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# Dan Dipiro's Mogul Book - Page 28

The gain in speed coming from the elevation (ignoring loss due to friction) is equal to the change in potential energy of elevation, which is mgh.  The work done to reduce this to a zero gain in speed is equal to the work you would do going up that hill.  If you are going down that hill, the energy that would (with no speed control) be added to your velocity is equal to the work you would have to do to go up that hill.  If you are going down that hill at 20 mph, how quickly you would have to work to account for all that energy is exactly how quickly you would have to burn energy if your were pumping that bike up the hill at 20 mph.  If you can pump your bike up that hill and maintain a 20 mph, congratulations; you can work hard enough to counteract all that speed you would otherwise be gaining as you go down the hill.

I'll let TE tell you how many horsepower it would take to go up a 20 degree slope at 20 mph (or how many brake horsepower it would take to prevent any speed gain going down a 20 degree slope at 20 mph).

Quote:
Originally Posted by Ghost

The gain in speed coming from the elevation (ignoring loss due to friction) is equal to the change in potential energy of elevation, which is mgh.  The work done to reduce this to a zero gain in speed is equal to the work you would do going up that hill.  If you are going down that hill, the energy that would (with no speed control) be added to your velocity is equal to the work you would have to do to go up that hill.  If you are going down that hill at 20 mph, how quickly you would have to work to account for all that energy is exactly how quickly you would have to burn energy if your were pumping that bike up the hill at 20 mph.  If you can pump your bike up that hill and maintain a 20 mph, congratulations; you can work hard enough to counteract all that speed you would otherwise be gaining as you go down the hill.

I'll TE tell you how many horsepower it would take to go up a 20 degree slope at 20 mph (or how many brake horsepower it would take to prevent any speed gain going down a 20 degree slope at 20 mph).

Basically, again no.  I think you watched the video TE had linked, but got lost.

Net net, it's way easier to keep speed at, say, 20 mph while going downhill than it is to pump uphill at 20 mph.  Jump on skis or a bike and check it out.  This is again a case where some hand's-on experience might go a long way in promoting some understanding.

Quote:
Originally Posted by CTKook

Quote:
Originally Posted by Ghost

The gain in speed coming from the elevation (ignoring loss due to friction) is equal to the change in potential energy of elevation, which is mgh.  The work done to reduce this to a zero gain in speed is equal to the work you would do going up that hill.  If you are going down that hill, the energy that would (with no speed control) be added to your velocity is equal to the work you would have to do to go up that hill.  If you are going down that hill at 20 mph, how quickly you would have to work to account for all that energy is exactly how quickly you would have to burn energy if your were pumping that bike up the hill at 20 mph.  If you can pump your bike up that hill and maintain a 20 mph, congratulations; you can work hard enough to counteract all that speed you would otherwise be gaining as you go down the hill.

I'll TE tell you how many horsepower it would take to go up a 20 degree slope at 20 mph (or how many brake horsepower it would take to prevent any speed gain going down a 20 degree slope at 20 mph).

Basically, again no.  I think you watched the video TE had linked, but got lost.

Net net, it's way easier to keep speed at, say, 20 mph while going downhill than it is to pump uphill at 20 mph.  Jump on skis or a bike and check it out.  This is again a case where some hand's-on experience might go a long way in promoting some understanding.

It is if you are not doing the work, which you are not if your bike has brakes.  Try fexing and extending on a snow bike going down that bump run without braking.

Energy cannot be created nor destroyed, but only converted to another form.  It is like transfering money from one bank account to the other.  Money coming out of the mgh bank goes into the 1/2mv^2 bank, unles you spend some of it.  The amount you spend per second has to be equal to the amount you take out of the mgh bank per second.

Aside:  The faster you go, the more energy you need to kill per second to avoid gaining speed.  Hence it is easier to control your speed, through friction, going down the same hill at 10 mph than 30 mph.

That may be, but the energy balance described by Ghost shows that the indirect effect of pumping has to be larger than the direct effect, regardless of how it feels to you.
Quote:
Originally Posted by Ghost

It is if you are not doing the work, which you are not if your bike has brakes.  Try fexing and extending on a snow bike going down that bump run without braking....

You may not realize it, but there are plenty of pump tracks and Squamish-style trails that aren't on completely flat ground.  So, believe it or not, it is possible to get direct experience in the matter both on skis and on a bike.

Again, and not surprisingly, it is easier to control speed on the descent than to pump uphill.  Even the geometry going uphill is a bit of a bear, but I digress.

Check it out.

Physics = spectator sport.

