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# CoM, BoS, Centrifugal G-Force, and "Inclination" - Page 2

Don't get hung up on the word centripetal; it just gives the direction of the force or acceleration.  It means towards the centre (of the circle you would get if you completed the arc all the way around).

There is a force exerted on the ski/skier body by the snow acting on the skis. It is directed towards the inside of the turn (aka centripetal).  It makes the skier turn (towards the inside of the turn).  No need to complicate it.

You know,  I'm starting to regret posting Docs video.

The flaw in that video in regards to the sport of skiing is that to prove the physics point it, uses a fixed radius embroidery loop.  Skiers must use their abilities in conjunction with the properties of the skis to create and continue that loop.

An important reason for my participating in this thread was to answer the question "why can't many wanna-be advanced skiers complete turns"

We can discuss physics till the cows come home.  Having said that, does anyone not think that having a basic understanding of how things work is important in formulating a teaching progression?

As ghost stated above,  "Don't get hung up on the word centripetal"  and to a certain extent I agree, but the fact is if you are traveling in a circle there is centripetal force.... if you are not there isn't...period..

So as many skiers approach perpendicular to the fall line where the battle between inertial forces and circular forces are at their peak, why does the inertial force often win out?

For these skiers, the impending return to inertial travel is that Centrifugal "Feeling" and for many is received as a warning of sorts that control is about to be lost and ironically the self preservation movements they make in reaction seal their fate.

When I am in a turn I treat that Centrifugal "Feeling" as a signal that I am doing things right...the more the better.  When carving,  I am a circle builder....a centripetal skier......

JESINSTR:  Don't feel bad that you posted that video and I ran with it.  I am being bombastic solely to subject my "proclamations" to examination and testing by others who have more knowledge and experience than I do.

Here's what I think.  So let's see what you and others say.  I can take it.  Hammer away.

Wikipedia recognizes two types of centripetal "forces" 1. a "pull" force (like a rope holds a ball on a rope swinging from your hand around your head); and 2. a "push" force (like that wooden crochet hoop with the 8 ball).

https://en.wikipedia.org/wiki/Centripetal_force

In the first instance, a larger mass (me) is swinging a smaller mass being held by a rope to stay in the scribed circle and preventing it from flying off outside the circle laterally by straight-line inertia.  (See also, Gravity is the pull force (i.e., rope connecting) larger Mass Earth to smaller Mass Moon.)

In the second instance, the hoop is pushing the 8 ball to the stay in the scribed circle and preventing it from flying off to the outside the circle laterally by straight-line inertia.

I believe that both of these phenomena can be explained as follows:

The rope and the crochet hoop are merely unbalanced forces (i.e., friction) which are keeping both the roped ball and the free ball from following the path of straight-lined inertia.

So in skiing we use the uphill face of a bump, the wall of a half-pipe, the edges of our skis in a high speed carved curve to create the unbalanced force to redirect our CoM from the straight-line where inertia would take it.

So, sincerely, it seems to me that there is only MASS, MOVEMENT, and INERTIA in skiing.

What we do with the terrain and with our skis is to redirect that inertia to a new curved path of travel.

What we feel with our bodies while doing it, are not "forces" in the physical world at all.  They are merely our psychological perception of our bodies being redirected from that straight-line inertial path.

So, I respectfully say, that at least at this stage of my new knowledge (which I hope you will test) that the concepts of centrifugal force and even centripetal force are not real and add nothing to the applied physics of skiing.

Which is merely the redirection of Mass from its straight-line inertial path of travel, to a curved path of travel by skis and terrain, and the management of the stresses that our bodies experience during that redirection by angulation, angulation-inclination, inclination, skeletal stacking, extending and flexing, etc.

Edited by Tim Hodgson - 9/13/16 at 9:18pm
Quote:
Originally Posted by JESINSTR
An important reason for my participating in this thread was to answer the question "why can't many wanna-be advanced skiers complete turns"

In order for you and your skis (combined mass of m) to turn at a given radius R and speed v, there must be a net force acting on you with a magnitude of m(v^2) / R, pushing you towards the inside of the turn.  Net force means the vector some of every force that is acting on you, including gravity and the force of the snow acting on the skis, and friction.  At the high c point of a turn gravity helps pull you towards the inside of the turn and the skis add some force to add up to give you that m(v^2) / R.  At the bottom of the turn, gravity is pulling you to the outside of the turn and your skis have to push hard enough to counter the gravity force just to keep you from turning downhill, your skis have to push more to make your turn.  In effect the the skis have to push as hard as they were pushing at the high c, and then an push added amount more that is equal to twice gravity contribution.  The gravity contribution depends on the slope (0 for horizontal surface, 0.5 g for a 30 degree slop IIRC).

