A new study (published in PLOS Biology) investigated how bats make sharp turns in the air, particularly when they have to grab the ceiling. It turns out aerodynamics have very little to do with it - it’s all about inertia. Just as a figure skater clutches his arms to his chest to increase his speed, bats pull in their wings to help them make turns.

You can read all about it (and see more video) in this piece by my friend Nsikan Akpan over at PBS Newshour.

NASA Tells Space Cowboy Concept To Mount Up

by Michael Keller

Earlier this month, NASA awarded $100,000 to a Washington-based company to develop their concept for a cowboy spacecraft. The firm, Tethers Unlimited, has up to a year to develop a proposal for a craft that can deploy a net and tether attached to a winch to capture an asteroid and stop it from spinning.

The nanosatellite-scale system, envisioned in the artist’s concept above, is called the Weightless Rendezvous And Net Grapple to Limit Excess Rotation (WRANGLER, of course). The proposed system will use two technologies to stop a much larger and more massive asteroid from spinning: the Grapple, Retrieve, And Secure Payload (GRASP) Technology for Capture of Non-Cooperative Space Objects that uses inflatable tubes to deploy a net; and a winch-mounted tether that can exchange angular momentum with the object.

If it works, it could be an important element to decrease the complexity and risk of NASA’s long-term plan to collect and redirect an asteroid. Once caught and despun, the celestial object would then be moved to a stable orbit beyond the moon so that astronauts can explore it.

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Fire Tornadoes, or Fire Whirls, form in situations where fires are present through similar mechanisms to tornadoes. Hot air is trying to rise, but it can’t get through a thicker layer of colder air, so it bursts through in one spot. When air compresses itself from a wide area into a small cone, the angular momentum of the air causes the full cloud to rapidly spin. This is perhaps the largest fire whirl I’ve ever seen. It was filmed last week by a firefighter working in Idaho.


Euler’s Disk is an interesting toy which allows you to get a real feel for a physical singularity.  If there was no friction, slipping, energy loss or air to complicate things- then the equations predict in a finite length of time it should spin infinitely fast. Of course this isn’t possible, and there are several explanations for what stops the disk from spinning quite abruptly at the end. [more]


Where does cosmic rotation come from?

“Before our Universe was filled with matter, radiation, neutrinos, dark matter or any of the particles that we currently find in it, it was in a rapidly expanding state, where the only energy found in our spacetime was the energy intrinsic to space itself. This was the period of cosmic inflation that gave rise to the Big Bang that we identify with the birth of what we call our Universe. During this time, as far as we can tell, there were quantum fluctuations produced, but they couldn’t interact with one another, as the expansion of space was too rapid to permit interactions mediated “only” at the speed-of-light. As far as we can tell, the expansion was the same everywhere and in all directions, with no particular preferred axis of any type.”

And yet, everything in the Universe today revolves and rotates. Where did this cosmic “spin” come from? Your excellent questions answered on this week’s Ask Ethan!

Climate change moves the world.

The rotation pattern of our globe seems to be changing, as mass is redistributed around the world due to the effects of global warming. The change is small, but clear, and is one of global warming’s most interesting but least threatening consequences. The entire planet has always wobbled as it turns due to its uneven shape, since the centrifugal force acting on all the spinning rock causes a big uneven bulge at the equator.

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This photo of the Amazon River taken by Astronaut Tim Kopra reveals the many meandering changes of the river’s course. Left untouched by human intervention, rivers tend to get more curvy, or sinuous, over time, simply due to fluid dynamics. Imagine a single bend in a river. Due to conservation of angular momentum, water flows faster around the inside curve of the bend than the outside - just like an ice skater spins faster with her arms pulled in. From Bernoulli’s principle, we know there is an accompanying pressure gradient caused by this velocity difference - with higher pressure near the outer bank and lower pressure on the inner one. This pressure gradient is what guides the water around the bend, keeping the bulk of the fluid moving downstream rather than bending toward either bank. 

At the bottom of the river, though, viscosity slows the water down due to the influence of the ground. This slower water, still subject to the same pressure gradient as the rest of the river, cannot maintain its course going downstream. Instead, it gets pushed from the outer bank toward the inner bank in what’s known as a secondary flow. This secondary flow carries sediment away from the outer bank and deposits it on the inner bank, which, over time, makes the river bend more and more pronounced. (Image credit: T. Kopra/NASA; submitted by jshoer)

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Yesterday I wrote about geckos’ sticky feet. But it isn’t their feet alone which make them so excellently adapted to living in trees- their tails also play a big role. They act as a third foot when one foot slips, they push them back against the tree when two feet slip, they even help them guide their falls, almost glide. A shown in the animation, the tail allow the gecko to self-right in mid-air in a tenth of a second (see animation). [more]

Climate change moves the world.

