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For most people, clouds are these fluffy white things floating up in the air, which may give the impression that clouds are light, but nothing could be further from the truth. Clouds are created when water vapor condenses into minute water droplets due to changes in pressure and temperature, and water has weight. Those large cumulus clouds that you see up in the sky actually weigh hundreds of tons! The reason why clouds don’t fall is due to in part the same reason why ships built out of steel don’t sink. The density of the clouds is lower than the density of the underlying air. In other words, a given volume of air below the cloud is heavier than the same volume of air in the cloud, thus the cloud floats on this air. In cumulus clouds, this happens because as the warm air rises from the land it expands and becomes less dense. Additionally, the water droplets that make up most clouds are microscopic, and the effect of gravity on them is negligible and easily counteracted by the updraft currents within the clouds. 
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Derek Muller from the YouTube Channel Veritasium demonstrates hydrodynamic levitation and explains the principles behind it. This guy is having too much fun! In the video below, I describe a new way to drink water with your hands. It involves first forming a cup shape using both hands to hold the water. The next step is curling the fingers of the hand mostly responsible for the side of the cup into a fist, which forces the water up. Finally, at the same time that the fist is made, you bring your mouth to the opening of the cup and suck creating a low pressure area inside your mouth. This will result in the atmospheric pressure pushing the water into your mouth. The water held between your hands is roughly enough for a mouthful. The physical forces acting upon bullets travelling through water are very different from those acting upon bullets travelling through air due to the much higher friction of the liquid medium. This fact allowed Norwegian physicist Andreas Wahl to shoot himself with a rifle in a pool and survive as shown in the video below. Firing guns underwater produces interesting effects that are explored in the video below by Destin from the YouTube channel SmarterEveryday. He shoots an AK-47 rifle underwater and explains some of the science involved in the effects. Several approaches have been used to increase the distance that bullets can travel underwater from the APS underwater riffle to supercavitating ammo. Most people are amazed by the multicolored fishes that they see in aquariums. Many wonder how such fishes can survive in the wild. After all, aren’t those bright colors on their bodies like a bull’s eye for predators? What must be remembered is that different wavelengths of light are absorbed by water to different extents. The most extreme case is that of red light. In the video below Kendall Roberg shows how colors change the deeper you dive. The video is about selecting the right fishing lure, but it demonstrates that as depth increases, the the color red is perceived as red to a lesser degree (because there is less red light to reflect) until it comes a point when it appears black. Other colors are also affected, but not as much. Most people are familiar with bubbles. Bubbles are a thin film of a liquid containing a volume of gas. But have you heard of anti-bubbles? Anti-bubbles are the opposite: a thin film of gas containing a volume of liquid. Dianna Cowern (Physics Girl), creates some anti-bubbles and explains what they are in the video below. At the beginning of her video while performing the classic milk and food coloring experiment, Dianna created some liquid drops that did not coalesce (merge) with the liquid on top of which they moving. This is a different physical phenomenon from anti-bubbles which Destin from SmarterEveryDay calls “walking water”. Destin investigates walking water in the video below with the help of Don Pettit, a chemical engineer and NASA astronaut. Although I think his video was awesome, the explanation that the droplets don’t merge because they are resting on a layer of air has been challenged by a competing hypothesis. 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. Water puts out fire. Everybody knows that, right? That is why firefighters hose down burning things with water, no? Actually, the truth is more complicated. There are certain fires that can actually be made worse by pouring water on them. Such is the case of oil or grease fires. You can see what happens when you add water to an oil fire in the video below by Greg Foot from BBC Earth Lab. The way water puts out a regular fire such as a wood fire is by covering it and depriving the fire of oxygen. But with oil the water displaces the oil rather than covering it. There are three potential explanations for this: 1) Water is denser than oil and will sink to the bottom pushing the oil upwards. This is why the water doesn’t cover the oil. 2) The mechanical force with which the mass of poured water hits the oil makes it splash. 3) Part of the water may be quickly converted to steam as a result of the heat, and will boil through the oil making it splash. An oil or grease fire will burn at the surface because only that area of the oil has access to oxygen. When water dropped on an oil fire causes the oil to splash, the mass of the oil underneath the fire is displaced upwards and more of the oil gains access to oxygen and the combustion reaction resulting in more oil igniting. Which of these explanations is the most accurate? To figure that out we can check what happens when trying to put out water with liquid nitrogen. Nitrogen is a gas that displaces oxygen and can put out a fire. When nitrogen gas is cooled down to - 320 °F, it becomes a really cold liquid with a density lower than that of oil, so liquid nitrogen should float on top of oil not sink under it like water. Therefore explanation #1 does not apply here. Also the mechanical force of a mass of poured liquid nitrogen would be similar to that of a mass of water poured into the oil fire (they are both liquids). Therefore any difference between liquid nitrogen and water would not be due to explanation #2. What happens when you pour liquid nitrogen into an oil fire was investigated by Cody from Cody’s lab in the video below. As you can see, it was actually worse than water! The liquid nitrogen turns to nitrogen gas so rapidly upon heating that it violently displaces the oil making it splash and creating an explosion with a loud bang. It seems to me that of the three explanations, the third one is most likely responsible for most of the effect we observe with both water and liquid nitrogen. And in case you are wondering who in their right mind would try to use liquid nitrogen to put out an oil fire in the real world, there is at least one known case of a scientist who tried this. It didn’t go well. If you ever face a pan of oil or grease that has caught fire, the easiest and safest way to put out the fire is to smother it with a cover such as the lid of a frying pan. This will deprive it of oxygen and extinguish the flames. So never try to put out an oil fire with water, or liquid nitrogen for that matter. 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. If two grenades fall near you, one on the ground and another in the water, what would you do? Would you dive in the water a certain distance away from the underwater grenade hoping that the water will shield you from the shrapnel, or would you dive to the ground hoping that not a lot of the shrapnel from the surface blast would hit you? Mark Rober teams up with Kevin (TheBackyardScientist) to figure this out. Their video is very good, so I will leave the explanation of the science up to them, but you can also check a written version here. |
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