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Conservation of momentum

post #1 of 26
Thread Starter 
A recent conversation brought up a question that I think deserves investigation.
Momentum, when do we want to conserve it and when do we want to lose some of it?

I was putting together a list of situations where we would want to lose a little momentum and it occured to me how infrequently we actually want to conserve all of our momentum. The slow line fast idea being the one example of trying to conserve momentum but even then skiing across the hill for speed control decreases our momentum somewhat. Seeking speed in a race also came to mind but even in that situation there are places we want to accelerate and places we need to slow down if we expect to stay in the course. Perhaps the only place would be in a wax race, or a speed skiing event.
So without actually quantifying the amount of momentum we are trying to lose, I think it is important to investigate the idea that in just about every skiing situation we are trying to maintain speed control by controlling our inertial momentum in both a forward and down the hill direction.
post #2 of 26
Jasp, for me it is more a question of how we want to use, control, and shape the direction of our momentum, as opposed to whether we want to conserve or loose it. However, an outcome of using, controlling, and directing our momentum would be loosing some of our momentum. To use/harness, control, and shape the direction of momentum is to reduce some of it's raw influence or power as I see it. We can't do one without the other. Yet using our big MO to our advantage still requires that we conserve it's power to a large degree, other wise we have nothing to use control or redirect. It becomes a delicate balance between allowing and efficient controlling.
post #3 of 26
JASP I guess I look at conservation of momentum a little differently. Instead of look at conservation of momentum from a physics standpoint I look at it from a conceptual standpoint.

I always want to ski the slow line fast. Once I select my line I am always trying to gain speed and therefore conserve momentum along that selected line. In truth, I am choosing overall to not gain the momentum in the first place through line selection but, once selected, I want the minimum amount of braking and degredation of momentum along that line.

I choose to dump momentum when I choose to brake.
post #4 of 26
Thread Starter 
Agreed Pierre,
Slow line fast is one way to limit acceleration and thus limit the amount of inertial momentum. I find this especially helpful early in the turn because it has such a profound effect on everything that happens later in that turn. That being said there are times that the slow line is not the best tactic for a situation, or a course setter forces a lower line and thus makes the racers to use a different type of turn. Which introduces the idea that some braking has to happen, either by line choice, or by other means. What are those other means and what situations do we use them?
RicB,
How we accomplish this is exactly where I want to go with this discussion.
post #5 of 26
JASP,

To prevent this discussion turning into a dogs breakfast you need define "momentum".

Physics types will see it as Mass X Velocity. Hence since I assume you are not looking at ways of reducing the earths gravitational pull, or a "mass" loss program that works within a single run...it seems to me, you are really asking about controlling velocity.....and again to avoid confustion, I wont be surprised if you will need to differentiate between velocity and speed....

Since you can keep your speed constant, but alter your line, thus altering your velocity...thus altering your momentum....alternativley, you can keep your line, dump your speed, and alter your momentum.....
post #6 of 26
KISS

Sometimes we turn to slow down. Sometimes we turn cuz it's fun. Sometimes we do both. Sometimes we put extra skid in our turns because we want to. Sometimes because we have the skills to do otherwise.
post #7 of 26
Thread Starter 
Velocity is part of it SD but momentum (MxV) is really the term that I want to use here. Mass remains constant, Gravity remains constant, velocity changes, directional vectors change.
Momentum is what carries us across the hill when we release a turn. Gravity will at some point decay that trajectory but momentum still carries us across the hill even though the skis are not on edge. It can also be exploited to carry the body across the skis during a transition phase. Like in RRX turns. Since we are not very far out of the fall line it would seem gravity would not pull us across the skis. So there needs to be another motive force in play.
So how could we exploit our natural tendency to go straight while we are constantly turning? When would we want to? When doesn't it make sense? How much momentum do we want for a maneuver? Relating it to tactical applications is what I am asking for here. Even within those applications is it possible that momentum has a different effect during different phases of the turn?
post #8 of 26
At first I thought to be correct you might be talking about kinetic energy, but I see by your further explanation that you indeed mean momentum, but with regards to redirecting it and not necessarily keeping it's direction conserved, just it's magnitude.

Upon further thought, I think this is appropriate. Newton's second law can be expressed as the sum of all the forces is equal to the rate of change of momentum, which accounts for direction change. Conserving momentum is important because the rate at which we withdraw it from the account so to speak is directly related to the forces we apply as we accelerate.

I need some sleep.......
post #9 of 26

I have been waiting for this question

I have had some thoughts on this subject for some time.

