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# Halfpipe physics

So, I was skiing Attitash on Sunday, a place where the chairlifts are situated for prime half-pipe viewing, and for some reason (ok, I'm a geek), I got to wondering about the physics of it all.

1) How come they always land back inside? i.e., I've never seen somebody jump out and crash onto the upper lip of the halfpipe (not that I want to -- sounds like a really painful way to crash). It seems like your body weight pressing against the wall as you go up would be enough to send you outside the halfpipe and into a very painful landing. Is there some law of physics here that I'm missing, or do the halfpipe junkies just know not to push against the wall as they're going up?

2) I saw a couple people who could ski straight up the wall of the pipe and just immediately stop when they reached the edge -- that is, they'd just ski up onto the top ridge of the pipe and stop, pop out of his skis, and start walking back up. No snowplowing, no sideslipping, they just stopped! How is it possible to just halt all the momentum they're carrying that's needed to get out of the pipe? I'll admit they know they're going to stop when the reach the top, but it would seem pretty hard to judge your momentum to the point that you're just barely going to clear the lip.

3) I've gone into pipes a couple times, but I've never dared to go more then 2/3rds of the way up the wall before deciding to turn them back down. At the moment where I hit the top of the arc on the wall, it feels like what I'd guess being weightless feels like. Am I dreaming, or is there really an instant of 0g's?

a) F doesn't really equal ma -- according to Einstein, Newton was wrong.
b) Einstein was wrong too -- according to Schrodinger (I think), God does play dice with the universe.
Well, I'm really into park skiing so I hope I can answer these questions.

1) The half pipe is made so that at the top (and near it) it is at vertical (90 degrees). This helps stop the skier/boarder from jumping out of the pipe. Also if you have every been on a steep jump you know that most of your weight presses on the jump right when you start going up. So most of your self pressing on it is lost by the top. And it does hurt if you land on a flat part.

2) I'm not really sure about this one...since I've never done it before. Maybe they just pressed themselves out...not real sure.

3) Yes, I'm pretty sure you go weightless when in a pipe or even a jump for that matter. But before you go weightless you'll experience above 1 G's like when you first hit the upward part.
1) In a perfect world, the top of a pipes' wall is straight up, 90 degrees. It will launch you straight up. If you get launched straight up, you come straight down, and land right where you want to be - top of the wall of the pipe.

In the real world, most pipes are slightly 'undervert' or less then 90 degrees. If you just ride straight up and do nothing, you'll land on the pipe deck. So jump! When a skier/boarder comes to the lip, they jump or "pop", how much depends on how undervert the wall, how fast they're going blahblabhalh basically what feels right. Since they're sideways on the wall, they're actually jumping back into the pipe instead of up, and this compensates for the undervertness launching you out of the pipe. Too little pop takes you out of the pipe and onto the deck. Too much pop launches you too far into the pipe and you land in the flats, which hurts more then hitting the deck since you are falling 18+ feet (in a well made superpipe at least).

2)It looks harder then it is, practice and you'll learn the right speed. Then just lean forwards and stand up at the lip. I didn't believe it was that easy, but it is. Yay

3) You have to get into the air for true 0g, 2/3rd s up the pipe wall, you are still being supported by the wall, though you will feel lighter.
You have to push off the wall of the pipe (extending knees and ankles) or you may come down on the deck or the lip. If you push off too much you will come down on the flat bottom of the pipe. To roll out of the pipe you can't be going fast like you would for a big air. Just enough speed to get to the lip, then flex knees and ankles instead of extending to absorb speed and to get over the lip.
Quote:
 Originally posted by KevinF: 1) How come they always land back inside? i.e., I've never seen somebody jump out and crash onto the upper lip of the halfpipe
Then you've never seen me in the pipe. I end up on the deck every other time I hit the pipe.
Quote:
 Originally posted by KevinF: 1) How come they always land back inside? i.e., I've never seen somebody jump out and crash onto the upper lip of the halfpipe (not that I want to -- sounds like a really painful way to crash). It seems like your body weight pressing against the wall as you go up would be enough to send you outside the halfpipe and into a very painful landing. Is there some law of physics here that I'm missing, or do the halfpipe junkies just know not to push against the wall as they're going up? Halfpipe is about maintaining pressure against the wall. If you add more pressure by pushing against the wall you will get pushed back and into the flat (not good). The top of the pipe is actually 88° to accommodate the rebound energy of twintips as the tip and binding clear the coping (the tail is still touching). So without doing anything, you will get pushed slightly inside. 2) I saw a couple people who could ski straight up the wall of the pipe and just immediately stop when they reached the edge -- that is, they'd just ski up onto the top ridge of the pipe and stop, pop out of his skis, and start walking back up. No snowplowing, no sideslipping, they just stopped! How is it possible to just halt all the momentum they're carrying that's needed to get out of the pipe? I'll admit they know they're going to stop when the reach the top, but it would seem pretty hard to judge your momentum to the point that you're just barely going to clear the lip. Energy is neither created nor destroyed, merely changed... By actively pulling your legs in towards your chest near the top, you will release a lot of the pressure and consequently speed. A strong push forward and extending of your arms, will pivot your body from horizontal to vertical position. It's all in the timing. 3) I've gone into pipes a couple times, but I've never dared to go more then 2/3rds of the way up the wall before deciding to turn them back down. At the moment where I hit the top of the arc on the wall, it feels like what I'd guess being weightless feels like. Am I dreaming, or is there really an instant of 0g's? Yes. You should play around with just riding back down the wall backwards (with twintips of course) so you're not preoccupied with spinning the skis around. This will help judge the timing of WHEN to spin since this is the moment of least resistance.
Quote:
 Originally posted by Pete Zehut:Then you've never seen me in the pipe. I end up on the deck every other time I hit the pipe.
I suspect you're pulling your legs in at the top... or the pipe wall is seriously undervert. This is quite common with older pipe cutters. The Zaugg Pipe Monster seems to be the cat's ass these days...
KevinF wrote: ' It seems like your body weight pressing against the wall as you go up would be enough to send you outside the halfpipe ..'

Kevin, this seems to me to be the same misconception as dogs the discussions about skiers' turning etc. There is no force throwing you out of the turn at right angles to your motion (or the vert curve of the halfpipe), but only a resistance to continuing the same turning radius, i.e. you are being forced to carry on straight ahead. To continue the turn you have to overcome this force.

So provided the pipe lip is vertical 90deg, you will go straight up and not out onto the deck. Likewise when you break out of a turn on piste, you go straight ahead on a tangent to the curve or arc you were carving.

You can demonstrate this by swinging a weight (conker) on a string around your head. Cut the string and the conker flies out on a tangent not a radius. The principle of the slingshot.

The confusion arises with talk of centripetal and centrifugal forces which are largely theoretical constructs.
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