Quote:
Originally Posted by E350

JESINSTR:  Don't feel bad that you posted that video and I ran with it.  I am being bombastic solely to subject my "proclamations" to examination and testing by others who have more knowledge and experience than I do.

Here's what I think.  So let's see what you and others say.  I can take it.  Hammer away.

Wikipedia recognizes two types of centripetal "forces" 1. a "pull" force (like a rope holds a ball on a rope swinging from your hand around your head); and 2. a "push" force (like that wooden crochet hoop with the 8 ball).

https://en.wikipedia.org/wiki/Centripetal_force

In the first instance, a larger mass (me) is swinging a smaller mass being held by a rope to stay in the scribed circle and preventing it from flying off outside the circle laterally by straight-line inertia.

In the second instance, the hoop is pushing the 8 ball to the stay in the scribed circle and preventing it from flying off to the outside the circle laterally by straight-line inertia.

I believe that both of these phenomena can be explained as follows:

the rope and the crochet hoop are merely unbalance forces (i.e., friction) which are keeping both the roped ball and the free ball from following the path of straight-lined inertia.

So in skiing we use the uphill face of a bump, the wall of a half-pipe, the edges of our skis in a high speed carved curve to create the unbalanced force to redirect our CoM from the straight-line where inertia would take it.

So, sincerely, it seems to me that there is only MASS, MOVEMENT, and INERTIA in skiing.

What we do with the terrain and with our skis is to redirect that inertia to a new curved path of travel.

What we feel with our bodies while doing it, are not "forces" in the physical world at all.  They are merely our psychological perception of our bodies being redirected from that straight-line inertial path.

So, I respectfully say, that at least at this stage of my new knowledge (which I hope you will test) that the concepts of centrifugal force and even centripetal force are not real and add nothing to the applied physics of skiing.

Which is merely the redirection of Mass from its straight-line inertial path of travel, to a curved path of travel by skis and terrain, and the management of the stresses that our bodies experience during that redirection by angulation, angulation-inclination, inclination, skeletal stacking, extending and flexing, etc.

Your almost there, except there are real forces that cause the redirection.  There is mass, movement and inertia and change.  The force is in fact equal to the rate of change with time of your momentum (momentum and inertia are closely related!).

Quote:
Originally Posted by JESINSTR

You know,  I'm starting to regret posting Docs video.

The flaw in that video in regards to the sport of skiing is that to prove the physics point it, uses a fixed radius embroidery loop.  Skiers must use their abilities in conjunction with the properties of the skis to create and continue that loop.

An important reason for my participating in this thread was to answer the question "why can't many wanna-be advanced skiers complete turns"

We can discuss physics till the cows come home.  Having said that, does anyone not think that having a basic understanding of how things work is important in formulating a teaching progression?

As ghost stated above,  "Don't get hung up on the word centripetal"  and to a certain extent I agree, but the fact is if you are traveling in a circle there is centripetal force.... if you are not there isn't...period..

So as many skiers approach perpendicular to the fall line where the battle between inertial forces and circular forces are at their peak, why does the inertial force often win out?

For these skiers, the impending return to inertial travel is that Centrifugal "Feeling" and for many is received as a warning of sorts that control is about to be lost and ironically the self preservation movements they make in reaction seal their fate.

When I am in a turn I treat that Centrifugal "Feeling" as a signal that I am doing things right...the more the better.  When carving,  I am a circle builder....a centripetal skier......

Wait.... what?  Is that what this thread is about?  How come a wannabe advanced skier has problems completing a turn?

We are discussing centripetal and centrifugal forces and inertial physics stuff to help wannabe advanced skiers complete their turns?

Quote:
Originally Posted by E350

Ghost

I am sorry, I don't see centrifugal or centripetal "forces" adding anything to our analysis.  And, in fact, these latter "forces" have always confused me.  Thus, I Proclaim that the "Unified Theory of Mass and Inertia In Skiing" along with the fom redirection corollary replaces and entirely obviates the concepts of centrifugal and centripetal forces and, thus, makes Bob Barnes' diagram in the first post of this thread unnecessary and, I dare say, invalid.*

Most people understand that to speed something up you have to push on it, but you also have to push on something to change its direction. When you go around a circle you are constantly changing directions, so something has to constantly push on you towards the center of the circle. This is called centripetal force. When you push on something it pushes back because of its inertia. So when something keeps pushing you toward the center of the circle, your body keeps resisting this constant change in direction. This push back away from the center of the circle is called centrifugal force.

