DARPA’s Cheetah Bot
AGES ago I posted a bit on DARPA’s plan to create a robot emulating the movement of a cheetah, whilst ambitious, previous projects such as Big Dog (created by Boston Dynamics) have been overwhelmingly creepy and effective. Now here’s the first video of the new cheetah bot, showing it run at speeds of up to 18 mph, which although not the organic cheetah’s record of 75 mph is still pretty damn good.
Microgravity Roller Coaster
Possibly one of the coolest applications of physics has to be in theme parks. Now engineers are going to try something a little more extreme than your run of the mill roller coaster. Specifically emulating zero gravity conditions while being strapped to a cart. But just to make things even more interesting they also wish to double the force of gravity in some parts of the ride. The roller coaster named “Vomit Comet” will be able to accelerate to speeds greater than 100 mph before shooting vertically upwards. At this stage the ride will slow slightly, giving its passengers the sensation of weightlessness for up to 8 seconds. The ride, designed by BRC Imagination Arts, could cost up to 60 million dollars due to the level of precision required, in fact due to the varying weight of passengers the physics of the ride will have to be recalculated every run.
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Superconductors.
The basic concept of a superconductor is that it is capable of sustaining an electrical current without resistance. Resistance in a circuit is what causes a loss of energy, so superconductors are the closest thing we have to perpetual motion. However, they only work at near absolute zero, or more specifically, anything colder than 91 Kelvin.
When a superconductor is cooled to these temperatures, any interaction with a magnet causes a repulsion, this effect is called the Meissner Effect. The induced field in the superconductor opposes the applied field of the magnet, therefore repelling each other. So if the two repel, how is it possible to achieve any levitation? When the magnet is moved closer the flux trapping effect is engaged and the superconductor not only repels the magnet, but attracts it as well. The magnetic flux lines from the magnet are trapped inside the superconductor causing the magnet to be held at a fixed position. This is only possible if there are imperfections in the crystalline structure of the superconductor.
Of course, the opposite effect of levitation also occurs. When the magnet is picked up, the superconductor remains in magnetic suspension, and hovers below the magnet. Another phenomena is that the levitated magnet will freely move at a fixed distance over the superconductor without friction, so the applications would benefit transport, which means we can all have hover cars now.
The Theremin
Possibly one of the coolest instruments in existence, the theremin is responsible for haunting, whining music everywhere such as the original Star Trek theme. The thing that sets the theremin is apart is it that to play it, you don’t need to touch it. The theremin works by having two antennas that “sense” where the player is relative to them. One controls the volume and the other the frequency and both are modulated by the distance between the antenna and the hand of the performer. The frequency is controlled using two oscillators, one is fixed while the other is free to vary, the difference between the two frequencies is what is audible and is an example of heterodyning (combining two frequencies to create a third). The frequency of one of the oscillators can be modified because the antenna and the hand act as a simple capacitor, as the hand moves closer the capacitance increases and the frequency increases.
As a bonus fact Vladimir Lenin took lessons in playing the theremin and commissioned 600 to be built and distributed around the Soviet Union.
Video of Fragile by Sting preformed on Theremin.
Stellar Explosions in the Lab
When an experiment consists of magnetism and plasma flying at 50 km/s you know it’s got to be good. A recent experiment conducted at Caltech involves these and more.
The experiment itself was conducted to better understand coronal mass ejections which form when magnetic fields “snap” and reconnect, expelling incredibly hot plasma in the process. Another phenomenon known as a Kink instability was also present. As the plasma is essentially 20,000 Kelvin gas of charged particles (with a current of 100,000 Amps) it generates a magnetic field as it moves. This magnetic field affects the charged particles in return, causing them to spiral and cork screw, which can be seen in this picture.
The Kink instability also begets another instability known as the Rayleigh-Taylor instability (which is also what causes the tendrils on the inside of the Crab Nebula) which forms when a dense fluid attempts to move through a less dense fluid, in this case the dense plasma through the lower density vacuum that trails it, causing the ripples seen.
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Soap Films and the Minimal Surface
One subject of particular interest to me is soap films, while they hold wonder for children they’re also amazing in scientific terms. The shape and structure of a soap films is determined by what configuration minimizes surface area, this is why bubbles are round. However other interesting shapes known as minimal surfaces arise such as the catenoid and helicoid. The catenoid is the shape formed by rotating a caternary around it’s axis of symmetry, the catenary in turn is the shape formed by a hanging chain. The helicoid is a minimal surface that can be formed from a catenoid without any deformation or stretching. Both of these shapes (along with the plane) have zero mean curvature and also minimize surface area and as such are energetically favorable shapes for soap films (with boundaries) to exist in.
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Why Do We Only See One Side of the Moon?