Why do some people glide or maintain a higher rate of speed over the same snow and terrain?

Wax?
I think it has allot more to do with drag. specificly the amount of drag created by your skis.

I think that two people of the same weight on the exact same skis, could travel over the same terrain at deferent speeds.

And the reason for this is BALANCE
I belive that the way you distribute weight over your skis and the way you adjust you COM over the skis has a direct berring on your ability to hold speed or momentum.

I would further speculate that those who glide well (Ski in Balance) have a much eiaser time in soft and variable snow conditions.

Bad and good balance (even distribution of pressure across the ski) is amplified when in snow conditions where drag is a greater factor.

The (How do I ski Sierra Cement Thread) comes to mind.

Well glad to get that off my chest!

Thoughts?
post #10 of 26
Quote:
Originally Posted by justanotherskipro View Post
So how could we exploit our natural tendency to go straight while we are constantly turning?
Well, as you say, transition: choose a release point that will use your momentum to carry you in the desired direction into the new turn. This means your release is clean.

Quote:
When would we want to?
Completion through initiation (transition).

Quote:
When doesn't it make sense?
The bottom of the turn.

Quote:
How much momentum do we want for a maneuver?
As much as is beneficial, or as little as is harmful -- it depends when in the turn.

Quote:
Relating it to tactical applications is what I am asking for here.
How about utilizing momentum to craete the directional movement of the CM across the skis to acheive early shovel pressure? Thus getting some turning done early, before the pull of gravity adds to the momentum and makes it more difficult. Early edge engagment will help reduce the amount of redirection and therefore pressure at the bottom of the turn.

Quote:
Even within those applications is it possible that momentum has a different effect during different phases of the turn?
Sure you can harness it at the top, and reduce the coupling of momentum and gravity at the bottom.

Is that the sort of thinking you were doing?
post #11 of 26
Thread Starter 
Yes E,
Only on a tactical maneuver level. Say for instance, Moguls.
While I've seen carvers and skidders motor through the bumps one thing that always catches my eye is the body moving with momentum regardless of what the feet and legs are doing. Contrast this with someone who blocks against the mogul tops (or uses a check to control speed). Their movements are like watching rush hour traffic. Stop, start, stop, start. No rhythm is ever established. In this example the lack of continuity forces them to use an excessive amount of fore aft levering, (or pivoting), to keep the skis even close to beneath the hips.
post #12 of 26
jasp, very good point.

To me is is all about skillfully managing momentum to serve your purpose. This is demonstrated in the rhythm and flow that is characteristic of really good skiers. When momentum is well managed a skier may appear slower than they are going and less dynamic that they actually are. This is because nothing is over done and evey movement is not just effective, but also efficient.

Whenever our CM is over deflected off of our intended line we resort to hitches, jerks and hoogles to compensate for having sent our energy some direction other than the direction we want to go. And the result is anything but smooth skiing.

For me this coorelates with the concept of balancing in the future.
A great example of managing momentum to work for your purpose and not against it.
post #13 of 26
Thread Starter 
Quote:
Originally Posted by Arcmeister View Post
jasp, very good point.

To me is is all about skillfully managing momentum to serve your purpose. This is demonstrated in the rhythm and flow that is characteristic of really good skiers. When momentum is well managed a skier may appear slower than they are going and less dynamic that they actually are. This is because nothing is over done and evey movement is not just effective, but also efficient.

Whenever our CM is over deflected off of our intended line we resort to hitches, jerks and hoogles to compensate for having sent our energy some direction other than the direction we want to go. And the result is anything but smooth skiing.

For me this coorelates with the concept of balancing in the future.
A great example of managing momentum to work for your purpose and not against it.
Nice picture Arc,
Especially the last sentence. Regardless of the tactical choice there is a way to exploit momentum and minimize the need for corrective maneuvers. A mentor of mine calls them deliberate, directionally disciplined maneuvers, or 3D skiing.
post #14 of 26
MTT,

Quote:
Why do some people glide or maintain a higher rate of speed over the same snow and terrain?
BigE hit it when he posted:

Quote:
How about utilizing momentum to craete the directional movement of the CM across the skis to acheive early shovel pressure? Thus getting some turning done early, before the pull of gravity adds to the momentum and makes it more difficult. Early edge engagment will help reduce the amount of redirection and therefore pressure at the bottom of the turn.
And if the directional movements are carried through the entire turn, there is much better glide even at the bottom of the turn. I had a lesson that was skiing behind me and asked why I seemed to accelerate at the end of the turn and he couldn't. I explained that I was applying directional movement to the skis and that allowed the momentum to be maintained. It also carries it into the new turn as the movements are redirected.