I like to ski moguls.  I like to complete my turns.  I like to skid.  I like to pump bumps.  I like to A&E.  I don't like to face plant.

Quote:
Originally Posted by LiquidFeet

Physics = spectator sport.

Only when people try to deny basic examples.  Very little of this even requires understanding physics, you can find a bumper or a kid running chainless and they likewise will get it.

The physics was supposed to make it interesting. But even last year we had someone claiming to have a phd strenulously arguing over centrifugal force with another phd. We've had similar in this thread. If this were a class in physics everyone would leave confused. The TA's would be overwhelmed with questions .

Sure the kid on the bike could keep pumping away but when he went to physics class his answers would depend on which prof he had. That's not supposed to be the case. Maybe in English Lit but not Physics.
Quote:
Originally Posted by Tog

The physics was supposed to make it interesting. But even last year we had someone claiming to have a phd strenulously arguing over centrifugal force with another phd. We've had similar in this thread. If this were a class in physics everyone would leave confused. The TA's would be overwhelmed with questions .

No, people who understand the pretty simple concepts would leave wondering why someone would assert some of the silly things asserted on here.

This really does come down to 2+2 = 4.  And, can be experienced directly for those curious.  For instance, for those off snow for the year, take brakes and chain off a hardtail or better yet go buy a cheap 20" bmx bike and run it chainless and brakeless on a pumptrack for training.  Find a pump track on a hill and then report back on whether it takes more effort to 1) go uphill, or 2) control speed on the downhills.  I personally am scared of numbers and letters and equations and such, but can tell which takes more effort.  And I am not violating physics I assure you.

For bonus point, even try to make speed just by getting tall in the flats.  J/k.

I take it you don't have any background in physics?
That's the issue. Not whether a child can swing or ride a bmx on a pump track. It certainly is not explained by a simple 2+2= 4. if so, we'd be done.
Quote:
Originally Posted by Tog

I take it you don't have any background in physics?
That's the issue. Not whether a child can swing or ride a bmx on a pump track.

Why do you care?  I introduced some of these basic concepts to the forum, again roughly two years ago, and had people claiming backgrounds in physics basically sneer and ask why I was using confusing terms like parametric oscillation (at that time, I think it fair to say that only Jack97 had heard of or understood the term, in terms of active posters in that thread).

Just like the interest in my waistline and my living quarters, my background seems an odd area of focus.  I thought the issue in this thread was Dan Dipiro's book, and more specifically whether absorption as used by him, Chuck Martin, and a great many others is an accurate term and concept.

Your desire for ad hominem may be your own personal issue.  Cognitively, however, you should be able to process what A&E is, and for that matter some of TE's posts are clear enough that I think I could walk a chainless, brakeless young adult with ADD through the principles underlying this.

If you really want to know what my waistline looks like or what my educational background is, at least offer to buy me dinner and give me some background noise first.  And, NTTAWWT, I still don't know that you need to know.

If you are simply trying to deflect attention from the fact that some very frequent forum posters were saying stuff that doesn't hold up to basic analysis, well, tough.  The physics here, pardon the pun, ain't rocket science.  And the terms and athletic use of those terms have been with us for quite some time, so in terms of bump skiing (the specific focus of the thread) and more broadly related activities such as some types of MTB, neither is this new or surprising ground for those with in depth experience in these activities.

Kook, you did not correct my earlier statement so I will assume that to mean I was correct in stating that you feel we can somehow soak up the speed, absorb speed...with our legs.  Is that correct?

If that is correctly stating your opinion, then can you please try to explain how that works, besides saying that you and any other 9 year old can ride in a pump track and experience what they believe to be soaking up speed with their legs.

Parametric oscillation has been shown to be totally irrelevant to bump skiing.  If you still think it is you're going to have to do a much better job of explaining why.  I realize you think you're a genius and the rest of us are neanderthals, but please try your best to bring us up to your level.

Lol CTK.
Since the issue at the moment is physics, and you claimed the physics was as easy as 2+2 = 4, it's relevant. But you answered by not answering. No, I don't care. The "do you have a background in physics?" is not an ad hominem attack. It's a simple question.

Anyone can throw a frisbee. Explaining how it flies is not easy.
Quote:
Originally Posted by Tog

The physics was supposed to make it interesting. But even last year we had someone claiming to have a phd strenulously arguing over centrifugal force with another phd. We've had similar in this thread. If this were a class in physics everyone would leave confused. The TA's would be overwhelmed with questions .

Sure the kid on the bike could keep pumping away but when he went to physics class his answers would depend on which prof he had. That's not supposed to be the case. Maybe in English Lit but not Physics.