Every 29.5 days the moon both completes one rotation about its axis and one orbit around Earth. This may strike you as an odd coincidence or even something of a miracle. It however is neither of these, but rather an almost unavoidable consequence of gravity. When the moon was first formed it rotated much faster than it did now and was also much hotter. The thing that may not be obvious to you is that not only does the moon cause tides on Earth, but the Earth causes tides on the moon. Of course not of water, but rather of the rock itself, this means that the moon bulges like an ellipsoid both towards us and away from us. It was this bulge that robbed the moon of angular momentum by converting it into heat through friction due to the moon’s contraction and expansion. In a way it’s this same principle that may lead to liquid under the surface of Titan by creating enough friction to melt water. Over time the moon will have this same effect on the Earth and it will have a day lasting 29.5 current days, but that’s a long way off!
Images: moon’s near side, moon’s far side.
A video showing the physics of gravity involved in Minecraft.
Parabolic Motion Bonus Fact!
According to relativity the parabola is the pathway that takes the mosttime. While this seems to contradict the principle of least action it needs to be stated that this is only relative to the object in motion. To a stationary observer the path is the least time taken, but to the ball in flight it takes the most time. This is because it’s a balance between the time slowing effects of increased velocity and time speeding up effect of lower gravity.
Principle of Least Action
The Principle of Least Action is a type of classical mechanics that stands separately from Newton’s laws. It also has the interesting effect of grasping a fundamental concept of the universe. Most of you will know that if left to its own devices an object or system will go towards the lowest possible energy state, a ball in mid air will fall to the ground instead of hovering awkwardly or an ice cube will melt in warm weather. What may not be so intuitive is that the same applies for time. An object in motion will take the pathway that reduces the time it takes, for example the movement of light through two mediums will occur with an angle of refraction that causes the light ray to take the shortest possible time. The concept of action is what happens when you multiply energy and time together and will always head towards being a minimum.
The interesting question that arose when the Principle of Least Action was first put forward was “how does the light know which path to take?” and this question also has an analogue in quantum physics. When a single electron is fired towards a sheet with two slits in it, we still get an interference pattern, which means that the electron is interfering with itself. The solution to this came from Paul Dirac and Richard Feynman and simply states that there is a probability that an object traveling from point A to point B takes every possible pathway. Therefore the light and electron do somewhat “know” which path to take because it some ways it could be said to have already gone through it.
Hydrogen Bonding
Hydrogen bonding is a common “force” in nature, it’s what holds your DNA and proteins together and what makes water so weird and wonderful. Without it you wouldn’t be you, in fact you probably wouldn’t be anything. Hydrogen bonds are both inter and intra molecular forces in that it can act between different molecules (in the case of DNA bases) or within the same molecule (such as single chain proteins). Hydrogen bonding arises from polarity within a molecule, for this to happen a hydrogen must be bonded to an electronegative atom such as oxygen, fluorine or nitrogen (or be part of something like CHCl3). This causes the probability of an electron being around the hydrogen to decrease thus leaving it with a partial positive charge whilst the electronegative species has a slight negative charge. The slightly positive hydrogen is then attracted to other electronegative atoms that neighbor it. This causes an attractive force between them and gives an organized structure such as the crystalline form of ice or the hexagonal shape of a snowflake.
Mirages
When it comes down to it a mirage is essentially just an invisible mirror. Which is still pretty damn cool. Mirages are simply the distortion of objects due to “layers” of air and come in three different types: inferior (where the mirage is under the real object), superior (where the mirage is above) and Fata Morgana (which is a complex, highly distorted mirage involving multiple images). The principle behind the mirage is rather a simple one. At different elevations air has different densities, typically due to heat, which in turn causes different refractive indexes. This difference in refractive indexes causes the light to bend as it crosses from hot to cold or cold to hot air due to refraction. As such the image you see is not where it really is but is simply light from the original object being bent towards you. In the example of the highway mirage above the “pool of water” is in fact simply the reflection of the sky being “bounced” towards the observer by a hot layer of air directly above the road.
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Zero Point Energy
Everything you see around you is vibrating. The hotter it gets the more it likes to jiggle about, in fact that’s how you perceive heat, but even at colder temperatures (approaching 0 Kelvin cold) the jiggling continues. As cold as you make it, particles are still bouncing around albeit ever more slowly. This phenomenon is called Zero Point Energy and simply states that even at the lowest energy state possible a quantum system still has some degree of energy. Zero Point Energy is due to the wave like nature of all matter as of course it would be impossible to have a wave with no energy, but it also relates to the Heisenberg uncertainty principle. If we were to say that a particle (or even field) has no energy we would be defining an absolute value, which of course we cannot have. Therefore the particle oscillates around this minimum value.
Zero point energy has been confirmed experimentally by the Casimir effect. Which demonstrates that if two uncharged metal plates are put close together in a vacuum that there will be a force acting to push them together. This is because the small distance between the plates excludes vacuum energies of large wavelengths so that more energy is located outside (or inside sometimes) the plates, thus forcing them together.
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