Besides clapping your hands and snapping your fingers, there is another way to make sounds with your hands. The approach I show in the video below exploits the facts that the palms of your hands are concave, and that when you place your hands opposite to each other and introduce a slight twist, the area around your palms acts like a seal turning both of your hands into suctions cups. Once you have arranged your hands in this position, you can press them against each other forcing out the air between them which will make a sound. The expelling of the air will create a low pressure area (not a vacuum, see below) between your hands, and when you break the seal, the air will rush back in and produce another sound. By pressing and releasing your hands repeatedly against each other you can rapidly produce rhythmical sounds.
It is important to understand that the reason air rushes in when you break the seal is not that the air is “sucked in” by the vacuum in between your hands. As I have explained in my blog, vacuums don’t suck; it is the atmosphere that pushes. A column of air hundreds of miles high above you excerpts a pressure of 14.7 pounds per square inch (at sea level) on your hands. When you move your hands or fingers and break the seal, it is all that pressure that pushes the air back in.
While clapping hands and snapping fingers are well known descriptions of how to make sounds with your hands, I don’t know what word to use to describe the method that I used in the video to produce sounds with my hands. If you know or want to suggest (or invent) a verb for it, please leave a comment below!
In this video you see a pilot pouring water into a cup while the plane is doing loops. At one point the water seems to be flowing away from the Earth below him or remains in the glass even while the plane is inverted. What happened to gravity? Aren’t things like water supposed to “fall”?
Gravity thankfully is alive and well. What is happening is that the looping motion of the airplane generates a force directed towards the bottom of the plane. This force, called centrifugal force, is strong enough to counteract the force of gravity and make the water stay in the cup or even “fall up” (from the point of view of the camera) while the plane is upside down during the loop.
You can see my demonstration of this force in the video below.
My demonstration is not perfect in that to spin around the cup my arm has to produce a twisting motion that sloshes the soda around and generates foam, but the principle is the same. In the case of the video of the airplane, the motor propels the plane, and the navigation controls inside the cabin lock it into a circular path which generates the force. In the case of my video, the muscles in my arm propel the cup, and my grip on it locks it into a circular motion which generates the force.
In the video below, Dianna Cowern (Physics Girl) employs science to scare another YouTube personality, Justine (iJustine). To this end she uses screaming dry ice, a Van de Graff Generator, and a plasma ball. The video is fun, and Dianna also explains the science behind the effects very well. Enjoy!
Since time immemorial human beings have observed the curious phenomena of non-coalescence of drops. This happens when a drop of a liquid comes in contact with a liquid surface and does not merge (coalesce) with the liquid surface right away. Rather the drop may remain as if floating on the liquid surface for periods of time ranging from seconds to milliseconds before finally merging with it. In the video below, I used a straw to pick up a volume of my coffee and gently add drops onto the surface of the coffee. The non-coalescence effect is observed in the drops to various extents, and it can be seen clearly in the part of the video slowed down to 240 frames per second.
Although this phenomenon has been investigated by several scientists spanning a time period of more than 100 years, we still don’t know for certain how it happens. The non-coalescence of drops depends on many variables including the nature of the liquid in the drop and the surface upon which it lands, the chemicals dissolved in them, the temperature gradient between the drop and the liquid, the charge of the drops, and the air pressure.
A current hypothesis is that those areas of the drop or liquid surface in contact with the air phase (interfacial) have a molecular organization that is different from the areas away from the air phase (the bulk phase). Thus the drop and the surface upon which it lands do not tend to mix right away when placed in contact with each other. However, as time goes by, the interfacial layer of the drop and the liquid surface tends to dissipate at the point of contact between them (which is no longer exposed to air), and after it has sufficiently thinned, the water drop coalesces with the liquid surface.
As I have explained before, water molecules due to their atomic makeup have one end with a partial negative charge (where the oxygen atom is) and another end with a partial positive charge (where the hydrogen atoms are). This gives rise to a phenomenon called surface tension where water molecules stick to each other (positive to negative) and to surfaces. This effect can be seen in the video below when I poured milk into my coffee before breakfast. The milk, which is more than 90% water, stuck to the side of the glass, and even thought I was tilting the glass more than 80 degrees, not a single drop of milk fell outside!
In case you are wondering, as in my previous Science Before Breakfast video, I had scrambled eggs with bacon and home fries for breakfast but no blueberry toast this time.
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.