or Connect
New Posts  All Forums:Forum Nav:

# Dan Dipiro's Mogul Book - Page 25

I will gladly be on the same side of the fence with JAMT any time!

(especially when physics are concerned)

Kook if you're referring to Mogul Logic....what I said is that Chuck's advice was sound but his understanding of physics on that particular point was flawed.  I have said it several times and say it again here.  what is your point exactly?

Quote:
Originally Posted by CTKook

The only problem is, what you think is helpful is not correct.  It would be helpful if people learned this stuff before engaging in misdirection on here.

@CTKook  I'm asking to learn, so help me out.          FWIW I have no problem with "pumping for speed"

-   down the halfpipe wall

- early rollers in a ski cross

- on a skateboard / bicycle in a halfpipe

(Again, asking for understanding, not to implicate or impugn)

When I see a snowboarder come back into a half-pipe, they generally flex / get shorter, and typically tilt their board to the uphill end of the pipe - they want speed for the next wall (and want to preserve length of the pipe)  They also do this when they first enter the pipe.

I would say this is pumping, and is used to increase speed.

The skier cross guys do the same thing on the early rollers in a ski cross.

However, if the have screwed up their run and want to just barely back

make it up the next wall, they will soften their legs as they go up, and stall just above the lip.

Visually, the moves look similar (largely flexed position, with one going down the wall the other up).

But the former is used to gain speed and the latter is used to stall the snowboarder at approximately the lip of the pipe.

Q1)   Do I have this right?

Q2)   What is the physics of each of these?

Thanks

Ok parametric people, who agrees with this assessment of a swingset?
Quote:
Jerzy Michal Pawlak, Phd High Energy Physics, Experimental
[The previous post mentioned extending and retracting legs on a swing]

Actually the leg work is only for balance, and the real work is done by the upper body. The way it works is by shifting of the center of mass of the body towards or away from the axis. Each time when the swing in is the lowest position (and has the highest speed) you should move your center of gravity up, closer to the axis of rotation. By conservation of angular momentum, this makes you move faster, so in effect you get higher up. At the turning point you move your center of mass away from the axis, and also further up in the direction of the swing.
Written 19 Dec, 2012. 420 views.
http://www.quora.com/Why-does-the-leg-pumping-thing-work-when-swinging
Quote:
Originally Posted by CTKook

This helpfully illustrates that you and BTS don't have an idea how this works.  Among other things, if this was the way it worked, you'd invalidate hundreds of millions of dollars in built and rideable skateparks and bikeparks -- but those parks are there and imminently rideable and pumpable.

Actually it proves that you don't understand. Also I question your reading comprehension since I clearly said that this was one example and that the effect is larger against a curved surface.

How come you never come with any real arguments? If you claim that something is wrong it would help if you said what.

Quote:
Originally Posted by Jamt

As long as the pressure is less when I squat than when I extend I still get a net gain. Same is true for the case with a curved bottom. Its just that the difference can be larger.

The only way to convert energy with the legs is to push with force through a distance.  When that force is gravity you increase your potential energy.  When that force is centripetal you increase your rotational energy.  The parametric pumping energy comes from pushing against the centripetal force.  Pushing against gravity only gives you the energy you give up when you squat at the top.  Just do conservation of energy and solve the equations.  I always found that was the best way to solve most physics questions.  I don't think this route of pressure is the best way to get to the answer, but with just gravity acting on you, the pressure will be the same whether you are standing or squatting.  Maybe you meant something else, but it's irrelevant, because conservation of energy has given us the right answer.

Quote:

However, if the have screwed up their run and want to just barely back

make it up the next wall, they will soften their legs as they go up, and stall just above the lip.

In that case the curved face of the wall is not efficiently redirecting towards the sky.  If they flex or soften and absorb some of the pressure that is being created by the rising curved wall in front of them (flexing absorbs pressure, not speed), then their CoM's momentum continues to move horizantally rather then being redirected vertically.  The wall continues to curve up there into more and more of an accute angle against the direction of their CoM's momentum.

Eventually the wall will literally become a wall that they are slamming into from more of a perpindicular angle where it is unable to efficiently redirect their momentum towards the sky.  And of course, gravity is always there..constantly trying to pull you towards the center of the earth

Quote:
Originally Posted by Tog
Actually the leg work is only for balance, and the real work is done by the upper body.

