Spinning

Hey, Vsauce. Michael here.

Do you want my head delivered to your door in a box? Well, too bad! I only have one head and I already called dibs on it. Plus, my neck is like pure muscle; this head ain't never coming off!

The next best thing is what actually comes in the Curiosity Box by Vsauce. It is chalk full of science gear designed by us. It's pretty amazing, and a portion of all proceeds goes to Alzheimer's research. So, a subscription isn't just good for your brain; it's good for all of our brains!

Now, if you subscribe right now, there's still time to get this box! The latest one is full of brain food, including the topic of this video: a gyroscope. If you hold a gyroscope in the palm of your hand upright like this, it will fall over. That's not very surprising.

You see, the trick is to take the included string and thread it through a hole, and then spin the disc around so the string wraps around and around and around and around. Once it's sufficiently wound up, hold the outside and give the string a firm pull. Ah yeah! Now the gyroscope appears to defy gravity.

The axis it spins around is like glued in place, almost cosmically. If you could get this to spin long enough, you could actually watch as that axis did move, but not because it actually moved—because you and Earth moved around it. That's actually where it gets its name: Gyroscope. You can use it to scope, watch the gyro gyration rotation of the Earth, as Léon Foucault did in 1852 when the device got its name.

But how does it work? Why are spinning things so stable? Also, watch this! If I give it a good spin and then hang it from its own string sideways, it will remain parallel to the ground but precess around.

To get to the bottom of what's up—or what up, as the cool kids are saying nowadays—begin by imagining yourself swinging a ball around your head on a rope. Even though the ball is following a curved path, its velocity is at any given instant straight tangent to its path. You can demonstrate this to yourself by letting go; the ball will shoot out at the velocity it had at the moment you let go.

It doesn't curve away or fly directly away. While rotating, the rope keeps the ball always at the same distance from the center, which means continuously changing the direction of the ball's velocity. Now, that requires force, and in this case, that force is delivered by tension in the rope—the same intermolecular forces that attract neighboring rope molecules to each other and are the reason the rope doesn't just disintegrate—also resist the ball's momentum acting on it, such that its velocity changes every next instant so as to always follow a circular path.

This center-seeking force that acts on the ball is called a centripetal force. It can be provided by a rope, an entire disc of material, something invisible like gravity, or from the other side by a wall or banked track.

Side note: notice that there is no force acting on the ball that goes directly outward, away from the center—what we often hear called a centrifugal or centripetal force. You know, the force you seem to feel when you lock hands with someone and you both spin around and you feel pulled straight back. Or when a car you're in turns quickly and you're suddenly thrown away from the center.

The thing is, this seemingly outward directed force isn't a force at all; it's actually just an object's inertia—the path the object would take if no other forces acted on it being blocked by a centripetal force.

Across sufficiently short time scales, an inertial path can seem like it's opposite to a centripetal force. But if allowed to follow its inertial path for a while, the difference becomes more clear.

Here is a ball in orbit. If the centripetal force is removed, it will continue along on a path tangent to its previous circular one. The outline represents where the ball would be if it hadn't been released. Now, at first, the separation between the two appears to occur directly away from the center, straight out. But if allowed to continue, a more interesting path emerges.

From the perspective of the outline, which is still moving counterclockwise, the solid ball appears to be receding and dragging behind clockwise. This is due to the Coriolis effect, which we will talk about soon.

But first, let's push some stuff around! It could really be anything, but I would love to use this ball—it's very cool, has fiber optic properties. It would be awesome to get these into curiosity boxes down the line. I will see what I can do!

Anyway, if I push the ball, it will move in the direction of my force—pretty simple! But if the ball is moving before I touch it, its velocity doesn't just disappear; instead, both combine, and the ball moves according to their sum.

The lower its initial momentum, the greater this angle. Now, let's go back to circular motion. Imagine a satellite orbiting Earth. Its velocity is, as we showed before, at all times tangent to its circular path. A centripetal force from gravity continuously swings its velocity around so that every 90° it starts pointing more and more in the opposite direction it used to be.

Just like with the ball, a downward force won't send it straight down. Instead, the vectors combine, resulting in a new path like this, 90° ahead. It reaches the maximum distance it will travel in the direction of our force before going back.

The key point here is that a downward force at this location didn't move the orbit like this; it moved the orbit like this, tilting it 90° ahead of where we acted. Now imagine the satellite as a small piece of our gyroscope. Pushing down here will cause the piece to assume a path that would take it like this.

But of course, since the disc is solid and all of its pieces quite rigidly push and pull each other, the gyroscope itself tilts like this—or if the gyro pivots around a point below its center like this.

If the gyro wasn't spinning, pushing it down here would just have caused this to happen. But when something is spinning, its component parts have other vectors you need to consider.

I learned a fun demonstration of this from a great video by Mahz Wandal. This is a cardboard disc I cut out myself. Now I can balance it on the end of a pen, just like this. If I blow on the side nearest me, it tilts to me—not very surprising. But if the disc is spinning and I blow in the same place, it tilts to the right.

And if I blow on the side nearest you, instead of tilting to you, well, it tilts to the left. If instead of applying my force in one location the whole time, I instead apply the force wherever the tilt is greatest—down—well, the tilt will glide around and around.

Watch! I've run out of air, but the point is this is exactly what is going on when a gyroscope here on Earth is spun up and tilted. The tilt glides around; this is called precession.

