Boston - 10/27/2012
Ice crystals are solid ice exhibiting atomic ordering on various length scales and include hexagonal columns, hexagonal plates, dendritic crystals, and diamond dust.
The highly symmetric shapes are due to depositional growth, namely, direct deposition of water vapour onto the ice crystal. Depending on environmental temperature and humidity, ice crystals can develop from the initial hexagonal prism into numerous symmetric shapes. — [**]
A sun dog or sundog, scientific name parhelion (plural parhelia) from Greek παρήλιον (parēlion), meaning “beside the sun”; from παρά (para), meaning “beside”, and ἥλιος (helios), meaning “sun”, also called a mock sun or a phantom sun, is an atmospheric phenomenon that creates bright spots of light in the sky, often on a luminous ring or halo on either side of the sun. — [**]
Physics of a Broken Swing Image
Surely by now, someone online has clearly shown this image to be fake. Instead, let’s use this as an example looking at the way people think about forces and motion.
- Forces and Circular Motion
Suppose something is moving around in a circle. Maybe it is a amusement park swing. Here is a force diagram for one of the riders:
There are only two forces on the swinger. There is of course the gravitational force pulling down. The only other force is the tension in the chain pulling on the swinger in the direction of the chain. Then why does the swinger move in a circular path? A component of the tension force pulls up to counter act the gravitational force. The other part of the tension from the chain pulls towards the center of the circle. It is this part of the tension force that makes the swinger move in a circle.
If you like, you can break all forces into two types. If a force is in the same (or opposite) direction as the motion (velocity) of an object, that force will cause the speed to either increase or decrease. If the force is perpendicular to the direction of the velocity, this force will cause the object to change directions. Of course you can have a force that both speeds up an object and causes it to turn.
Really, that is it. That is the only physics that you need to get this swinger to move around in a circle. Sure, there is a relationship between the angle the swing is at and the speed that the swinger moves, but for now we can leave that alone.
- What Would Really Happen?
If the chain suddenly breaks, what happens next? Well, the force diagram becomes a little bit simpler. It would just look like this:
This gravitational force would cause the velocity to change in the downward direction. So, clearly, it would fall down since before the chain broke it wasn’t moving in the vertical direction at all. But what else would it do? Here is a diagram of the swinger from the top view.
In this view (after the chain broke), you can’t see the only force pulling on the swinger – the gravitational force is pulling down. Since there aren’t any forces pushing the swinger to the left or right, from the top the swinger would just go in a straight line.
In the faked image, the falling swinger is clearly moving away from the swing in a path perpendicular to the way he was originally moving.
- Why Would a Faker Get This Wrong?
Many people seem to think that if an object is moving in a circle there is a force pushing outward from the center of the circle. This is the fabulous and mythical centrifugal force. Even though the centrifugal force is fake, it can still be useful in some ways. However, the point is that if you are in a stationary frame then there IS NO force pushing outward.
One of the most commonly observed fluid instabilities is the Rayleigh-Taylor instability, which occurs between fluids of differing densities. It’s most often seen when a denser fluid sits over a lower density fluid. In the video above, this is demonstrated experimentally: a lower density green fluid mixes in with the clear, higher density fluid. This is the classical case in which each initial region of fluid is uniform in density prior to the removal of the barrier. But what happens when each zone has its own variation in density? This is the second case. Before the barrier is removed, each region of the tank has a varying—or stratified—fluid density. In this case, the unmixed fluids are stably stratified, meaning that the fluid density increases with depth. At the barrier interface, the two separate fluids are still unstably stratified—with the denser fluid on top—so when the barrier is removed, the Rayleigh-Taylor instability still drives their mixing. Because of the stable stratification within the original unmixed fluids, the mixing region after the barrier’s removal is more limited. (Video credit: M. D. Wykes and S. B. Dalziel; via PhysicsCentral by APS)
How Do Jellyfish Sting?
the science of cnidocytes and nematocysts
jellyfish don’t sting through electricity or by touch. Jellyfish sting through a special type of cell called a Cnidocyte, there are three types of cnidocytes currently known. Spirocysts which entangle their prey, Ptychocysts which build tubes for tube anemones and the most well known Nematocysts. Nematocysts consist of a toxic barb which is coiled on a thread inside the cindocyte, when triggered the barb is ejected almost instantly taking only 700 nanoseconds to fire and firing with a force of five million g’s. A cindoctye can only fire once, and must be replaced when fired a process that could take 2 days.
From the page:
…For purposes of one of my talks next week in Oxford, I thought it would be useful to actually summarize those laws on a slide. Here’s the most compact way I could think to do it, while retaining some useful information. (As Feynman has pointed out, every equation in the world can be written U=0, for some definition of U — but it might not be useful).
These wave-like Kelvin-Helmholtz clouds can form due to shear between different layers of air in the atmosphere. When one region of air has a higher velocity than the other, their interface forms a shear layer, which can break down in this wavy pattern. In this case, the lower layer of air was moist enough to form condensation and clouds, making the pattern visible to the naked eye. (Photo credit: Gene Hart; via Flow Visualization)
Attached to the body via hinged metal and leather straps this iron arm still allowed a certain range of movements. The hand is fused facing inwards, but the wrist joint can move vertically – as if to shake someone’s hand. A hollow metal globe acts as a substitute elbow joint. This may have been controlled by springs and catches, which are now missing. Surgical amputations are referred to by Hippocrates, they were for many years a main function of the surgeon. This artificial arm dates from the 1500s. During this era, most limbs were amputated due to war injuries or accidents.
