GetAClass - Physics

All you need is a dark box and a tiny hole. The light from outside passes through it and—here's the magic—draws a perfect, living picture on the wall. Upside down. This is a Camera Obscura, and it's the reason your smartphone can take photos. The physics of light is amazing.

1 day ago | [YT] | 0

GetAClass - Physics

A needle can float on the surface of water due to surface tension forces. Surface tension effects play a role on small scales, where they dominate over the effects of gravity.

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1 week ago | [YT] | 15

GetAClass - Physics

Let's take a rubber belt and make a belt drive out of it. If the rollers are cylindrical, the belt gradually slips off them to one side. Let's try to fix the situation and pull the belt onto the concave rollers. Very strange — the belt slips off them even faster. But on convex barrel-shaped rollers, the tape stays perfectly in place! Moreover, if we lower the tape on both rollers, turn on the motor, the tape quickly rises and aligns itself with the centers of the rollers! And it's not because of their fast rotation: if we slowly rotate the rollers, we can see that the belt aligns itself in just a couple of turns.

How can we explain this amazing effect, which contradicts our assumptions based on common sense? It turns out that the uneven stretching of the rubber causes the skewed belt to creep toward the thicker part of the roller under the action of friction. Watch our video “Self-centering belt drive,” be amazed with us, and don't forget to like it!

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2 weeks ago | [YT] | 11

GetAClass - Physics

The Crab Nebula is one of the most interesting objects located not so far from the Solar System, at least by cosmic standards. The Crab Nebula has long been the subject of attention from astronomers: in 1731, it was observed by John Bevis, and in 1758, it was rediscovered by Charles Messier, who initially mistook it for a comet but then discovered that it did not move against the background of the stars. Messier began compiling a catalog of nebulae so that they would not distract comet hunters, and the Crab Nebula was entered into this catalog under number 1. The name “Crab” was given to it by William Parsons in 1844: it seemed to him that he saw branches resembling crab claws. But four years later, observing the nebula with a much better telescope, Parsons realized that there was no resemblance to a crab, but the name had already stuck.

And in 1921, astronomers discovered that the Crab Nebula was gradually increasing in size over time! We also took photographs of the nebula taken in 1950 and 2000, aligned them with fixed stars, and found that over 50 years, the distances between characteristic elements had increased by about 5%. This means that the nebula originated quite recently, about a thousand years ago. Indeed, in 1054, Chinese astronomers observed the brightest star in the morning sky in the constellation Taurus for almost a month, comparable in brightness to Venus. And it is in the constellation Taurus that the Crab Nebula is located! This led to the assumption that the Crab Nebula was formed as a result of a giant cosmic explosion—a supernova.

To learn how the spectrum of the nebula was used to determine its size and distance, and what amazing object remained at the site of the supernova after its explosion, watch our English-language video “Crab Nebula's space mysteries”! Watch, reflect with us, and don't forget to like!

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2 weeks ago | [YT] | 5

GetAClass - Physics

Today we present to your attention what is perhaps the most amazing of all existing electric motors! First of all, its exceptionally simple design is striking: two ball bearings are fixed on a common axis, their outer rings are fixed to supports, and power is supplied to these same supports — and that's all, not counting the flywheel, which ensures the smooth operation of this machine!

Apply a constant voltage of just over one volt to the motor, and a current of about 30 amps flows through the circuit, because the resistance of such a circuit is very low. Give the flywheel a gentle push, and the motor starts to accelerate, gradually picking up speed. If we push the flywheel in the other direction at the start, the motor will rotate in that direction, accelerating to the same frequency of 10 revolutions per second. This motor works perfectly underwater at even higher currents due to better cooling of the bearings, although they do rust quite quickly.

And here's another surprising fact: this electric motor was described back in 1961 in a patent application filed by Kosyrev, Rabko, and Velman from Novosibirsk, but since then, no theory has emerged that satisfactorily explains how it works! In an article by Soviet electrical engineers Polivanov, Netushil, and Tatarinova, published in the journal Elektrichestvo in 1973: www.booksite.ru/elektr/1973/1973_8.pdf
a spark hypothesis was put forward, which linked the appearance of torque to the pressure of the plasma discharge occurring behind the bearing balls as they moved. However, at moderate currents, we did not see any spark discharge even in complete darkness.

The second, electromagnetic explanation links the appearance of torque to the interaction of currents flowing through the balls with the magnetic fields induced in them. But alas, the conclusions of this theory directly contradict the experiment. The third hypothesis is more plausible and explains the operation of the motor by the uneven thermal expansion of the balls. You can find out all the details in our video “Ball bearing electric motor”! Watch, think along with us, and don't forget to like!

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3 weeks ago | [YT] | 10

GetAClass - Physics

Today we will talk about the stability of ships or, as sailors say, their buoyancy. In school physics class, we study Archimedes' principle and learn a simple condition for floating objects: the average density of the object must be less than the density of the liquid in which it is immersed. But how exactly will such an object float?

Let's conduct an experiment with a simple model ship: take a square bar with an average density equal to half the density of water and immerse it in water with its bottom edge. The bar immediately tips over and floats with its edge facing down! But if we take a block of the same size made of foam, whose density is 40 times less than the density of water, it floats stably not only on its side, but also on its square base, rising high above the surface of the water! What is going on here?

