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# High-speed skiing - Page 3

Bob Barnes: "Anyway--there is one other aspect of the bicycle vs. skier problem: the bicycle does not only have to accelerate in a straight line--its wheels must also accelerate their rotation. So the rotational inertia of the wheels must enter into the discussion. This is a variable the skier does not have. Wanna take it from here, PhysicsMan?"

cold water: "The Angular momentum (of the rotating wheel) will only slow your acceleration. It has no effect on your terminal velocity."

"Cold Water" got it exactly right. No more comment needed.

Tom / PM
re "lubricating film of water":

Friction is a really complicated topic with no simple, "one-phenomena" answers to questions.

For example, whether or not the snow crystals contact the base and contribute to dry friction depends on the thickness of the water film.

However, the thickness of the water film depends on the coeff of dry friction in the front of the ski where the process of warming up the snow to the melting point begins. Everything also depends on the ambient temp, the speed of the skier, the water content, base structure, compaction of the snow, etc., etc.

I'm not going to try to go over this topic here because it is covered quite well in the book, "The Physics of Skiing". It makes a great read for anyone interested in the details of phenomena like this, and is virtually a prerequesite for advanced discussions.

BTW, If I remember correctly, this book even shows a graph of the various components of friction related to the layer of water as a function of velocity (for one set of assumed conditions). Certain frictional components increase with speed, certain decrease, and their sum shows a complicated behavior with speed (I think a minima). Read the book guys, most of what you are getting hot under the collar about about is in there.

Tom / PM
Geez... I'm strictly a recreational skier. I doubt I often break 30 mph. I skied a few runs with Hugh Grierson (sadly R.I.P) in New Zealand and watched him do the skiing leg of the Mt Hutt to Methven race a couploa years ago. That boy skied fast. I think his brother was a speed skier?

My boat goes 47.5 mph on flat water according to the GPS and I have towed barefoot water skiers at 42 mph. I doubt I've been faster than that on skis.

For PhysicsMan:
I'd think the big limiting factor is wind resistance. That must be a function of altitude. It's a lot harder to ski fast at, say, the bottom of Le Massif 100 feet above sea level in Quebec than at the top of Chamonix. I gave up integrals for lent and I never took an aero course so I don't really know what I'm talking about.
Fastest clocked speed
76 mph at the end of the season barbecue speed trap.(ca 1993) on 212 TNCs.

That was fast.

I think I went faster once, but maybe it just felt like it 'cause I didn't have any clothes on. -- End of the season summit to base naked tuck run, ca 1991...next time I'll make sure I'm in front...

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When the going gets weird, the weird turn pro...
GeoffD: "...I'd think the big limiting factor is wind resistance. That must be a function of altitude. ..."

I forgot in which post I said this, but what I probably was referring to was serious DH type speeds, not recreational speeds.

The wind resistance is indeed a function of altitude - it varies linearly with the density of the gas, but is a much stronger function of velocity (ie, the square of velocity).

If I remember the numbers correctly, the density of air varies by something like 30% from sea level to the highest lift served altitudes, so if you compensate for the reduced aero drag at higher altitudes with a higher speed, the speed increase will only come out to something like 15% (eg, 50 mph vs 57 mph) - not huge, but noticeable.

Tom / PM

PS - Loved your line, "I gave up integrals for lent". I may have to use it myself sometime.<FONT size="1">

[This message has been edited by PhysicsMan (edited September 09, 2001).]</FONT>
I gotta pick up that book Physics of Skiing. I have a collection of over 200 books on skiing. I could file it in with John Howe, Juris Vagners and others under insominia remedies!<FONT size="1">

[This message has been edited by Robin (edited September 08, 2001).]</FONT>
Quote:
 Originally Posted by Ott Gangl ...I was pretty amazed to find most of the intermediate skiers were easly hitting 3O-35mph.<<< Has any intermediate skier here tried a pole plant at 35mph? Is your arm still attached? ...Ott