For what it's worth, I agree with the engineer.  He understands conservation of energy.  A point Kook doesn't seem to get.

I'll try one more time

The amount of gravitational potential energy that you will have at the top of the run over and above the amount you will have at the bottom of the run is your mass times the elevation change times gravitational acceleration.  If you do not find a way to get rid of that energy on the way down, it will be converted to Kinetic energy.  We usually get rid of most of it by friction.   It is the same amount of energy you would have to supply to go up the run.   You would have to run up the run at the same speed to suppy the same amount of energy as quickly as you need to get rid of the energy being converted to kinetic energy at any given speed on the way down.

Quote:
Originally Posted by CTKook

Why do you care?  I introduced some of these basic concepts to the forum, again roughly two years ago, and had people claiming backgrounds in physics basically sneer and ask why I was using confusing terms like parametric oscillation (at that time, I think it fair to say that only Jack97 had heard of or understood the term, in terms of active posters in that thread).

Parametric oscillation is a type of driven oscillation where you change one or more parameters of the differential equation describing the system, and it is used in harmonics. To say that you use it in bump skiing provides very little information.

In the swing and ramp example the parametric frequency is double that of the oscillation, what is it in bump skiing?

It seems the only argument you are using is that you made a post two years ago.

You are welcome to provide proof either way.

As TE said, it is easy to make mistakes, but until I'm proven wrong I'll stick with my reasoning.

So Ghost I like what you wrote...  let's go back to the stair case example.  When you walk down stairs you are getting rid of potential energy with each step down the stairs, or rather converting it to kinetic energy.  If you move down the mountain its not a question of whether kinetic energy will be realized.  It will.  its only a question of how quickly that conversion will take place, in terms of energy conservation.

If we are resisting this conversion to kinetic energy, we feel that as muscular effort which is a certain kind of contraction that is different from walking up stairs, but nonetheless, its engagement of muscles and outputting work, lowering our load towards the center of the earth but in a resisted way.  Resisting against what?  Resisting against the acceleration of gravity.  That results in pressure under our feet.

Walk down stairs or run down stairs.  Running down feels like less resistance.  Less work in terms of what we do with our human body.

What we are resisting against is that actual conversion from potential to kinetic, and this resistance is the key to using pressure to control speed.

I posted this ramp before but no one responded.

Can someone give a motivation why this is wrong?

A The skier starts out with bent legs, zero velocity and energy mgh

B the skier has converted the energy to speed and starts extending his legs

C The skier has extended the legs, since the work is orthogonal to the direction of travel the speed has not changed and the potential energy is mgdeltah

D the ramp flattens out

E The skier has reached the same height as in the beginning, but since the altitude difference between C and E is h-deltah there is still mgdeltah of kinetic energy left. Now the skier retracts his leg so there is virtually no force acting on him except gravity.

F the skier lands on the ramp with bent legs. This position is higher than A. It is less high than h+deltah because some energy was lost, but it is larger than H. In the example I calculated yesterday it was h+0.8deltah (it depends on the timing and rate of the retraction)

G the skier is at the bottom again and the speed is higher than at B since the starting height at F was higher than A.

Please provide detailed motivation. Just saying it is not possible by conservation of energy is not enough. Energy is added between B and C.

An by the way, I agree with Kook about one thing, its harder to walk up stairs then to walk down.  One reason could be that with each step up the stairs we are both working against gravity that is trying to pull us back (this is the same resistance we have to fight going down except we are not allowing any kinetic energy to be realized, we are resisting entirely against all energy conversion, which takes effort) and at the same time we are raising ourself to a new level of potential energy with nothing but our own leg output.  So yea...its a lot more effort involved in terms of muscle output. More pressure, etc.

The problem with the energy conservation model for analyzing this activity, in isolation, is that it does not account for the effort we have to make as humans with muscles to resist against gravity at all times, while also raising or lowering our CoM per the energy conservation model.  In the case of flexing and extending it becomes even more complicated by the fact we are not a rigid body, we're manipulating pressure and ground reaction forces.

Quote:
Originally Posted by borntoski683

An by the way, I agree with Kook about one thing, its harder to walk up stairs then to walk down.  One reason could be that with each step up the stairs we are both working against gravity that is trying to pull us back (this is the same resistance we have to fight going down except we are not allowing any kinetic energy to be realized, we are resisting entirely against all energy conversion, which takes effort) and at the same time we are raising ourself to a new level of potential energy with nothing but our own leg output.  So yea...its a lot more effort involved in terms of muscle output. More pressure, etc.

