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Making a wine glass produce sound by rubbing the tip of a finger on its rim is a very old parlor trick. It happens because, as the finger moves over the rim, it encounters resistance (friction) from the wine glass. As a result of this resistance, the finger deforms the wall of the glass a bit, but as soon as this happens, the resistance yields, the deformation disappears, and the finger moves on. This cycle is repeated hundreds of times per second as the finger travels over the rim of the glass. The back and forth motion generated by these repeated deformations of the glass (vibrations) pushes and pulls the air adjacent to the surface of the glass generating compression waves much in the same way that waves are generated if you repeatedly smack the surface of the water in a pool. The sound that you hear is these waves reaching your ear. To perform this trick you need the friction between the rim of the glass and your finger to be just right. If there is too much or too little friction between your finger and the glass, the sound will not be produced. This is why the finger is normally made wet by dipping it in a glass of water or other liquid before moving it over the glass.
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I filmed the video below from a train on my way to the town of Kutna Hora in the Czech Republic. The way the tracks seem to “travel” next to each other and then merge is really cool. This is due to the fact that the wooden blocks of the train tracks (called sleepers or ties) seem to be moving in the same direction that the train is moving even though in reality they are receding as the train moves forward. This effect is called "temporal aliasing", and it is the same effect that you see in the spokes of the rotating wheel of a wagon. In the case of the camera, this happens because it does not record reality in a continuous fashion. Rather the camera samples reality a number of times per second, and the samples are put together to generate the video. This is similar to how the old celluloid movies would generate the perception of motion from different successive frames each displaying different stages of a movement. If the sampling rate is not appropriate to record a moving object, the object will appear to move in a direction that is illusory, such as is the case of the wooden blocks in my video. Whether the human eye/brain system takes samples of reality like a camera is controversial, and there is evidence both in favor and against this hypothesis. The interesting thing about my video is that I could not see this forward motion of the wooden blocks with my own eyes, because the blocks became a blur when the train was moving fast. I could only see it with the camera! Just before eating my breakfast at a local restaurant, I fancied testing some scientific principles related to pressure and temperature differentials and buoyancy. First, I inserted my straw into the hot coffee and blocked the top of the straw with my finger. In this way I was able to remove the straw with a column of hot coffee inside it leaving the bottom open. Some people claim that this occurs because the gravity pulling down on the column of coffee produces a vacuum inside the straw, and the vacuum sucks the coffee into the straw preventing it from flowing out. This is not true. Like I have explained before, vacuums don’t suck. The pull of gravity tends to create a low pressure area inside the straw, and the push of the atmospheric pressure against the bottom of the column of coffee is enough to counter gravity and keep it inside the straw. I then proceeded to place the tip of the straw with the coffee inside the cold milk. The hot coffee is less dense than the cold milk. Under the influence of a gravitational field, liquids that are less dense will float on top of liquids that are denser. You can see in the video that, even though the coffee is in direct contact with the milk at the bottom of the straw, the column of coffee inside the straw remains by and large unperturbed (with the exception of some coffee at the coffee-milk interface mixing with the milk due to equilibration of the temperatures of the liquids). Then I did the opposite. I placed a column of milk inside the straw in contact with the coffee. Because the hot coffee is less dense than the milk it starts flowing up the straw almost immediately, while the cold milk that is denser flows in the opposite direction. After this I ate my breakfast: scrambled eggs with bacon and home fries and blueberry toast (with no butter). When an airplane flies, it pushes the air molecules in front of it creating a compression wave. As the airplane travels faster, the air molecules are pushed together further and further forming more densely packed compression waves. In the early days of modern aviation, planes approaching the speed of sound were battered by these compression waves bad enough that they could be torn apart. This led to the notion that there was a “sound barrier” that prevented flight at speeds faster than sound. Although these problems were eventually overcome with better airplane design (which allows some planes nowadays to fly at several times the speed of sound), the name “sound barrier” stuck. Thus when an object accelerates past the speed of sound, many people refer to it as breaking the sound barrier. When the airplane hits the speed of sound (770 miles per hour) the compression waves merge with one another and create a shock wave which people on the ground hear as a very loud noise called a sonic boom. But you don’t have to fly a plane to generate a sonic boom. This can be done with a whip. The sound that is produced when a whip cracks is the sonic boom produced when the tip of the whip exceeds for an instant the speed of sound. However, using a whip properly requires some practice. As it turns out you can use a regular towel to generate a sonic boom. Most people do this by wetting the towel, rolling it up, and snapping it like a whip, which again requires some preparation and practice. But you can also do it like I demonstrate in the video. Hold a dry towel close to the edges, and flip it upwards and then downwards very fast with a curved motion. The edge of the towel will break the sound barrier and generate a loud crack (sonic boom). I have included a section in the video slowed down to 240 frames per second to better visualize the acceleration of the edge of the towel and the generation of the sonic boom. If you try this, please be mindful of safety. Flip the towel above your head away from your eyes (tilt your head down, don’t look at the towel while you are flipping it), and wear some protection in case the edge of the towel comes in contact with your head. |
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