I think he's trying to say that most of the mass that is changing positions is from the upper body.  I don't think he's trying to say it doesn't take energy from the legs.  I suppose you could bend at the waste, but I would think you would get the most deviation of the COM, by squatting.  And anyone can say that squatting while standing on a swing that's swinging will take some leg strength, and that is where the energy is coming from.

Quote:
Originally Posted by borntoski683

I will gladly be on the same side of the fence with JAMT any time!

(especially when physics are concerned)

Kook if you're referring to Mogul Logic....what I said is that Chuck's advice was sound but his understanding of physics on that particular point was flawed.  I have said it several times and say it again here.  what is your point exactly?

Again you are expressly misrepresenting what you did say.

Here's what you did say:

"bzzzzzz wrong.

He says 50% of speed control comes during the absorption process.  He does not say that speed control comes from the absorbing process.   Watch it more carefully.  I mentioned this days ago FWIW."

When in fact this is completely incorrect, as people who watch the video can see.

Thank you for repeating your pattern of misrepresentation, though, it helps bear out that this is a persistent issue, even after the misrepresentation has been pointed out previously.

Quote:
Originally Posted by Jamt

Actually it proves that you don't understand. Also I question your reading comprehension since I clearly said that this was one example and that the effect is larger against a curved surface.

How come you never come with any real arguments? If you claim that something is wrong it would help if you said what.

Keep spinning.

For passive readers, just search parametric and driven oscillation on the forum, and you'll see I explained this stuff over a year ago, and had some of the same people claiming then it wasn't the way it works, either.  Jamt, repeating incorrect analysis doesn't make it any less incorrect.

Quote:
Originally Posted by The Engineer

I think he's trying to say that most of the mass that is changing positions is from the upper body.  I don't think he's trying to say it doesn't take energy from the legs.  I suppose you could bend at the waste, but I would think you would get the most deviation of the COM, by squatting.  And anyone can say that squatting while standing on a swing that's swinging will take some leg strength, and that is where the energy is coming from.

He (the author of the quoted material about the swing) is leaving out driven oscillation, when for most people on a swing that's over half of what they use to get the swing swingin'.

Quote:
Originally Posted by borntoski683

In that case the curved face of the wall is not efficiently redirecting towards the sky.  If they flex or soften and absorb some of the pressure that is being created by the rising curved wall in front of them (flexing absorbs pressure, not speed), then their CoM's momentum continues to move horizantally rather then being redirected vertically.  The wall continues to curve up there into more and more of an accute angle against the direction of their CoM's momentum.

Eventually the wall will literally become a wall that they are slamming into from more of a perpindicular angle where it is unable to efficiently redirect their momentum towards the sky.  And of course, gravity is always there..constantly trying to pull you towards the center of the earth

For the record, after this I will no longer comment on physics statements by Borntoski.  My silence is neither an endorsement nor a criticism.  Though, there is an interesting perspective in this amongst a little bit of nonsense.

Quote:
Originally Posted by Tog

Ok parametric people, who agrees with this assessment of a swingset?
Quote:
Jerzy Michal Pawlak, Phd High Energy Physics, Experimental
[The previous post mentioned extending and retracting legs on a swing]

Actually the leg work is only for balance, and the real work is done by the upper body. The way it works is by shifting of the center of mass of the body towards or away from the axis. Each time when the swing in is the lowest position (and has the highest speed) you should move your center of gravity up, closer to the axis of rotation. By conservation of angular momentum, this makes you move faster, so in effect you get higher up. At the turning point you move your center of mass away from the axis, and also further up in the direction of the swing.
Written 19 Dec, 2012. 420 views.
http://www.quora.com/Why-does-the-leg-pumping-thing-work-when-swinging

Almost, and it turns on what you consider upper body.  The basic idea -- that energy is put into the system by using muscular effort to move the center of mass in and out from the axis of rotation -- is correct.

It is true that your legs don't weigh nearly as much as your torso plus head, so flapping them doesn't do much.

But the potential suggestion that the minor change in height from leaning is misleading.  The big change in height comes from bending the chain/rope by exerting a force between you hands on the chain and your bottom on the seat, using muscles in the arms and the core.  Pumping on a swing with sold rods instead of a chain is much, much harder.

The whole angular momentum thing is a red herring in my opinion -- not wrong, just not useful.. Energy balance is the way to go.

I'm also trying to decide whether there is a non-negligible tangential force.  I know there is some, since the bend in the chain at your hands means the tension in the rope is not quite parallel to the line from the axle to your center of mass.

