字幕列表 影片播放 列印英文字幕 The 11th Curiosity Box is currently shipping. I'm talking ferrofluid, a trammel of archimedes. I can't give every item away but you will also be receiving one of these if you're a subscriber. This is a hui stick. Aka a gee-haw whammy diddle. Aka the ouija windmill. Oh yeah it's spooky. It's just a little rubbing stick and a propeller stick that has a propeller on one end and notches on one side. Now when you rub the rubbing stick along the notches the propeller stick will vibrate and the propeller will spin but the direction it spins in will obey your command. All you have to do is say the magic word. Hui. Hui! Hui! Hui? Who? E? How does it work? Well that's what we're gonna talk about today on Michael's Toys. To see how the Hui stick works you need to come closer or I could come to you. Now here are my hands and let's pretend that they are your hands. When you hold the propeller stick you hold the rubbing stick across the notches in the other but you don't just rub the rubbing stick across the notches like this. No no no. The trick is that you hold the stick like this so that when it's engaged with the notches you can press against the propeller stick from one side with your index finger or from the other side with your thumb. So as you rub across the notches you are also dragging a finger and constantly pressing a finger against one side of the stick or the other. And that is what reverses the direction. Watch this. I will begin. Let's begin with my thumb. I'll try to make them far apart so it's very obvious when I switch. Here I go. I am pressing. I 'm constantly pressing against the stick from the right with my thumb and now here goes the notch action. Nice! We've got some rotation and that is in a counterclockwise direction but now I'm going to switch so that my finger, my index finger, presses against the left side of the propeller stick. You ready? Boom. Whoa! Reversal! Pretty cool huh? But how does it work? Well to begin this investigation let's talk about how the propeller stick moves when the notches are being rubbed. Well that should cause up and down motion. Vertical, straight up and down. Except not exactly. No matter how hard you try, there will always be a little bit of error. Even if it's just the width of an atom, horizontally and that's really all it takes for a little bit of rotation to be induced. You can try this at home. Take something like a pencil and a roll of tape or some other ring-shaped object. Put the pencil through and try to move the pencil only up and down. No matter how hard you try, rotation will always result. Only if you're able to move the pencil up and down right through the exact center of mass of the ring so you only push it up and down will you not get rotation but the tiniest little error from there introduces a torque which causes rotation and then a feedback loop makes it stronger and stronger. This is exactly what's happening with the hui stick. I don't know if you can tell but the shaft that connects the actual propeller blades to the propeller stick is much thinner than the propeller hole. Here's an example that's been deconstructed. You see that shaft there's quite thin and the hole in the propeller's pretty big. This means that as that shaft bounces around it really does bounce around inside the propeller's hole and can push and drag through friction the propeller around. To see how exactly those vibrations of the propeller stick work we got some great slow motion footage. Here's how we made it. I took a laser measure that shoots out a laser beam, there's the dot, and I gripped this in the same hand as the hui stick and projected the laser point on a wall across the room so that when I stroked the notches they would both, the laser measure and the hui stick, they would both vibrate together and I could look at how it vibrated. Now when I don't even press a finger against the propeller stick I still get pretty chaotic motion. The propeller stick doesn't just vibrate straight up and down. It does move in circles sometimes but it's quite a mess. It's definitely not controlled. But if I press my index finger against the left side the laser pointer moves clockwise. And if I press my thumb against the right side the laser pointer rotates counterclockwise. And because the shaft has room inside that propeller hole to move around its circular motion easily translates into rotation for the propeller. But why does putting a finger on the side lead to circular motion in the first place? Well there have been many many papers written about how a hui stick works and they don't all agree with each other. Many explanations just simply say that it happens and they don't get into the actual mechanism but I've done a few experiments on my own and I'd like to present some of my findings and this is what I believe is important in the functioning of a proper hui stick. I believe that it doesn't just matter which side left or right you press a finger against. It also matters which side top or bottom your finger is pressing against. And here's why. Let's pretend that this circle is a front on view of the propeller stick like this right? So we're looking straight at it and the whole in the middle is the shaft in the middle that the propeller sits on. Now if this is that hui stick as I rub across the notches the stick is pushed up and down. It's pushed down when the stick is on the top of a notch and then as the stick flies back up because of course my hand is holding it and producing a torque his way to keep it in contact with that rubbing stick it goes up and then it's pushed down when the stick is on top of a notch. Then it comes back up when I'm in a notch and so on up and down. But when I drag a finger along the hui stick my finger is slightly above. Right? I'm not reaching down below the middle of the stick. I'm on top. And look at what happens. Two torques. Two torques because when I press a finger against the stick like this I'm creating a torque that causes rotation in this direction. But of course the stick