What's happening is actually very similar to my breath moving around, except instead of my breath, it's gravity. No matter how perfectly upright a gyroscope is, it will not stay that way forever. Any deviation, no matter how slight, will allow gravity to form a torque.

A torque occurs whenever a force rotates an object around a pivot point. If the disc is spinning, the effect of that torque will move 90° ahead in the rotation, like so. But now the torque is operating this way, so its effect will have to be felt 90° ahead, and so on.

And well, what do you know? The gyroscope precesses! Now, if it wasn't for friction and air resistance and other forces meddling with this disc, it would just precess at the same angle forever. But of course, we live in a world with friction and air resistance and all that good stuff. So, the disc spins down.

In the same way that a slower-moving ball is deflected with a steeper angle, as the disc slows down, each piece of it is given a steeper and steeper orbit around. The gyroscope tilts further and further down until it hits the ground and eventually slows to a stop.

Take a look at this bicycle wheel—it's like our gyroscope but bigger. I have a string tied to its handle right here, so when lifted like this, a torque is applied by gravity, like so. It's equivalent to a force up here in this direction and a force on the wheel down here in this direction.

If I let go, those two forces will do their work. Okay, pretty predictable. But if I get this wheel spinning, there will be other vectors at play. A piece of the wheel moving, say counterclockwise, along with the rest of the wheel down here, we have a velocity tangent to its circular path. But also a torque pushing it this way that will cause its orbit—like in our gyroscope—to move like this.

This force will have its maximum impact 90° ahead in the rotation. So, the wheel will turn like this, but, well, there's still a torque here pushing the bottom of the wheel this way, moving that 90° ahead. We see that the wheel will just keep turning, and in fact, that's what it will do if I can get it up to a fast enough speed.

Then, ta-da! An even bigger spinning object is the Earth, and we are all on it, strapped in by friction and gravity for the ride—a ride with some surprising consequences. A helicopter can't just lift up off the ground, remain motionless, and allow the Earth to rotate under it. It doesn't work that way.

This is because the helicopter, the ground it used to be on, and the air all around it are all also traveling with the Earth's spin. But if you had a magic paper airplane that could be thrown really far and you decided to throw it directly north to your friend, Earth's spin would come into play. No matter how dead-on your aim was, every time you would find that the plane drifts a bit east, as if pushed by some mysterious force.

Likewise, your friend to the north would find that their plane thrown south directly at you would always drift a bit west. This is called the Coriolis effect. You are both on Earth, and you're both always north and south of each other. But in a day, your friend nearer the pole travels a shorter total distance around. It's less distance in the same time.

Your friend is moving slower than you. Your plane, meanwhile, is spinning at your faster velocity, and it continues to have this velocity after you throw it. As it moves toward the pole, it finds itself amongst slower and slower things, so it pulls ahead of them and drifts east in the direction of Earth's spin.

Your friend's plane, meanwhile, finds itself amongst faster and faster moving things that it falls behind. There's a vertical version of the Coriolis effect as well. An object suspended very high, right up above you, will actually fall a bit to the east if dropped, and anything you throw straight up at it will curve away to the west a bit.

The higher-up object, whose circular path around is bigger, covers more distance in the same time than you. These velocities don't just disappear when things are dropped or thrown up, so you both miss your marks.

It's also why the directly outward pointing force you feel when spinning is fictitious. Your inertial tendency to move in a straight line takes you further from the path you're on to where things have to have higher velocities to rotate at the same rate. If you fly out long enough, you'll notice yourself falling behind your original rotation pace, not with it, always directly out.

Finally, lose weight fast but not mass with this one weird trick discovered by a Hungarian nobleman and physicist! People who think the Earth is flat hate him! It's called the Utoš effect. In the early 1900s, Baron Roland von Utoš was looking at gravity measurements taken on ships at sea and noticed that readings were lower when ships were traveling east and higher when they were traveling west.

Further investigations found that yes, in fact, when traveling on foot, in a car, on a bike, in a plane—doesn't matter—you weigh less when traveling east and more when traveling west. In fact, an airplane flying east at the equator experiences an apparent weight reduction of about 0.9%. So, if you want to quickly lose a bit over a pound (about half a kilo), get on that plane!

What causes this effect? Well, when you're spinning with the Earth, you have a linear velocity, but gravity, a centripetal force, is always resisting this. If gravity was turned off and the Earth remained whole, you'd be flung off on a tangent that rose above your usual path. For this reason, your inertial tendency to take this tangent path is a sort of lift.

It's not enough to take you off the surface. Earth doesn't spin fast enough for that to happen, but just as I can lift something that's on a scale and make it weigh less without completely removing it from the scale, so too does your inertial path lift you just a bit.

Traveling east adds to the velocity Earth's spin gives you and provides more of this kind of lift in a direction away from the surface, making you lighter. Conversely, traveling west decreases your velocity in that direction, lowering its lifting effect on your weight.

I will be thinking about this and maybe even weighing myself very precisely while I'm traveling the country with Brain Candy Live! You got your tickets already, right? Don't wait! It is very, very exciting, and I cannot wait to get a chance to see you all in person, be more interactive with this kind of stuff, and do so on a much bigger, louder, personal scale.

Also, subscribe to the Curiosity Box for more mind dynamite! It's fantastic and helps a very, very good cause. I so fully stand behind it! I'm very proud of it! And you know what? Go subscribe to Dong while you're at it! My latest video there is all about some of my favorite free physics simulators on the internet.

You are all great! I appreciate your time, and as always, thanks for watching! [Music]