Quote:
Originally Posted by The Engineer

For the record, after this I will no longer comment on physics statements by Borntoski.  My silence is neither an endorsement nor a criticism.  Though, there is an interesting perspective in this amongst a little bit of nonsense.

Quote:

In that case the curved face of the wall is not efficiently redirecting towards the sky.  If they flex or soften and absorb some of the pressure that is being created by the rising curved wall in front of them (flexing absorbs pressure, not speed), then their CoM's momentum continues to move horizantally rather then being redirected vertically.  The wall continues to curve up there into more and more of an accute angle against the direction of their CoM's momentum. - bts683

I don't get this horizontal bit. Why would not the skier continue on a tangent to the curve? Which at the top of the pipe is near vertical.
Quote:
Originally Posted by CTKook

Quote:
Originally Posted by The Engineer

I think he's trying to say that most of the mass that is changing positions is from the upper body.  I don't think he's trying to say it doesn't take energy from the legs.  I suppose you could bend at the waste, but I would think you would get the most deviation of the COM, by squatting.  And anyone can say that squatting while standing on a swing that's swinging will take some leg strength, and that is where the energy is coming from.

He (the author of the quoted material about the swing) is leaving out driven oscillation, when for most people on a swing that's over half of what they use to get the swing swingin'.

Driven oscillation is a different question.  It doesn't affect the energy balance during one individual pumping cycle.  It only affects whether or not you get a cumulative effect from successive cycles, or a random jumble.

TE, I am starting to think you are a troll and friends with Kook in some way.  You have made numerous fallacious statements.  I have no idea why are you trying so hard to discredit me, but whatever..  I stand by absolutely everything I have said...

Quote:

@CTKook  I'm asking to learn, so help me out.          FWIW I have no problem with "pumping for speed"

-   down the halfpipe wall

- early rollers in a ski cross

- on a skateboard / bicycle in a halfpipe

(Again, asking for understanding, not to implicate or impugn)

When I see a snowboarder come back into a half-pipe, they generally flex / get shorter, and typically tilt their board to the uphill end of the pipe - they want speed for the next wall (and want to preserve length of the pipe)  They also do this when they first enter the pipe.

I would say this is pumping, and is used to increase speed.

The skier cross guys do the same thing on the early rollers in a ski cross.

However, if the have screwed up their run and want to just barely back

make it up the next wall, they will soften their legs as they go up, and stall just above the lip.

Visually, the moves look similar (largely flexed position, with one going down the wall the other up).

But the former is used to gain speed and the latter is used to stall the snowboarder at approximately the lip of the pipe.

Q1)   Do I have this right?

Q2)   What is the physics of each of these?

Thanks

Q1.  Kinda.  I'm not sure what you mean about "typically tilt their board to the uphill end of the pipe" though, and you leave out the pump (and depending on the size of the pipe, some of them aren't real pumpable anymore on snow).

Q2.  Timing is everything.  In one case the rider or skier is pumping, and the other, using the normal terminology, is absorbing.  How much is from the up/down and how much is from fore/aft will depend on the rider/skier and on the feature, so the specific answer will vary a bit between individuals, but I would refer you back to the swing explanations (the swing is also attached to the earth, which is why it works, so while the rider being on snow looks different from a swing, the analysis is more or less the same as regards this issue).

Quote:
Originally Posted by borntoski683

TE, I am starting to think you are a troll and friends with Kook in some way.  You have made numerous fallacious statements.  I have no idea why are you trying so hard to discredit me, but whatever..  I stand by absolutely everything I have said...

LMFAO.  Classic Dewdman!

I've never had any interaction with TE outside the bounds of this thread (unless there may have been some other thread in which we both posted, which is certainly possible, but I don't recall any back and forth).  That's the beauty of this physics stuff.

Quote:
Originally Posted by mdf

Driven oscillation is a different question.  It doesn't affect the energy balance during one individual pumping cycle.  It only affects whether or not you get a cumulative effect from successive cycles, or a random jumble.

I'm not following some of what you are saying, so it made me think you are missing something about parametric oscillations, but maybe I'm just not understanding  Let me summarize, and I apologize if I'm covering something you already know.  There are two main ways to swing a swing.  The most common is to sit and lean back while kicking the legs out while moving forward, and then legs back, body forward while moving backward.  There's also another method you can do by standing on the swing.  You squat at both high ends and stand up as you move through the lowest point.  Without any other forces pushing any other direction than just straight down on the swing, you can pump the cycle so that the amplitude keeps growing and growing.  This works by pushing your center of mass against the centripetal force to increase rotational energy.  Everytime you stand up while swinging through the lowest point you add energy into the rotation.  It relies solely on having angular momentum.  You have to get the swing going first by traditional swinging, but once you have some angular momentum you can just keep pumping it by standing and squatting with no other typical swing forces.

