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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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B2 中上級

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