The rotation pattern of our globe seems to be changing, as mass is redistributed around the world due to the effects of global warming. The change is small, but clear, and is one of global warming’s most interesting but least threatening consequences. The entire planet has always wobbled as it turns due to its uneven shape, since the centrifugal force acting on all the spinning rock causes a big uneven bulge at the equator.

Keep reading

Can You Slow Down a Day Using Angular Momentum?

Could you do this? Could a spinning human slow down the Earth? Theoretically, yes.

It’s All About Angular Momentum

In an introductory physics course, there are three big ideas. There is the work-energy principle, the momentum principle and then the angular momentum principle. I’ll skip the work-energy principle since it doesn’t matter too much here. You might be familiar with the momentum principle. Basically, it says that the net force on an object changes its momentum. I can write it like this:

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Everyone remembers playing with soap bubbles as a child, but most of us probably never became as adept with them as magician Denis Lock. In this video, Lock shows off some of the clever things one can do with surface tension and thin films. My favorite demo starts at 1:25, when he constructs a spinning vortex inside a bubble. He starts with one big bubble and adds a smaller, smoke-filled one beneath it. Then, using a straw, he blows off-center into the large bubble. This sets up some vorticity inside the bubble. When he breaks the film between the two bubbles, the smoke mixes into the already-swirling air in the larger bubble. Then he pokes a hole in the top of the bubble. Air starts rushing out the deflating bubble. As the air flows toward the center of the bubble, it spins faster because of the conservation of angular momentum and a miniature vortex takes shape.  (Video credit: D. Lock/Tonight at the London Palladium/ via J. Hertzberg)


Life from Dust: How Stars Form

We know that the stars exist as sure as we do, but how did they form? How did our sun and solar system get here?

The spaces between the stars in a galaxy aren’t completely voids. Interstellar space is often filled with cold clouds of dust and gas called nebulae, many light years across. The majority of the gas is molecular hydrogen and helium, along with some heavier elements formed in other stars, and the cloud is so huge that it contains enough mass to form thousands of suns.

When such clouds are disturbed—like when a nearby star goes supernova, or when galaxies collide—shockwaves can squeeze the gas and dust together. They form a denser, spherical cloud, which gravitationally attracts more and more material until it collapses in on itself. The movement of all the individual particles of gas and dust cause the cloud to spin in a certain direction, and due to the conservation of angular momentum, the cloud spins faster and faster as it collapses. Think of an ice skater who twirls slowly when their arms are thrown out, and faster when they pull their arms in. (Definitely try this at home, kids: instead of ice skates, use your spinning desk chair.)

The collapsing cloud soon flattens into an accretion disc, which spins around a dense core. As gravity keeps pulling in mass, the core becomes hotter and more pressurised until temperatures of 10,000 degrees Kelvin trigger nuclear fusion reactions. Hydrogen atoms (one proton, one electron) fuse together to form helium (two protons, two electrons)—and in the process, release huge amounts of energy.

Thus, a star is born. At this stage it’s called a protostar, because it hasn’t finished collapsing; it’s still much larger than its final state. It takes a few more million years for the outward force of fusion to balance with the inward force of gravity, and therefore become a stable star.

(Image via Wikipedia Commons)

Meanwhile, the accretion disc is still spinning around this new nuclear reactor. The dust particles will soon start to stick together via electrostatic forces, forming tiny clusters, which pull in more material to form pebbles, then rocks, then boulders, then planetoids. These-not-quite-planets cross orbits and collide in periods of violent bombardment, and eventually, they too stick together and whole planets are formed out of the amalgamation of millions of specks.

Our solar system formed this way around 4.6 billion years ago; our sun ignited in a nebulae’s dust. Different types of stars are formed depending on the initial mass of the protostar; some are huge and hot and short-lived, others are small and cool and burn for tens of billions of years. Our sun sits somewhere in the middle, an ordinary yellow star with a lifetime of 10 billion years.

We weren’t around to see the formation of our own solar system, but by looking out into the universe we can see other stars being born elsewhere. From our vantage point on a speck made of specks, we can look into the void between stars and see life rise from the dust.

