Avalanche!
December 1, 2006 11:39 AM Subscribe
Often when I hear a description of an avalanche (or landslide, etc...), there is mention of the snow or debris falling at a couple of hundred miles an hour. From a physics standpoint, how is this possible?
I am under the impression that unless acted on by an outside force that propels it faster or slows it down, an object will fall at 9.8m per second squared, so how do those forces of nature reach these excessive speeds?
I am under the impression that unless acted on by an outside force that propels it faster or slows it down, an object will fall at 9.8m per second squared, so how do those forces of nature reach these excessive speeds?
This is a guess... but I don't think that the speed is of the individual object, but of the entire phenomenon. If the avalanche is caused by an initial shock, that shock wave travels very fast, but the individual chunks of snow don't have to. Think of a tsunami: the wave of water travels 500 mph, but the molecules of water in that wave never reach those speeds.
posted by peeedro at 11:45 AM on December 1, 2006
posted by peeedro at 11:45 AM on December 1, 2006
Velocity and acceleration are different things. Acceleration due to gravity is 9.8 m/s^2. Unimpeded, the velocity an object can attain under acceleration is unlimited(-ish).
posted by pealco at 12:00 PM on December 1, 2006
posted by pealco at 12:00 PM on December 1, 2006
Best answer: Avalanche dynamics is an interesting topic. This article does a good job of summarizing most of the major stages. It looks like in some cases, avalanches may act more as a fluid than a collection of solids and speeds vary from 10 to 90 m/sec.
posted by vacapinta at 12:03 PM on December 1, 2006
posted by vacapinta at 12:03 PM on December 1, 2006
According to google 100 miles per hour is only 45 meters per second. Just fuging the numbers in my head, about 5 seconds of free fall should do it. Of course the avalanche is probably not doing 9.8 m/s^2 but even about 5m/s^2 (accounting for drag and slope) might do the trick with 10-20 seconds of fall. Pyroclasic landslides are also lubricated by the production of gas and steam on the way down. (In fact, there is one known case of a pyroclastic flow that bounced back when it hit water.)
Just by comparison, peregrine falcons have a reported speed of between 50-80 m/s during swooping dives.
posted by KirkJobSluder at 12:10 PM on December 1, 2006
Just by comparison, peregrine falcons have a reported speed of between 50-80 m/s during swooping dives.
posted by KirkJobSluder at 12:10 PM on December 1, 2006
The bird thing was just to point out that 45m/s isn't that big of a deal for natural objects accelerated by gravity.
posted by KirkJobSluder at 12:24 PM on December 1, 2006
posted by KirkJobSluder at 12:24 PM on December 1, 2006
Agree with vacapinta above.
Turtle Mountain in Alberta was a rockslide that acted similarly to an avalanche. I remember my geology prof (Dr. Hugh Dresser) saying that the debris pattern and distance indicated that the air trapped underneath the rock reduced the friction, creating an effective fluid which in turn, caused a greater terminal velocity and resultant distance.
Fascinating dynamics, but when you see a boulder the size of a house about a mile away from where it used to be, its also kinda intimidating.
posted by fox_terrier_guy at 1:17 PM on December 1, 2006
Turtle Mountain in Alberta was a rockslide that acted similarly to an avalanche. I remember my geology prof (Dr. Hugh Dresser) saying that the debris pattern and distance indicated that the air trapped underneath the rock reduced the friction, creating an effective fluid which in turn, caused a greater terminal velocity and resultant distance.
Fascinating dynamics, but when you see a boulder the size of a house about a mile away from where it used to be, its also kinda intimidating.
posted by fox_terrier_guy at 1:17 PM on December 1, 2006
Keep in mind that nobody is out there measuring the speed of the thing when they occur. They might come along after the fact and measure where the rocks fell and estimate a speed. More likely, the article you're reading is citing an eyewitness account, which can vary widely ("It must've been moving hundreds of miles an hour!").
posted by frogan at 7:22 PM on December 1, 2006
posted by frogan at 7:22 PM on December 1, 2006
No, they are out there measuring them.
Please explain to the class how someone can know exactly where and when the next landslide/avalanche will occur, how this real-time measurement strike team is organized, and the radar guns they're using.
As I said, researchers may investigate a landslide after it happens by taking measurements ... or even cause an avalanche to observe the subsequent effects ... but your average joe eyewitness wouldn't be at all accurate.
posted by frogan at 10:39 PM on December 1, 2006
Please explain to the class how someone can know exactly where and when the next landslide/avalanche will occur, how this real-time measurement strike team is organized, and the radar guns they're using.
As I said, researchers may investigate a landslide after it happens by taking measurements ... or even cause an avalanche to observe the subsequent effects ... but your average joe eyewitness wouldn't be at all accurate.
posted by frogan at 10:39 PM on December 1, 2006
Please explain to the class how someone can know exactly where and when the next landslide/avalanche will occur, how this real-time measurement strike team is organized, and the radar guns they're using.
Avalanches happen in predictable places. Usually on slopes of about 30-45 degrees, on leeward slopes where significant snow loading has recently occurred. Lots of slopes avalanche regularly and predictably, so scientists who study avalanches simply go to these places with video cameras. You don't need a radar gun to work out the speed, just a decent movie where you can see how quickly the avalanche moved down the slope.
Avalanches are pretty well understood and can be predicted, just like the weather can be predicted. And when they don't happen on their own, it's really pretty easy to trigger one artificially too.
posted by dseaton at 5:32 AM on December 2, 2006
Avalanches happen in predictable places. Usually on slopes of about 30-45 degrees, on leeward slopes where significant snow loading has recently occurred. Lots of slopes avalanche regularly and predictably, so scientists who study avalanches simply go to these places with video cameras. You don't need a radar gun to work out the speed, just a decent movie where you can see how quickly the avalanche moved down the slope.
Avalanches are pretty well understood and can be predicted, just like the weather can be predicted. And when they don't happen on their own, it's really pretty easy to trigger one artificially too.
posted by dseaton at 5:32 AM on December 2, 2006
This thread is closed to new comments.
posted by exogenous at 11:43 AM on December 1, 2006