Quote:
Originally Posted by CTKook

I've never had any interaction with TE outside the bounds of this thread.  That's the beauty of this physics stuff.

well at any rate, both of you seem incapable of civil discussion FWIW

Thanks @CTKook

(the tilting of the board probably should have been left out - that has to do with preserving the length of the pipe and going straighter / more square up the "next" wall)

Could you live with this (where down the wall things are still good but up the wall they've decided their done and just want to stall out at the top of the pipe)

Going down the wall the rider is flexing = pumping (in this context) to gain speed

On the way up their "last wall" the rider is flexing = absorbing (in this context) to lessen speed.

Inline

Quote:

Originally Posted by The Engineer

The only way to convert energy with the legs is to push with force through a distance.

>>Yes

When that force is gravity you increase your potential energy.

>>Yes

When that force is centripetal you increase your rotational energy.

>>Yes, but consider that in our case without a flat bottom the force is both gravity and centripetal

The parametric pumping energy comes from pushing against the centripetal force.

>>and gravity, but that is a generalization. Parametric means you are changing a parameter in the differential equation describing the oscillation with a frequency that is usually different from the harmonic one.

Pushing against gravity only gives you the energy you give up when you squat at the top.

>>When I squat at the top I have both gravity and centripetal. The gravity component is almost orthogonal and the centripetal is very small. This means I give up almost no energy at the top.

Just do conservation of energy and solve the equations.  I always found that was the best way to solve most physics questions.

>>I did a simplified version of this. But I'm sure that you are aware that these differential equations are not easy to solve and they also depend on a lot of model assumptions.

I don't think this route of pressure is the best way to get to the answer

>>Force times distance is energy so you have to consider what forces there are when you squat and when you stand up. If one is larger than the other you have changed the energy balance.

but with just gravity acting on you, the pressure will be the same whether you are standing or squatting.

>>This is where we don't agree. When I stand up I work against gravity, when I squat I work towards gravity*sin(theta)+mv^2/r, which is much smaller

Maybe you meant something else, but it's irrelevant, because conservation of energy has given us the right answer.

>>Yes it has. I hope you reconsider what that answer is though

Quote:
Originally Posted by The Engineer

Quote:
Originally Posted by mdf

Driven oscillation is a different question.  It doesn't affect the energy balance during one individual pumping cycle.  It only affects whether or not you get a cumulative effect from successive cycles, or a random jumble.

I'm not following some of what you are saying, so it made me think you are missing something about parametric oscillations, but maybe I'm just not understanding  Let me summarize, and I apologize if I'm covering something you already know.  There are two main ways to swing a swing.  The most common is to sit and lean back while kicking the legs out while moving forward, and then legs back, body forward while moving backward.  There's also another method you can do by standing on the swing.  You squat at both high ends and stand up as you move through the lowest point.  Without any other forces pushing any other direction than just straight down on the swing, you can pump the cycle so that the amplitude keeps growing and growing.  This works by pushing your center of mass against the centripetal force to increase rotational energy.  Everytime you stand up while swinging through the lowest point you add energy into the rotation.  It relies solely on having angular momentum.  You have to get the swing going first by traditional swinging, but once you have some angular momentum you can just keep pumping it by standing and squatting with no other typical swing forces.

Even here, angular momentum is not a particularly fruitful way to do the calculations.  You have to have an initial movement to have something to be in phase with, sure.  But the simple energy balance is an easier way to look at it.  When you stand up at the bottom, you raise your center of mass relative to where it would be passively, adding energy into the system.  When you crouch at the top, the direction of crouching is not aligned with gravity, allowing you to reset your legs without as much loss of potential energy.

Can you do the calculation with angular momentum instead?  Probably, but why?

Quote:
Originally Posted by Tog

I don't get this horizontal bit. Why would not the skier continue on a tangent to the curve? Which at the top of the pipe is near vertical.

Tog, thanks for the civil inquiry!

