angular-momentum

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.

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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]

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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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!

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]

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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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.

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