doesn't keep rotating that way because my steadying hand pushes the other way. It produces a counter torque that keeps the stick in one position. And it is the interplay between these two torques that causes spin in a particular direction. And here's why. Let's go back to our big circle. If this is the hui stick and my finger is, let me take this cap off, and my finger is here. Let's say my finger's up here. Producing a push, a force, a torque in that direction and my steadying hand is producing a torque in this direction. When the notches are rubbed and the stick goes up and down when it goes down it temporarily disengages from my finger's torque and now the torque from my steadying hand wins out and it slightly moves this way before going back up and reengaging with my finger and through the finger's constraint getting pushed this way only to be pushed back down away from the finger where the torque from my steadying hand plays a much more dominant role and so we get rotation like this. Rotation that is counterclockwise when seen from the front. Let's see if that can be confirmed by experiment. I'm gonna put my finger in the upper right quadrant as seen from the front. You ready? Upper right quadrant as seen from the front. Will I get motion that is to you counterclockwise. Yes I do. Now, if instead I put a finger, let me erase this so that we have a nice clean diagram. If I instead rub my finger along the other side on the top so my finger is right here. That's a big magnified finger. If I put my finger here and I push this way now what happens is that as the notches are rubbed and the propeller stick, oh wait, if my finger is pushing this way then my steadying hand is going to produce a counter torque in the opposite direction. That's very important. So the hand holding the back of the stick is always kind of turning it this way. So oh yeah let's grab this one. Now when the propeller stick moves down because the top of the notch has been reached by the rubbing stick it disengages with that finger's torque and is pushed this way. Then it flies back up, reengages with the finger, and that torque brings it back this way and so on until we get clockwise rotation. Will a finger dragged along the upper left side result in clockwise rotation? I think we already know the answer to this question. I'm going to rub my thumb along that side, what is for you the upper left side and we will get clockwise rotation. Pretty amazing. But if this is true that means that it should matter whether I'm above or below the middle of the stick because if we restart our diagram here putting a finger not on the right above or the left above but instead down below like say here, pushing against the stick this way means that we have to produce a torque that is moving in this direction with our steadying hand. Boy I'm not a very good drawer but I think the point should be pretty clear. Now when the stick is pushed down by the rubbing of the notches it engages with this finger and is pushed that way but then it flies back up and now that counter torque from that steadying hand pushes it a little bit to the right and so on and so we get clockwise motion. We got counterclockwise motion when my finger was in the upper right quadrant but in the lower right quadrant we should see clockwise. Let's try and experiment. I'm going to begin by putting my finger on the lower right quadrant. I'm going to press against that quadrant as I rub the notches. We should see some rotation and we do. We see clockwise rotation but now I'm going to move my finger up to the upper right quadrant right now. And it reverses direction. Likewise if I rub my thumb along the upper left quadrant I get rotation in that clockwise direction but if while I do this I manage to move my thumb to the lower left quadrant I reverse directions. Pretty cool. Pretty cool. If this explanation is true then we should not have controllable rotation direction when a force is applied that the up and down vertical motion of the vibrating stick cannot escape. To try that out I created a hui stick comb. It's just like a usual rubbing stick but its got these teeth on it that allow me to put a torque directly 90 degrees from the up and down motion caused by the rubbing of the notches and because these teeth are quite long going up and down does not disengage the propeller stick from that torque like it would with a finger. Now let's see what happens here. I'm going to start by applying the torque on what is for you the left side of the stick. And we get some rotation but watch what happens when I swap. No reversal. Okay here comes the force on the left side and now on the right. I am really pushing but the propeller stick cannot escape from the force and so it cannot take advantage of the interplay between the opposite direction of one hand's torque and the steadying hand's opposite torque which is what I believe leads to the rotation. Please when you get your hui stick perform your own experiments. It is a blast and it's really good exercise especially for someone with weak hands like myself so have fun, stay curious, and as always, thanks for watching. By the way the shirt that I have been wearing this entire time comes in box eleven as well. I talk about cardinal and ordinal numbers in how to count past Infinity and I love cardinal numbers. Cardinal. It's a great word and it's a great bird. Cardinals are a very common sports team mascot. In fact my middle school, Blue Valley Middle School was and is the cardinals. So we started thinking what if we made a shirt that looked like a jersey or like it represented a sports team but not just the cardinals but the transfinite cardinals. Now the smallest transfinite cardinal, the smallest amount of infinity is of course Aleph-Null and there's the symbol for Aleph-Null. Aleph-Null is how many whole numbers there are. It's how many integers there are. It's how many even numbers there are and it's how many odd numbers there are. So I wear this shirt with an infinite sense of pride. And I hope you do to.