The problem with the energy conservation model for analyzing this activity, in isolation, is that it does not account for the effort we have to make as humans with muscles to resist against gravity at all times, while also raising or lowering our CoM per the energy conservation model.  In the case of flexing and extending it becomes even more complicated by the fact we are not a rigid body, we're manipulating pressure and ground reaction forces.

It is simply because eccentric muscle contractions expend less energy than concentric. You can get really sore muscles though.

Definitely could be something to be said for eccententric contractions being more efficient then concentric (note to self, flex to release!)

But I think if we are just standing statically on flat ground, we are using our muscles, resisting gravity.  Try to stand with slightly bent legs for a little while, it takes work.  The only reason it seems easy is because typically we stack up onto our bones.  Just standing here on ground zero we are expending muscular effort resisting gravity's efforts to pull us to the center of the earth, converting potential into kinetic.  We feel the pressure under our feet standing here as the ground reacts.

And when we go uphill, we are not only expending that muscular effort, but also we are creating new levels of potential energy by moving up through no other means other than our muscular work (its not kinetic energy being converted to potential).

Quote:
Originally Posted by CTKook

I thought the issue in this thread was Dan Dipiro's book, and more specifically whether absorption as used by him, Chuck Martin, and a great many others is an accurate term and concept.

I thought I covered that here ...with terms.....no physics either !!!!

http://www.epicski.com/t/39753/dan-dipiros-mogul-book/570#post_1873776

Quote:
Originally Posted by Jamt

Quote:
Originally Posted by borntoski683

An by the way, I agree with Kook about one thing, its harder to walk up stairs then to walk down.  One reason could be that with each step up the stairs we are both working against gravity that is trying to pull us back (this is the same resistance we have to fight going down except we are not allowing any kinetic energy to be realized, we are resisting entirely against all energy conversion, which takes effort) and at the same time we are raising ourself to a new level of potential energy with nothing but our own leg output.  So yea...its a lot more effort involved in terms of muscle output. More pressure, etc.

The problem with the energy conservation model for analyzing this activity, in isolation, is that it does not account for the effort we have to make as humans with muscles to resist against gravity at all times, while also raising or lowering our CoM per the energy conservation model.  In the case of flexing and extending it becomes even more complicated by the fact we are not a rigid body, we're manipulating pressure and ground reaction forces.

It is simply because eccentric muscle contractions expend less energy than concentric. You can get really sore muscles though.

Also easier going down than up because there are always inefficiencies and loss due to friction.  When going up we have to add enough energy to both get up and overcome the inefficiencies and friction, but when going down we only have to supply enough energy to cover our inifficiencies and what is left after friction losses.

Friction is an interesting one.  Friction creates more pressure, more resistance, more slowing...I don't particularly think that makes it any easier on our body.  It makes the slowing effect more efficient, but when more friction is present, we feel more load, muscles work more.  Friction gets the job done quicker, but the body still has to work for it and will generally feel this sensation as pressure.

Quote:
Originally Posted by borntoski683

Friction is an interesting one.  Friction creates more pressure, more resistance, more slowing...I don't particularly think that makes it any easier on our body.  It makes the slowing effect more efficient, but when more friction is present, we feel more load, muscles work more.  Friction gets the job done quicker, but the body still has to work for it and will generally feel this sensation as pressure.

Yes, but we apply that load through proper skeletal stacking with advantageous leverage, as oppesed to bent legs.

hehehe  better duck that sounds like pumping!

Agree, skeletal stacking is extremely powerful skill base in skiing and hiking and many things.  When I climbed Mt Rainier we had to learn how to do a lock step thing which basically accomplished exactly that.

Quote:
Originally Posted by Uncle Louie

I thought I covered that here ...with terms.....no physics either !!!!

http://www.epicski.com/t/39753/dan-dipiros-mogul-book/570#post_1873776

Well, again, if you want to say that for purposes of EpicSki as a forum we will throw out the terms used most commonly by the people focused on the activity, and instead substitute different terms, then fair enough.

I do note that the thread is titled "Dan Dipiro's Mogul Book," and not only does he use the term absorption, but the term has been around since the 70s (at least).  If he talks to Chuck Martin who talks to TE who talks to Jack97, and each mentions absorption, they all know what is being said.  It is used by the USST, by PSIA, by international muckety mucks.  So, why throw that out?  There's no athletic need to do so, nor any communication benefit from doing so.