(Image Credit: Tarantula Nebula, HubbleSite)

How Do the Planets Stay in Orbit?

The manner in which stars and planets form is not well understood, but according to the most popular hypothesis (nebular) of cosmic evolution; all planetary and stellar systems are born from the gravitational collapse of giant interstellar clouds.

Solar system formation happens when part of a molecular cloud begins to contract under its own gravitational force. As gravity forces the dense core to become smaller, it spins faster and faster –due to the conservation of angular momentum. The collisions between particles in the molecular cloud gradually reduce random motions, only circular orbits remain and as a result the contracting cloud flattens into a spinning pancake shape (protoplanetary disk) with a bulge at the center (protostar). All orbital motion of the planets – including the spin, is due to this original angular momentum.

The reason why the planets stay in their orbits is because the force of gravity between the planet and the Sun provides a centripetal force: the Sun pulls the planets in orbit around it with the force of gravity that is strong enough to divert the planets from a straight line path. At the same time, the angular momentum of the planets gives them a tangential velocity that prevents the Sun from pulling them in. Isaac Newton’s law of universal gravitation states that everything with mass generates gravity, and like all objects with mass, planets have a tendency to resist changes to their speed and direction. This tendency is called inertia.The combination of the centripetal acceleration that gravity provides and the tangential velocity that inertia provides is what keeps the planets in stable orbit.

Newton’s laws of motion and law of gravity explain Kepler’s three laws of orbital motion which govern the orbits of planets, satellites, comets, asteroids, stars and even galaxies. Kepler’s laws apply to any object, orbiting another object, under the influence of gravity. So, what are these universal laws?

Kepler’s first of three laws states that each planet moves in an elliptical orbit, with the Sun at one focus (The Law of Ellipses):

Kepler’s second law: the orbital speed changes regularly as the distance from the Sun changes, meaning that when the planet is far away from the Sun, it does not move as fast as when it is close to the Sun (The Law of Areas). This implied that whatever force was moving the planet weakened with distance, and this led to the realization of what became Kepler’s first law: that the planets move in an ellipse:

Kepler’s third law (The Law of Harmonies) can be derived from Newton’s laws of motion and the universal law of gravitation. The third law shows that orbital period increases with orbital radius:

Kepler’s laws clearly show the effects of gravity on orbital motion. Using these laws and calculus (which Newton invented), Newton was able to prove that the planets orbit the Sun because they are influenced by its gravity.

Why does the Earth rotate?

You probably know since you were a child that the Earth rotates on it’s axis and complete a rotation in about 24 hours. Have you ever wondered why it rotates at all? And why all the other planets do the same?

You’ll be surprised to know that the dynamic of this is still not well understood by current planetary models, but luckily the physics is not hard, so we can have a glimpse of what happens that makes planets start rotating.

First of all we have to remember that any planetary system is formed from what’s called a protoplanetary disk.

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We now have to introduce the concept of angular momentum, which is a measure of how much something is spinning relative to a fixed point. A very important thing is that if we measure the angular momentum in the center of mass of a system, the quantity is conserved. It’s like to say that the total spinning of a system must be conserved.

So, in the protoplanetary nebula case, we can measure the angular momentum of the system relative to the proto-sun at it’s center (that is the center of mass) and we have a quantity that is conserved.

Now, what all this means in practice? It means that when parts of the protoplanetary disk condense to form a planet, they have a certain angular momentum that, being conserved, give the planet it’s spin.

So the planets, and then the Earth, were formed already spinning. They don’t stop just because in vacuum there’s no friction to slow down their spin.


A new twist on the Doppler shift

Light with orbital angular momentum will undergo a frequency shift when it bounces off a rotating object.

The well-known Doppler shift is typified by the audible rise and fall in the pitch of a siren as a fire engine races past. That Doppler shift applies to light too, and it is perhaps best exemplified by the redshift of the spectral emission lines from distant stars. Indeed, it was in the context of the color of light from binary stars that Christian Doppler first proposed the phenomenon in 1842; three years later Christoph Buys Ballot verified the effect for sound waves.

For light, the angular-frequency shift Δω = ω0v/c, where ω0 is the unshifted frequency and v, the relative speed between source and observer, is presumed to be much less than the speed of light c. If the velocity is not parallel to the line of sight between source and observer, then the frequency shift picks up a factor of sin α, where α is the angle between the line of sight and the velocity vector. One then speaks of a reduced Doppler shift Δω = (ω0 sin α) v/c.

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