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Photographer's Note

I am afraid you are already bored with my Balkan photos, so I jump for a while to my deep archive, inspired by photos and note by Rob:).
Another iconic rock in Yosemite National Park, besides El Capitan, is Half Dome. You can see it , looking like a dome cut by half, on the right from the middle. El Capitan is for the bravest, instead Olmsted Point here is for every one. The rocks are smooth and one can walk around on them, it as a lot of fun!
Rob wrote about the rock fall, so maybe he (and other people) will be interested in other rock fall there.

On July 10, 1996, two successive and dramatic rockfalls occurred near the Happy Isles Nature Center in Yosemite National Park, California. In each fall, a huge chunk of granite first slid down a steep slope and then went into projectile motion, hitting the ground about 550 meters farther down.
The impacts produced seismic waves that were recorded on seismograph machines as far away as 200 kilometers. More surprising, however, was the damage the chunks produced even farther down in the valley, over 300 meters from where they landed: Over 1000 trees were downed, a bridge and a snack bar were demolished, one person was killed, and several
other people were hurt. How could the granite chunks
cause so much destruction in places they did not reach?
The impact of each granite chunk at the bottom
of its fall produced pressure variations in the air, which traveled away from the impact point as a sound wave. The wave, said to be an air blast from the impact, consists of a compression of air followed by an expansion of air. If you were standing in that air blast, you would be violently shoved, first
in one direction and then in the opposite direction by the pressure variations, which were effectively a source of strong winds. The air blast from the second chunk, which had about three times the mass of the first chunk, was especially destructive, creating winds up to 430 kilometers per hour through the trees (this is comparable to winds near a tornado). In fact, the air blast from the second chunk was supersonic (it was a shock wave), because dust stirred up by the first impact reduced the speed of sound in the air from its normal value of 340 meters per second to about 220 meters per second, and near the impact point the speed of the air blast was greater than that.


This description is from the book Jearl Walker, The Flying Circus of Physics. I was an editor of the Polish edition of it. I found there many fascinating stories about physics in everyday life.

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Walking on the rocks

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