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• - 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.

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

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# 説明されています。5つの楽しい物理現象 (Explained: 5 Fun Physics Phenomena)

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林宜悉 に公開 2021 年 01 月 14 日