To stress, it is a common term, apt, has been around for quite some time.  I'm not sure I see the benefit in throwing the term out.  You can't even claim it hurts people feelings or is politically incorrect.  Perhaps the solution is to have this thread focused on Dan Dipiro's Mogul Book and add the qualifier "practical mogul skiing discussion."  Then we could also have a new thread with no standard terms allowed, and for purposes of that thread say that it must be accepted that pumping the face is the only means of speed control other than edging.

I agree Kook, I have nothing against the term "absorption".  However, what I feel we are absorbing is pressure.  There are good reasons for doing this on the face of the mogul

Where we disagree is how the absorption of pressure effects speed control.  Loss of pressure means loss of resistance to the aforementioned conversion of potential to kinetic energy.....ie...go fasta'

However, in the grand scheme of things, if pressure absorption on the face enables us to keep our skis on the snow on the backside, then we will be able to maintain control, maintain edge engagement and use edge engagement to control our speed on the backside.  So yes...absorbing has an indirect effect on allowing us over all to control our speed, but the act of absorbing itself does not slow us down.

In terms of using pressure for speed control, then we can embrace some of the pressure on the face, rather then absorbing all of it.  This can be done in such a way so that we are absorbing all of it as we crest the top, absorbing less of it near the bottom of the face.  This lack of absorbtion means more pressure, more resistance, and direct effect on the conversion to kinetic speed energy.  Absorbing it also basically spreads out the period of time over which the total pressure from the face of the bump experience will hit us.  This makes it easier on the body to accept and embrace that pressure and resist the energy conversion.

But if you eliminate the pressure too much with too much aborption of pressure, then you will go fasta'

Quote:
Originally Posted by Ghost

For what it's worth, I agree with the engineer.  He understands conservation of energy.  A point Kook doesn't seem to get.

I'll try one more time

The amount of gravitational potential energy that you will have at the top of the run over and above the amount you will have at the bottom of the run is your mass times the elevation change times gravitational acceleration.  If you do not find a way to get rid of that energy on the way down, it will be converted to Kinetic energy.  We usually get rid of most of it by friction.   It is the same amount of energy you would have to supply to go up the run.   You would have to run up the run at the same speed to suppy the same amount of energy as quickly as you need to get rid of the energy being converted to kinetic energy at any given speed on the way down.

This is both funny and kinda....I am at a loss.

Let's think this through.  You seem to be implying that, for instance, someone going through bumps with no turning at 30 mph is working exactly as hard as they would work to pump straight UP the slope at 30:.

Quote:
Originally Posted by Ghost

... If you are going down that hill at 20 mph, how quickly you would have to work to account for all that energy is exactly how quickly you would have to burn energy if your were pumping that bike up the hill at 20 mph.  If you can pump your bike up that hill and maintain a 20 mph, congratulations; you can work hard enough to counteract all that speed you would otherwise be gaining as you go down the hill....

You and BTS/aka Dewdman and others seem unable to do the basic math, so suffice it to say the only problem with what you assert are the basic physics.  And, if you had again the basic physical literacy of having done these things, you would know that this is not the way it works.

The problem is completely clouding up a discussion of bump skiing with lots of nonfactual, clearly erroneous ideas like those you are putting forward.

At least the safety issues here are not as severe as for some other topics where bad info can be much more serious.

Quote:
Originally Posted by Jamt

It is simply because eccentric muscle contractions expend less energy than concentric. You can get really sore muscles though.

Again, no.

This:

Quote:

Originally Posted by borntoski683

An by the way, I agree with Kook about one thing, its harder to walk up stairs then to walk down.  One reason could be that with each step up the stairs we are both working against gravity that is trying to pull us back (this is the same resistance we have to fight going down except we are not allowing any kinetic energy to be realized, we are resisting entirely against all energy conversion, which takes effort) and at the same time we are raising ourself to a new level of potential energy with nothing but our own leg output.  So yea...its a lot more effort involved in terms of muscle output. More pressure, etc.

The problem with the energy conservation model for analyzing this activity, in isolation, is that it does not account for the effort we have to make as humans with muscles to resist against gravity at all times, while also raising or lowering our CoM per the energy conservation model.  In the case of flexing and extending it becomes even more complicated by the fact we are not a rigid body, we're manipulating pressure and ground reaction forces.  [end quote]

Is what Jamt's comment relates to.

Anyone who runs or hikes regularly, much less skis or pumps on a bike, has experienced that going up requires way more work than going down.  You burn more calories, etc. etc.  This is both quantifiable, and easily experienced.  It isn't simply from eccentric contractions being more efficient.

Nothing magical happens by posting here that insulates people from this reality.

Edited by CTKook - 5/7/15 at 4:46pm
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