www.z2systems.com ...Read On
www.z2systems.com ...Read On
We were half right with our explanation. Geosynchronous is a term used to describe the orbit of a satellite that moves at the same speed that the Earth rotates about its axis. However, because this orbit can be titled over the Earth like an angel with a lopsided halo, the satellite can appear to move north and south in the sky throughout the day, though it always stays over the same line of longitude.
A geostationary orbit, the one we often think of when we hear the word "geosynchronous," is when a satellite is in a geosynchronous orbit over the equator. In this kind of orbit, the satellite appears to be stationary over the Earth.
In the same way that a square is always a rectangle but a rectangle isn't always a square, a satellite in a geostationary orbit is always in a geosynchronous orbit, but not the other way around.
Communications satellites are often in geosynchronous orbits so that the antennas of ground stations can remain constantly pointed at the same spot in the sky. Weather satellites are also common geostationary orbiters so that they can constantly monitor the same spot on the Earth.
Back to our lunch discussion: How fast, we wondered over our kebabs, would a geostationary satellite have to be moving to stay stationary in the sky? The space shuttle orbiter, we know, orbits at around 8,000 meters per second (18,000 miles per hour) but it does a complete orbit in about 90 minutes. Would a geostationary satellite be going faster or slower?
To find out, I did a little math. To find the speed of an object, we divide the distance it crosses by the time it takes to cross that distance. (Speed equals distance divided by time.) The speed of the Earth's rotation is 465 meters per second, which we get from dividing it's circumference, 40,075 km, by 86,4000 seconds (the number of seconds in a day).
To find the circumference of the geostationary satellites' orbit, we add the radius of the Earth, 6,378 km, to the height of the satellite's orbit, 35,786 km, (which we obtained from Wikipedia) to get 42,164 km. We then multiply that number by 2*pi (the equation for the circumference of a circle is the circle's radius times 2*pi) to get 264,924 km.
Because the satellite has the same orbital period as the Earth's rotation, we divide the orbital circumference by 86,400 seconds and we get 3,066 meters per second (or 6,858 miles per hour) -- quite a bit slower than the space shuttle. Still, that's way faster than the average bear. feedproxy.google.com
["A Sunday on La Grande Jatte" by Georges-Pierre Seurat. Click on the photo to see the individual dots of paint in the park scene. In her talk, Imogen Clarke used the painting to show that while the individual dots are discontinuous, the painting as a whole is continuous, much like the role of atoms in our lives.]
Last week, the American Institute of Physics (a society that, among a few others, shares our building) hosted a conference on the history of physics. A hundred or so graduate students and early-career physicists gathered to talk about topics stretching from the first uses of X-rays in medicine to the development of theory in physics.
Imogen Clarke, a PhD student of science history at the University of Manchester in the U.K., spoke Friday morning about the transition from classical to modern physics at the turn of the last century. In a talk titled "A 'Conservative Attitude'? Continuity, Discontinuity and the Contested Rise of 'Modern Physics'" Clarke noted how the changes in thinking in the physics community in the early 1900s mirrored the broader cultural attitude shift of the time.
The painting above, "A Sunday on La Grande Jatte" painted in 1885 by Georges-Pierre Seurat, is an example of the cultural merging of continuity and discontinuity at the turn of the century, Clark said. Seurat's scene shows that while the painting appears continuous, the individual dots of paint are actually discontinuous, much like the role of atoms in our lives. Atoms on their own are discontinuous, they are individuals, but when combined they form people and rocks and trees - continuous objects.
Pablo Picasso's "Le Guitariste" (at right) from 1910 and George Braque's "Fruit Dish and Glass" (at left) from 1912, both examples of cubism also echo the trend of the time to look at things in a new way. In cubist art, objects are broken apart into many pieces and reassembled at angles that show them from difference perspectives, often removing the perception of depth. The style examines objects as a sum of their parts rather than as a whole.
In 1913, Neils Bohr published his model of the atom in which he said that an electron could drop from a high-energy orbit around a nucleus down to a lower orbit and - in doing so - emit a photon, a particle of light. The model helped bring about the creation of quantum mechanics. Quantum mechanics explains how matter and energy interact on the very small scale - on the scale of atoms.
Before quantum mechanics became widely accepted, it was thought that light propagated through a medium called "aether." Experiments, however, failed to verify the presence of the aether. Thanks to those experiments and discoveries like Bohr's, quantum mechanics, and the interactions between matter and energy, gradually became accepted physics. feedproxy.google.com
The forward flip is one of the most difficult moves you can pull on a motocross bike. While backwards rotating flips off a ramp are almost natural, rotating forward presents some serious challenges.
For one thing, when you flip backwards, you pull back on the handlebars and push on the foot pegs as you leave the ramp. Forward flips require you to do the opposite, which is a challenge because it's not easy to pull on foot pegs.
For another, when you do a backflip, you crane your neck back and look for the ground in order to nail the landing, as you can see in this video of Travis Pastrana's double backflip.
A forward flip requires you to lean forward and hunch over the tank, which means you're essentially riding blind as you hope to time things just right.
Of course, Strong pulled it off so well that he made the front flip look easy. In addition to the initial forward rotation he gets of the ramp by pushing on the bars and kicking back and up on the pegs, he gets very low over the tank. That reduces his rotational moment of inertia, which makes him spin faster in the same way that rotating ice skaters spin faster when they pull their arms and legs inward. In fact, at the beginning he leans so far forward that his head goes down to the right of the tank. His rotation slows a bit as he lifts up early in the jump, but then he ducks down again to spin faster and nail the landing
Finally, if you look closely you can see one additional piece of the puzzle: Strong hits the brakes during the jump. That makes both the front and back wheels stop spinning. Because angular momentum is conserved, stopping the wheels from rotating forward makes Strong and his bike rotate faster. (For a backflip, you should do the opposite - goosing the engine to rotate the wheels faster forward helps the bike and rider to rotate more rapidly backward.)
It might sound simple enough, but here's how it looks when you don't get it right.
Paris Rosen had a couple problems with his attempt. He didn't get much initial forward rotation off the ramp, which got him off to a bad start. Once he was in the air, he lost contact with the bike pegs. Instead of pulling in close to reduce his rotational inertia, he kept his torso well above the tank, and halfway through the move he drifted away from the bike, which increased his rotational inertia still more and further slowed his rate of spin. On the bright side, it appears that Rosen managed to hit the brakes and stop the wheels from spinning, but there's just not enough angular momentum in the wheels to make up for the other challenges he was facing at the time.
Now that you've seen how things went wrong for Rosen, take another look at Strong's successful flip. It's simply amazing how well he put it all together.
(To be fair, Jim Dechamp pulled the front flip first, but Strong is the first person to land it in an official competition. Here's a video of DeChamp practicing front flips into a foam pit.) feedproxy.google.com
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