You can view centrifugal force as just inertia and changing directions, but when you start trying to solve problems it gets real complicated real fast, because the change of direction occurs in an infinitely small time. So every time you set up an equation using just linear parameters, you'd have to do an integral, thus rotational physics is born, and the problems are much simpler to solve using rotational equations.

Once you understand rotational physics it really is the easiest way to solve rotational problems and understand what's happening to predict what will happen. In this discussion forum, it's hard to define success when talking about these physics problems, so maybe one perspective seems as good as another, but you'll have very little luck trying to design a mechanical system involving rotation and have it do what you want it to do without using rotational physics. But, obviously you don't have to be able to solve rotational physics problems to be able to ski well, so whatever floats your boat.

Ghost:  Thank you for bearing with me.  I guess you could say that by holding my skis on edge in a carve that I am a "force."

But I don't see how a bump or a half-pipe wall is a "force."

Yet both of these redirect my body Mass from its straight-line inertial (or even Gravity-based fall line) path of travel.

So, again I see the only force in skiing as inertia (created by movement or gravity or the combination i.e., sum of both).

And all we do in skiing is redirect inertia.  Period.

No?

Edited by Tim Hodgson - 9/13/16 at 8:48pm

The Engineer:

PSIMAN

https://vimeo.com/9872613

He skis better than me.  Has no muscles and no brain, yet the terrain redirects his inertia just fine -- i.e., clearly there is no "force" either in PSIMAN or in the terrain is there?

I am not trying to predict the future via a mathematical formula.

I am a ski instructor - about as smart as PSIMAN - trying to first understand for myself and then to convey to others a crude but correct theory of applied skiing physics if I deem it helpful in a particular lesson, with a particular type of student -- not to most students.

Edit:  maybe I'll have a beer.

Edited by Tim Hodgson - 9/13/16 at 10:05pm

The most common mistake in physics when setting up force equations is to include forces that are acting on a different body.

Gravity is a force that is acting all the time.  We are so used to it that we don't even notice it most of the time, if we aren't falling.  Gravity is pulling PSI man.

The other very common force is the contact force.  When we stand on the ground, gravity is pulling us down, but we don't go down, because the ground won't let us go down; it is pushing us up, just as much as gravity is pulling us down, so no net force and we just stand there with no acceleration.   When PSI man's ski hooks up, or our skis hook up, there is a contact force acting on the ski and us that turns us (the centripetal force), and keeps us from sinking into the snow.  This force acts at the ski-snow interface and must be included in the equations that calculate our acceleration, if you care to calculate it.

Not only is the snow pushing on the ski, but the ski is pushing on the snow.  That force is pointing to the outside of the turn (centrifugal) and down.  However unless you want to calculate the the acceleration of the earth due to your pushing it with your skis, don't put it into a equation.  It has no effect on your motion, because it isn't pushing or pulling you.   It may be equal in magnitude to the force that IS pushing you, but it is not the force pushing you; the centripetal one is.

Hmmm...

So, the bump and the half-pipe are in fact a "force."

Per Wikipedia they would be a centripetal "push" force.

And they are centripetal (meaning circular) forces because any deflection from a straight line is part of an arc?

So that all earth push forces are by their nature centripetal?

Ghost, you are a very patient person.

Thanks.

Quote:
Originally Posted by E350

Hmmm...

So, the bump and the half-pipe are in fact a "force."

Per Wikipedia they would be a centripetal "push" force.

And they are centripetal (meaning circular) forces because any deflection from a straight line is part of an arc?

So that all earth push forces are by their nature centripetal?

Ghost, you are a very patient person.

Thanks.

Very few motions in skiing are truly circular. Perpendicular force components change your direction. The tangent component changes you speed.

This is the simple case where the force acts through the CoM, in reality it seldom does, which is why you have to add rotational aspects as well.

Quote:
Originally Posted by E350

The Engineer:

He skis better than me.  Has no muscles and no brain, yet the terrain redirects his inertia just fine -- i.e., clearly there is no "force" either in PSIMAN or in the terrain is there?