First, what causes the rider to follow the tangent of the curve?  The reaction force of the snow.  More pressure equals more reaction force, less pressure equals less reaction force.  So if you flex and reduce some of the pressure, you will be at least temporarily reducing the reaction force.  With less reaction force, the surface will be redirecting you upwards less then it would with more reaction force.   So if you flex your legs lower down on the wall, reducing pressure and reducing the redirection...then your CoM is getting closer to the wall, its not running in tangent anymore.  Its heading on a collision course for the wall.  As you continue to move on more of a horizontal path that does not match the wall, the more acute the angle of the wall becomes compared to the direction of travel by the CoM.  Eventually you will either be unable to flex your legs anymore and your body is going to compress against the wall at whatever angle it happens to be when the pressure finally comes.

But the angle of the slope that you're running into has an effect also.  There is a reason that good jump faces are curved rather then an acute and abrupt angle for the jump.   When you run into an acute angle, there will be a braking effect and the conservation of speed will be compromised.

In ski turns, we can consider similar effect.  If you have a small steering angle, the speed is conserved, the skis are redirected on a new path, in small increments, with an angle of deflection that is always very small, one tiny deflection after another.  If you create a larger angle of deflection, it becomes more of a braking effect.  The ultimate braking effect would be if the skis are perpendicular to the direction of travel, then its basically a hockey stop.  But the skis don't have to be perpendicular to be compromising the speed.

Quote:

Thanks @CTKook

(the tilting of the board probably should have been left out - that has to do with preserving the length of the pipe and going straighter / more square up the "next" wall)

Could you live with this (where down the wall things are still good but up the wall they've decided their done and just want to stall out at the top of the pipe)

Going down the wall the rider is flexing = pumping (in this context) to gain speed

On the way up their "last wall" the rider is flexing = absorbing (in this context) to lessen speed.

No.

You flex while dropping in or while re-entering.  You pump the transition for speed.  If you want to pump the next wall, you flex again before you get there, and again pump the transition.

If you want to absorb speed, you drop in tall, and get a sick feeling in your stomach from the moment of free-fall, then absorb by flexing into the tranny.  If you want to kill more speed, you get tall again, and flex into the next transition, too.   You can then even get tall on the upper part of that wall, and flex into the tranny yet again on the way down, at which point your speed, on snow, will be pretty well gone.

Quote:
Originally Posted by borntoski683

TE, I am starting to think you are a troll and friends with Kook in some way.  You have made numerous fallacious statements.  I have no idea why are you trying so hard to discredit me, but whatever..  I stand by absolutely everything I have said...

So, the problem I have is that you challenged my premise and backed it up with a very basic mistake in physics.  I would have been willing to continue the discussion with some verification, so that we can have a real discussion that gets to the right answer.  But, you had no interest in modifying your thinking or your statements based on verifiable facts.  This is a non-starter.  I understand that you don't have a physics background.  We don't have to discuss physics.  I actually like you quite allot and enjoy talking about skiing with you.  But, there's absolutely no point what so ever discussing physics if you can't limit your conversation to things that are true whenever possible, whenever verifiable.  You really have an interesting personality trait going on there.  I can't relate, but it's fascinating.  You're definitely a skiing expert and want to participate in the conversation.  You know what.  I've decided I like it.  Don't let me stop you, but I don't want anyone to think I'm targeting them without targeting you.  So, I just had to put that out there, that I won't comment anymore about the physics, but still enjoy reading.

Quote:
Originally Posted by CTKook

No.

You flex while dropping in or while re-entering.  You pump the transition for speed.  If you want to pump the next wall, you flex again before you get there, and again pump the transition.

If you want to absorb speed, you drop in tall, and get a sick feeling in your stomach from the moment of free-fall, then absorb by flexing into the tranny.  If you want to kill more speed, you get tall again, and flex into the next transition, too.   You can then even get tall on the upper part of that wall, and flex into the tranny yet again on the way down, at which point your speed, on snow, will be pretty well gone.

Thanks @CTKook

Pump the transition for speed = extension?

Quote:
Originally Posted by CTKook

No.

You flex while dropping in or while re-entering.  You pump the transition for speed.  If you want to pump the next wall, you flex again before you get there, and again pump the transition.

If you want to absorb speed, you drop in tall, and get a sick feeling in your stomach from the moment of free-fall, then absorb by flexing into the tranny.  If you want to kill more speed, you get tall again, and flex into the next transition, too.   You can then even get tall on the upper part of that wall, and flex into the tranny yet again on the way down, at which point your speed, on snow, will be pretty well gone.

This is correct. Now you just have to understand why what I said is also correct.

New Posts  All Forums:Forum Nav:
Return Home
Back to Forum: Ski Instruction & Coaching