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# mechanics of accelerating out of the turn

I had a conversation with a seasoned racer yesterday who explained how one gets extra speed out of a turn. He's much more experienced than I am, and I trust that he's right, but remembering my physics 101 leaves me puzzled. Can someone clarify the mechanics of this?

He said that simply tipping the skis on edge and carving a turn generates extra speed.

I said that it uses up extra energy to get something to move in a curve instead of a straight line, so extra speed would have to come from some sort of pushing action by the skier. I was thinking that the skier must push against the snow during the top of the turn, before the skis reach the fall line, in order for the skier to accelerate out of the turn.

He said no, just tip the skis and they will accelerate. His example was a drill he did of skiing along a flat cat-track with his hands clasped behind him, keeping his torso quiet, yet maintaining speed by keeping his skis on edge. Clearly he was reducing friction by keeping less ski in contact with the snow, which would allow him to maintain his original speed longer. But if no extra speed were being generated by the tipping, he would still slow down. I watched him; he didn't slow down, so maybe he's right.

What do people think?
Just turning won't do anything.

The short answer. A skier's skis travel in an approximately circular path. The skier's center of mass is somewhere along the radius connecting the skis and the center of the circular arc they form. By moving the center of mass along this radius (bending knees or standing upright), the skier loses or generates speed.

I can relate this to some other physical examples and avoid the physics and equations.

Ever seen the physics demonstration where someone spins on a stool with arms extended then pulls them in and spins faster? You have seen figure skaters tuck their arms in to spin faster in the same fashion. The way inline skaters and skateboarders generate speed in halfpipes (called "pumping") is the same physical principle (snow halfpipers do this in addition to the speed they get from the pipe sloping downhill). On a swing, you generate speed by pulling on the ropes and lifting your feet as high as possible -- shifting center of mass in towards the center of rotation.

Generating speed in a ski turn is based on the same principle.

I can also explain it using conservation of angular momentum and the underlying physics, but it would be rather verbose to be thorough...
I think he tricked you. His lower centre of mass translated his potential energy due to gravity into kinetic energy "somehow". As he tipped and bent the skis into a turn he probably leaned forward ever so slightly as he put more pressure on the tails than on the tips. F=ma cannot be denied, if he is accelerating forward then the integral of force applied must net out to be a forward impulse.

There are a couple of speed-gaining turns threads somewhere around here. Try a search.
I won't say that forward momentum cannot be made by turning; hockey players often generate backward momentum while skating in parallel turns going backwards. I will say that in all likelyhood, there are many other things to concentrate on before this will help you.
Quote:
 Originally Posted by Barnstormer You have seen figure skaters tuck their arms in to spin faster in the same fashion. Generating speed in a ski turn is based on the same principle.
OK, I think I get it. Just lean in farther, i.e. move the center of mass farther towards the center of the turn, a.k.a. lengthen the radius of the turn, and you will go faster; no need to push against the snow. Principle in action is conservation of angular momentum. Same as in crack the whip: the longer the whip with you at the outside edge, the faster you move. Right?
Quote:
 Originally Posted by Ghost I will say that in all likelyhood, there are many other things to concentrate on before this will help you.
I know. I'm just curious.
Conservation of angular momentum means that you will spin faster with more mass near the centre. I don't want to think about transforming the angular momentum into forward momentum after the turn; I'm on holidays!
There is a subtle difference between the spinning figure skater and the carving skier examples although they both conserve angular momentum (neglecting friction).

In the case of the spinning figure skater, angular momentum won't translate to linear momentum because the axis of rotation is through the center of mass of the body. If the figure skater were holding a baseball and let go, the baseball would fly off at a tangent with some linear momentum. Individual pieces of the skater's body have linear momentum, but they sum to zero.

In the case of the carving skier, the axis of rotation is someplace else in space, not through the skiers center of mass. The skier simultaneously has angular momentum relative to that axis and linear momentum tangent to the radius of the turn (or in whatever direction their center of mass is moving). Since the mass of the skier is (hopefully) constant, an increase in the instantaneous linear momentum is the desired increase in speed.
LiquidFeet,

This is a topic that comes up every now and again. Might be nice if we had a link in the 'training' section that goes directly to PhysicsMan's post #27 in the thread:
Accelerating out of a turn.

The whole thread is kinda interesting but a direct answer to your question starts at post #27.

.ma
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