There most definitely is a force, and Ghost did a good job of explaining it.  There's one more concept that you really have to wrap your head around that helps explain much of it.  Entire fields of engineering live mostly to exploit this concept.  Forces that are 90 degrees to each other do not interfere.  With circular motion the centripetal force is always 90 degrees to the direction of travel, so even though there's always a force applied, the instantaneous linear speed doesn't change.  Let's take an easy example of the earth going around the sun.  The centripetal force is gravity.  The centrifugal force in reaction to this comes from the earth's inertia.  When you push on a rock, that rock will push back and accelerate away from you, but when you stop pushing that rock doesn't suddenly accelerate towards you.  Similarly, when gravity is pulling on the earth, if it suddenly stopped pulling, the earth wouldn't accelerate in the opposite direction.  What it would do is keep going the way it was going.  It seems to me that some people think centrifugal force isn't real, because it doesn't accelerate you in the direction it's pointing when the centripetal force is removed.  But, I can't think of an example of where that would happen for any reactionary force, so there's no reason to single out centrifugal force as different from other reactionary forces that people accept.  When the centripetal force is removed the centrifugal force immediately stops as well, because it was there only in reaction to the centripetal force.  When you stop pushing on a rock, it stops pushing on you immediately.  The centripetal force is real, because it takes a force to change directions.  The centrifugal force is real, because every force has an equal and opposite reaction force which you feel as pressure on your hands every time you push on something.

In the earth and sun example, the forces are easy to understand from gravity and inertia.  In a train example (or skier), I agree it's harder to wrap the head around.  Maybe an example to help understand it would be a ball bouncing off the ground at an angle.  The only reason that ball can change directions is because it runs into the ground and the ground pushes back with a force from the impact (due to the balls inertia and the earth's inertia).  When a train goes around a curve the rails push on the train to change it's direction.  This is why those train wheels have a lip that stick down below the rail, so that the rail can push on it.  Otherwise, the train would just keep going in the same direction and fly off the tracks.  If there wasn't a force there, the engineer wouldn't have to design the thickness of that lip to keep from breaking off, nor the thickness of the railroad ties to keep the the track from breaking apart.  Using the speed and mass of the train, the engineer can calculate the centrifugal force that will be applied on those components to design them to handle the load.  If those forces didn't exist, we could make those components out of something lighter to save money.

Quote:
Originally Posted by E350

The Engineer:

I am not trying to predict the future via a mathematical formula.

Predicting the future is pretty much all we do with trying to understand physics.  When you understand why it's happening you can understand what will happen when the same conditions repeat in the future, because the laws are consistent. This is what we do with engineering.  We predict that if the lip of that train wheel in the example above is greater than a certain thickness, it will not break in the future when the train comes around the curve at a certain speed.  Or in skiing, if you apply a little more counter you can predict that .......

My training on the subject of paired forces pointed out that in the case of circular travel the paired forces are not centripetal and centrifugal but the force applied by the object in motion and the reaction force. I push the world to the right, the world pushes me to the left. Being somewhat less massive than the world I get moved a lot to the left while the world gets moved just a tiny bit to the right.

fom

No e350 they are not a force. They are a mostly stationary object that we run into. At least for our puposes. We can add in the Earth's rotation, travel around the sun, travel through the Milky way, and the Universe for that matter. We can add in the internal muscular forces we create as well. In the end what we as humans feel is being pulled somewhere. Our response is to resist that pull through our actions. Standing is one of those actions, as is skiing. What is unique to skiing is how we play with Gravity. The first half of every turn we cooperate with Gravity by allowing it to pull us into the fall line. We can add to that by tipping and engaging the ski edges (allow the snow to deflect us into the turn). As we turn into the fall line Gravity pulls us straight forward and unless we use those engaged ski edges I mentioned, we would continue to go straight down the hill.
So we resist Gravity by turning away from it. The skis tipped on edge help deflect us there as described earlier. But only if we use our muscles to strongly stand on the skis. We time our cooperation and resistance to Gravity in just the right amounts to control our direction of travel.

As far as the physics goes the interaction between the snow and skis is a pushing match but we do all the pushing and the snow simply resists that.

jasp,

I'm being picky here but the snow does more than resist, it pushes back with the same amount of force that you are pushing on the snow with. It is this ground reaction force that I use to move myself about on the slope.

fom

In that way the snow isn't a force but can react to a force applied to it. By tipping the skis the push and push back is no longer vertical, or even parallel to the snow surface. It is sideways in the same direction as the top of the tipped skis.