Only two forces act on a floating body: gravity and Archimedes' force. But these forces are applied to different points: gravity to the center of mass of the body, and Archimedes' force to the center of mass of the water displaced by the body, which is commonly referred to as the center of buoyancy. When deviating from the equilibrium position, the ship is acted upon by the moment of these two forces, and if this moment returns the ship to its equilibrium position, it floats stably, and if the moment increases the deviation, the ship capsizes. The difficulty here is that when deviating from the equilibrium position, the shape of the volume of water displaced by the body changes, and therefore the position of its center of buoyancy also changes.

However, for a square bar with a density equal to half the density of water, mathematics is not required. Such a bar is always half submerged in water, so its center of mass is always at the water level, and its position does not change with any rotation. If we place the bar on the water with its lower edge, the center of buoyancy will be exactly under the center of mass of the bar. But at the slightest tilt, the center of buoyancy shifts to the side, a overturning moment acts on the bar, and this equilibrium position becomes unstable.

With other density values, it is difficult to answer the question of stability even for homogeneous bars with a rectangular cross-section. We modeled the floating of such bars in the GeoGebra program, performed direct mathematical calculations, obtained a stability criterion, constructed a stability diagram for different bar densities and size ratios, and then tested our theoretical conclusions experimentally.

You can learn about all this, as well as the concept of a ship's metacenter, in our English-language video “Ship stability and balance on water.” Watch, think along with us, and don't forget to like!

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1 month ago | [YT] | 7

GetAClass - Physics

This time, we will look at a vertical system consisting of two weights and two springs, held in place by a frame.

The heavier weight is placed on the lower crossbar of the frame, and above it are a stiffer spring, a second weight, and a second spring resting on the upper crossbar.

The question is, will the pressure on the lower crossbar change if the weights and their springs are swapped?

At first glance, it seems that the force should not change when the “components” are rearranged, but this is not the case, and the experiment confirms it!

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1 month ago | [YT] | 10

GetAClass - Physics

Derek Muller on the Veritasium 42 channel showed an amazing wind-powered car: https://youtu.be/jyQwgBAaBag that can travel faster than the wind speed. First, the wind pushes the car from behind, its wheels start moving, and the transmission transfers this rotation to the propeller, which pushes the air backward like an airplane propeller and accelerates the wind car.

But if the wind car has reached the speed of the wind, the air relative to the propeller is stationary and can no longer accelerate it. And if the car goes faster than the wind speed, the airflow will hit the propeller not from behind, but from the front, and it seems that now it will slow down the wind car. And yet this remarkable device really does travel faster than the wind speed! How is this possible?

In today's video, we will begin to figure this out and look at some similar, but simpler and more understandable devices, the operation of which, nevertheless, raises new questions. Watch our new video “Against the wind and faster,” think along with us, and don't forget to like it!

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1 month ago (edited) | [YT] | 14

GetAClass - Physics

We filmed this video based on an experiment by our subscriber Ivan Kovalev: he throws a steel ball from a ball bearing onto a large anvil, and the ball bounces many times in one place completely vertically and does not deviate to the side at all, as if it were held by a magnet! However, there are no magnets here, so why doesn't the ball move to the side?

It turns out that this effect is caused by a small depression on the surface of the anvil, which was formed by its long use. We modeled this phenomenon in the Algudo program, letting the ball fall onto the surface of a sphere with a slight deviation from the vertical diameter and ignoring energy losses during impact. When the ball fell from a height greater than half the radius of the sphere, its movement was chaotic, and the trajectory filled the entire area below the initial height. However, at lower heights, the ball's motion became regular, and the trajectory “focused” near the vertical diameter, with the ball bouncing in a narrower area the lower the initial height was.

Watch our new video, “Jumping on an Anvil,” ponder stochastic dynamics with us, and don't forget to like it!

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1 month ago | [YT] | 6

GetAClass - Physics

Take a hard-boiled egg, place it on the table, and spin it vigorously—the egg will stand on its blunt end! As a result of rapid rotation, the stable position of the egg on its side and the unstable position on its end seem to switch places! A spinning egg can also stand stably on its pointed end. These experiments are somewhat reminiscent of the rotation of a regular spinning top, the turning over of a Chinese spinning top, and the transition of a Celtic boat's oscillations into rotation. But why does the egg's center of gravity tend to rise?

In the absence of friction, the egg's momentum is conserved, and in a vertical position, it would rotate faster due to the reduction in inertia, like a figure skater pressing their arms against their body. The kinetic energy of the egg's rotation also increases, and then, according to the law of conservation of energy, the potential energy should decrease and the center of gravity should lower, not rise, as we observe in the experiment. So the egg can only become vertical due to the action of friction, which reduces the angular momentum!

High-speed photography clearly shows that the frequency of the egg's rotation does not change at all during the rise, which means that the angular momentum does indeed decrease, as does the kinetic energy of rotation. Then the frequency slowly decreases until the upper position of the center of gravity ceases to be stable.

To further explain this phenomenon, we need to choose an adequate model of friction. Does rolling friction act on the egg, and does an instantaneous axis of rotation always pass through the fixed point of contact with the table? Or does the egg slide across the surface of the table, in which case we need to talk about sliding friction, which can behave very unusually in such a situation? In any case, the behavior of a Chinese spinning top strongly depends on the properties of the surface on which it rotates.

And another thing: why does the egg stand upright only at a sufficiently high rotation speed?

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1 month ago | [YT] | 10