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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.
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I am fascinated by water striders. These insects are uniquely adapted for their life on the surface of the water. I filmed several of them in a creek. You can’t really see the actual insects very well in the video below, but you can see two things that give away their presence. The first is that each insect generates two small round shadows and a larger round shadow on the surface of the water. These shadows are created by the way light is altered by the bending of the surface of the water by the strider’s rear (small shadows) and front (large shadow) legs. These insects exploit the surface tension of water to stay afloat coupled with adaptions in the tips of their legs that make the legs water repellent and also help them trap little bubbles of air resulting in high buoyancy. The second is that these insects use their middle legs as oars generating an explosive movement that propels them forward giving rise to waves that radiate from the strider in a concentric pattern. A multitude of striders in one place can create quite a display of circular waves giving rise to cool interference patterns as shown in the video. Since time immemorial there have been individuals who claim to have “special powers”. They maintain they can tap into the “vibrations” or “energies” or “essences” of nature, or they even claim to have a special relationship with the “divinity”. And these individuals can demonstrate their powers or even use them to help people…for a fee, of course. At the same time there have been other individuals that have stood up to these fakes and exposed the tricks they use to lie and manipulate. For example, in the 1920s the magician Harry Houdini exposed many mediums who claimed they could communicate with the dead even offering thousands of dollars to anyone who could provide believable evidence of their powers (no one could). In more recent times others have taken to this task. The magician James (The Amazing) Randi exposed many of these individuals who claimed to have special powers such as the psychic James Hydrick in 1981. Randi, like Houdini, also offered a monetary prize that ended up reaching an amount of one million dollars to anyone who could present conclusive evidence of psychic powers in carefully controlled conditions (no one succeeded). Randi was also instrumental in exposing the televangelist Peter Popoff in 1986. Magicians Penn and Teller, who are disciples of James Randi, have continued this tradition of debunking paranormal or pseudoscientific claims in their famous program “Penn & Teller: Bullshit!” that ran from 2003 to 2010. Below you can see their debunking of the psychic Rosemary Altea. The best way to deal with these characters who claim to have special powers is to keep a closed mind and to contract the services of a seasoned magician! The domino tumble is a great example of the conversion of potential energy into kinetic energy (energy of movement) and the internet is filled with videos of elaborate domino tumbles involving thousands of dominoes such as the one below. Less well known variants of the domino tumble are the double domino and domino amplification effects. The double domino effect occurs when the regular forward wave of toppling dominoes is followed by a second wave in the opposite direction. This occurs because the first wave leaves the dominoes lying partly on top of each other with some of their potential energy still intact. When the terminal domino falls to the ground, it fails to provide the support needed for the domino immediately before it which falls to the ground too followed by the next one and so on. The process and the math involved are explained in the video below by Matt Parker from standupmaths. The domino amplification effect is a far more interesting for me because with a tiny input energy you can release a huge amount of energy in a process that is somewhat similar to a chain reaction. This is possible because a domino can topple over another domino that is 1.5 times larger. Thus by placing dominoes of increasing size one after the other you can topple a gigantic domino using a much smaller one achieving amplifications factors in the billions. The process and the physics involved are explained in the video below. The domino amplification effect was featured in an episode of Mythbusters shown in the video below. |
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