Edit:  In the time it took my meager brain to absorb Engineer and Ghost's posts above, and to compose the following reply, there have been 4 posts.  Those even better than my post below, describe the conceptual battle that I am having.  Thanks JASP and fom!.

Quote:

Originally Posted by The Engineer

There most definitely is a force, and Ghost did a good job of explaining it.  There's one more concept that you really have to wrap your head around that helps explain much of it.  Entire fields of engineering live mostly to exploit this concept.  Forces that are 90 degrees to each other do not interfere.  With circular motion the centripetal force is always 90 degrees to the direction of travel, so even though there's always a force applied, the instantaneous linear speed doesn't change.  Let's take an easy example of the earth going around the sun.  The centripetal force is gravity.  The centrifugal force in reaction to this comes from the earth's inertia.  When you push on a rock, that rock will push back and accelerate away from you, but when you stop pushing that rock doesn't suddenly accelerate towards you.  Similarly, when gravity is pulling on the earth, if it suddenly stopped pulling, the earth wouldn't accelerate in the opposite direction.  What it would do is keep going the way it was going.  It seems to me that some people think centrifugal force isn't real, because it doesn't accelerate you in the direction it's pointing when the centripetal force is removed.  But, I can't think of an example of where that would happen for any reactionary force, so there's no reason to single out centrifugal force as different from other reactionary forces that people accept.  When the centripetal force is removed the centrifugal force immediately stops as well, because it was there only in reaction to the centripetal force.  When you stop pushing on a rock, it stops pushing on you immediately.  The centripetal force is real, because it takes a force to change directions.  The centrifugal force is real, because every force has an equal and opposite reaction force which you feel as pressure on your hands every time you push on something.

In the earth and sun example, the forces are easy to understand from gravity and inertia.  In a train example (or skier), I agree it's harder to wrap the head around.  Maybe an example to help understand it would be a ball bouncing off the ground at an angle.  The only reason that ball can change directions is because it runs into the ground and the ground pushes back with a force from the impact (due to the balls inertia and the earth's inertia).  When a train goes around a curve the rails push on the train to change it's direction.  This is why those train wheels have a lip that stick down below the rail, so that the rail can push on it.  Otherwise, the train would just keep going in the same direction and fly off the tracks.  If there wasn't a force there, the engineer wouldn't have to design the thickness of that lip to keep from breaking off, nor the thickness of the railroad ties to keep the the track from breaking apart.  Using the speed and mass of the train, the engineer can calculate the centrifugal force that will be applied on those components to design them to handle the load.  If those forces didn't exist, we could make those components out of something lighter to save money.

Predicting the future is pretty much all we do with trying to understand physics.  When you understand why it's happening you can understand what will happen when the same conditions repeat in the future, because the laws are consistent. This is what we do with engineering.  We predict that if the lip of that train wheel in the example above is greater than a certain thickness, it will not break in the future when the train comes around the curve at a certain speed.  Or in skiing, if you apply a little more counter you can predict that .......

Engineer and Ghost:

"[T]here's no reason to single out centrifugal force as different from other reactionary forces that people accept.  When the centripetal force is removed the centrifugal force immediately stops as well, because it was there only in reaction to the centripetal force."

If I accept their existence, I totally get that statement.

"This is why those train wheels have a lip that stick down below the rail, so that the rail can push on it.  Otherwise, the train would just keep going in the same direction and fly off the tracks."  You mean not in the same circular direction but in a straight-line inertial direction like the 8 ball being released from the centripetal push force of the crochet hoop, right?

I think the reason that the Doc guy in the video said that centrifugal force is a fraud and does not exist is because once the hoop was removed the centrifugal force disappeared.  Which is what you are saying too I guess.  You are saying the centrifugal force is merely a reactionary force to the actionary centripetal force and, as a result, you can't have one without the other.  And once the centripetal force is gone, the centrifugal force also disappears.  I get what you are saying there if I accept there existence.

"[C]entripetal force is real, because it takes a force to change directions."  This is where, obviously, I have the problem.  Having just learned about straight-line inertia I want to hold onto that concept like a baby blanket, and because I understand it, I want to use it to explain all manner of things. I will continue to mull this over.  (And I am not an internet "Help Vampire."*)   I help people for a living elsewhere and on the snow.  I hate to frustrate you guys and gals, but in this thread it is all about me and my growth as a ski instructor.