This short film offers an artistic look at the phenomenon of the water bridge. When subjected to a large voltage difference, such as the 30 kV used in the film, flow can be induced between water in two separated beakers. This creates a water bridge seemingly floating on air. There are two main forces opposing the bridge: gravity, which causes it to sag, and capillary action, which tries to thin the bridge to the point where it will break into droplets. These forces are countered by polarization forces induced at the liquid interface due to the electrical field separating the water’s positive and negative charges. This separation of charges creates normal stresses along the water surface, which counteracts the gravitational and capillary forces on the bridge. The artist has done a beautiful job of capturing the unsteadiness and delicacy of the phenomenon. (Video credit: Lariontsev Nick)
The Cavendish experiment, performed in 1797–98 by British scientist Henry Cavendish, was the first experiment to measure the force of gravity between masses in the laboratory, and the first to yield accurate values for the gravitational constant. Because of the unit conventions then in use, the gravitational constant does not appear explicitly in Cavendish’s work. Instead, the result was originally expressed as the specific gravity of the Earth, or equivalently the mass of the Earth; and were the first accurate values for these geophysical constants. The experiment was devised sometime before 1783 by geologist John Michell, who constructed a torsion balance apparatus for it.
The apparatus constructed by Cavendish was a torsion balance made of a six-foot (1.8 m) wooden rod suspended from a wire, with a 2-inch (51 mm) diameter 1.61-pound (0.73 kg) lead sphere attached to each end. Two 12-inch (300 mm) 348-pound (158 kg) lead balls were located near the smaller balls, about 9 inches (230 mm) away, and held in place with a separate suspension system. The experiment measured the faint gravitational attraction between the small balls and the larger ones.
The two large balls were positioned on alternate sides of the horizontal wooden arm of the balance. Their mutual attraction to the small balls caused the arm to rotate, twisting the wire supporting the arm. The arm stopped rotating when it reached an angle where the twisting force of the wire balanced the combined gravitational force of attraction between the large and small lead spheres. By measuring the angle of the rod, and knowing the twisting force (torque) of the wire for a given angle, Cavendish was able to determine the force between the pairs of masses. Since the gravitational force of the Earth on the small ball could be measured directly by weighing it, the ratio of the two forces allowed the density of the earth to be calculated, using Newton’s law of gravitation.
The experiment was popularly known as weighing the Earth because determination of G permitted calculation of the Earth’s mass.
One of history’s most notable experiments, always a favorite!
Happy Birthday Stephen Hawking!
“It surprises me how disinterested we are today about things like physics, space, the universe and philosophy of our existence, our purpose, our final destination. Its a crazy world out there. Be curious.” — Prof. Hawking
Image: Noted physicist Stephen Hawking (center) enjoys zero gravity during a flight aboard a modified Boeing 727 aircraft owned by Zero Gravity Corp. (Zero G). Hawking, who suffers from amyotrophic lateral sclerosis (also known as Lou Gehrig’s disease) is being rotated in air by (right) Peter Diamandis, founder of the Zero G Corp., and (left) Byron Lichtenberg, former shuttle payload specialist and now president of Zero G. Kneeling below Hawking is Nicola O’Brien, a nurse practitioner who is Hawking’s aide. At the celebration of his 65th birthday on January 8 2007, Hawking announced his plans for a zero-gravity flight to prepare for a sub-orbital space flight in 2009 on Virgin Galactic’s space service. Credit: NASA
Stephen William Hawking, CH, CBE, FRS, FRSA (born 8 January 1942) is a British theoretical physicist, cosmologist, and author. Among his significant scientific works have been a collaboration with Roger Penrose on gravitational singularities theorems in the framework of general relativity, and the theoretical prediction that black holes emit radiation, often called Hawking radiation. Hawking was the first to set forth a cosmology explained by a union of the general theory of relativity and quantum mechanics. He is a vocal supporter of the many-worlds interpretation of quantum mechanics.
He is an Honorary Fellow of the Royal Society of Arts, a lifetime member of the Pontifical Academy of Sciences, and a recipient of the Presidential Medal of Freedom, the highest civilian award in the United States. Hawking was the Lucasian Professor of Mathematics at the University of Cambridge between 1979 and 2009.
Hawking has achieved success with works of popular science in which he discusses his own theories and cosmology in general; his A Brief History of Time stayed on the British Sunday Times best-sellers list for a record-breaking 237 weeks. Hawking has a motor neurone disease related to amyotrophic lateral sclerosis (ALS), a condition that has progressed over the years. He is almost entirely paralysed and communicates through a speech generating device.
How do animals change color?
the science of chromatophores
Animals like cuttlefish and chameleons are able to quickly change color in-order to blend in with their surroundings. They can do this due to a special type of cell called a chromatophore. Chromatophores work by moving vesicles that contain different color pigments into different forms by contracting and expanding them, so a different color comes to the “surface” when moved, giving the animal a different color. This can either be controlled by the animals nerves or happen hormonally.