On steeper terrain, the directional movements also help the momentum carry through the transition eventhouth the turn is held onto a little longer.

RW
post #15 of 26
Thread Starter 
Ron the last sentence is a little unclear. Do you mean to say we turn across the fall line more (as a unit), or are you talking about the skis continuing across the fall line while the body is moving towards the middle of the next turn (projecting the body into the new turn)?
post #16 of 26
Windup as the CM seeks the apex maybe?
post #17 of 26
Thread Starter 
Sounds reasonable E, that's a very good way to describe the leg steering (femoral rotation) continuing while the body has already started to move towards the next apex.
I was hoping to get Ron to expand on the idea of hanging onto the turn a little longer. That sentence explains the stategy but it didn't include the tactical maneuver he uses to accomplish it. I would assume he meant the wind-up but I want to give him the opportunity to clarify that point.
In my experience many of my students would take the idea of hanging onto the turn a little longer to mean turning across the hill as a unit more. Which if you think about it might be something to add prior to the wind-up. Overdone it would inhibit our ability to use momentum to link turns but in the right amount it would add a little bit of speed control.
post #18 of 26
jsp,

Quote:
On steeper terrain, the directional movements also help the momentum carry through the transition eventhouth the turn is held onto a little longer.
To clarify, the turn shape is carried across the fall line further (or even a little up the hill on very steep terrain), the momentum must be carried through the transition using a strong directional movement into the new turn. There is a greater distance for the CM to move to redirect during the transition than on flatter terrain where the turn shape is less developed.

Hope this explaines.

RW
post #19 of 26
Thread Starter 
Thanks Ron, it does.
post #20 of 26
Kinetic energy is like money in the bank. The more you have the more you can do with it. I like having lots of it. Momentum goes hand in hand with kinetic energy, except it imposes some restrictions as well. What speeds up must slow down, and you can only change your line by so much, the faster you go the harder it is to change your overall direction of travel and the longer it will take.

Edit: to add. Kinetic energy is like freedom, it let's you do stuff. Momentum is like responsibility. You can't have one without the other. (1/2mv^2 needs mv)
post #21 of 26
"skiing the slow line fast" is the epitome of conservation of momentum!
post #22 of 26
Thread Starter 
In another thread we were discussing a release that would cause the tips to move downhill without causing the tails to move uphill. Not exactly slow line fast but IMO another example of directing our momentum towards the apex.
post #23 of 26
Thread Starter 
I've been asked to add some clarification of my last post:
In a wedge or wedge Christie maneuver, the uphill ski being stemmed to create a wedge, or being push out to create the wedge, assumes the CoM is relatively inactive and the feet are moved to create the converging skis stance. Another example of this is to pivot the skis to move the tips of the skis towards the inside of the new turn. As a result of the tips being pivotted downhill, the tails move uphill. These examples demonstrate a lack of commitment to the new turn.

The corrective prescription is to get the student to move with the skis more and to move their body downhill enough to facilitate getting the new inside ski flat. Coaching a student to "give themselves to gravity" is just one of the phrases we have used over the years to describe moving our body in this manner. I believe telling them to use their momentum to move with the skis gives them a more precise objective and more precise (disciplined) body movements start happening earlier in their skiing. When they get to the level where they can begin to ski the slow line fast they are better prepared to perform that style of skiing.
post #24 of 26
JSP,

Quote:
These examples demonstrate a lack of commitment to the new turn.
I agree with you if you are talking about the traditional stem christie type of turn transition or a rotary push-off turn transition where the outside ski is pivoted around the new inside ski.

The modern wedge christie done to speed has a good committment to the turn. A more advanced pivot of the new outside (actually both skis) ski such as the spivot, that racers use, is also very committed and momentum, in both casses, is carried through to the new turn.

The drill "stem steps" is a great drill for displacing the new outside ski on an effectife edge and also carring momentum into the next turn (if done correctly).

RW
post #25 of 26
Thread Starter 
Ron, Are you talking about a downhill diverging step?
post #26 of 26
jasp,

Quote:
Ron, Are you talking about a downhill diverging step?
A stem step is very similar to the white pass turn. The turn is iniated on the new inside ski as as that ankle flexes, the new outside ski makes contact with the snow somewhere around the fall line.
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