and there'll

also be some other impurity ions in the water. So what happens when you hold this negatively

charged cup up against the water's stream is it will repel the negative charges, the

negative ions in the water, some of which will go back up into the tap. And that means

the water coming down will be slightly positively charged. And once it breaks up into droplets

those droplets have a positive charge that they can't get rid of, so now those positive

droplets are attracted to the negatively charged cup. And you can see those droplets swirling

around the cup because they are so attracted to it. So this is not actually a very good

demonstration of the polar nature of water. Even non-polar substances with some ions dissolved

in them will deflect in exactly this way. So this is actually showing us that water

droplets are charged, they are charged by induction, and it is not due to the polar

nature of water that they are attracted to electrically charged objects. Now you know.

In the teabag rocket we showed that if you light a teabag on fire from the top it will

actually take off into the air. That happens because as the teabag is burning all the air

inside it heats up and so it expands so it gets less dense and it's pushed up by all

the cooler air around it. You know, sometimes people talk about how hot air rises. I mean,

that is what hot air does, but only because the cooler air around it is pushing it up.

In essence it's like a buoyant force because the hot air is now less dense than the cooler

air around it. So, when the teabag burns right down to the bottom the remaining ash is so

light that it gets swept up in that convection current, and you get a teabag rocket. So do

you agree with all of my explanations, and did you get them right the first time? Let

me know in the comments and thanks for watching.