So, here goes again, why must we call a wooden crochet hoop, a rock, a half-pipe wall, an uphill bump face, the surface of the Earth a "force" to explain what happens when an moving Mass encounters of object of greater Mass.  The moving little Mass can't deflect, or move through, the larger immobile solid Mass, so the inertial direction of the more little Mass is merely redirected (the fom corollary) to a new straight-line inertial direction.  If the larger immobile Mass is concave, the little Mass' direction will be redirected in a circular path.

But if the larger immobile Mass is convex - the direction of the little Mass will not follow the curve of the larger Mass.  Rather, the little Mass' path of travel will be shot off into a straight-line inertial direction (into space, like me in the bumps).

It seems to me that the relative disparity of the Mass of the two objects sufficiently explains the physical result we observe.

Without having to resort to stating that the lifeless immobile wooden crochet hoop, rock, half-pipe wall, uphill bump face, or the surface of the Earth have some sort of mystical "force" to explain what is to my mind (and to fom) simple redirection of Mass by an object of greater Mass.

* Man there is a great discussion/warning of internet "Help Vampires" over at the Sportsmobileforum.com explaining that they frequent forums asking stupid questions and toying with the well-meaning helpful members and eventually suck the fun out of being a member of such forums. Resulting in the death of the victim forum by causing the most helpful members to flee.

Is that what happened to Bob Barnes?

**  Engineer:  Can I assume that meant to include the bracketed term in the following quote:  "Similarly, when [Sun's] gravity is pulling on the earth,"

P.S.  I am a Christian, so clearly I am capable of "faith."  But intuitively, in my bones, heart, and soul, imbuing rocks, bumps, half-pipe walls and like objects with "force" is, at least at this point, a leap too far.

350,

It's simple. Matter is full of demons. Demons don't like to be pushed around so when you push on them they push back. As is usually the case whoever is biggest pushes the other around.

fom

Quote:
Originally Posted by justanotherskipro

In that way the snow isn't a force but can react to a force applied to it. By tipping the skis the push and push back is no longer vertical, or even parallel to the snow surface. It is sideways in the same direction as the top of the tipped skis.

jasp,

Your second sentence is pure gold. How I use the tool allows me to generate and direct the force and in conjunction with muscles and bone apply that force to the com to move me where I want to go.

fom

https://en.wikipedia.org/wiki/Maxwell%27s_demon

fom:  Now you are just trying to hurt my brain.

Here's an equation for you:

Naturalism < Hinduism < Buddhism < (Christianity + Ayn Rand's Objectivism = Love with Reality while on Earth and faith for even more After)

Quote:
Originally Posted by E350

Engineer and Ghost:

"[T]here's no reason to single out centrifugal force as different from other reactionary forces that people accept.  When the centripetal force is removed the centrifugal force immediately stops as well, because it was there only in reaction to the centripetal force."

If I accept their existence, I totally get that statement.

"This is why those train wheels have a lip that stick down below the rail, so that the rail can push on it.  Otherwise, the train would just keep going in the same direction and fly off the tracks."  You mean not in the same circular direction but in a straight-line inertial direction like the 8 ball being released from the centripetal push force of the crochet hoop, right?

I think the reason that the Doc guy in the video said that centrifugal force is a fraud and does not exist is because once the hoop was removed the centrifugal force disappeared.  Which is what you are saying too I guess.  You are saying the centrifugal force is merely a reactionary force to the actionary centripetal force and, as a result, you can't have one without the other.  And once the centripetal force is gone, the centrifugal force also disappears.  I get what you are saying there if I accept there existence.

"[C]entripetal force is real, because it takes a force to change directions."  This is where, obviously, I have the problem.  Having just learned about straight-line inertia I want to hold onto that concept like a baby blanket, and because I understand it, I want to use it to explain all manner of things. I will continue to mull this over.  (And I am not an internet "Help Vampire."*)   I help people for a living elsewhere and on the snow.  I hate to frustrate you guys and gals, but in this thread it is all about me and my growth as a ski instructor.

So, here goes again, why must we call a wooden crochet hoop, a rock, a half-pipe wall, an uphill bump face, the surface of the Earth a "force" to explain what happens when an moving Mass encounters of object of greater Mass.  The moving little Mass can't deflect, or move through, the larger immobile solid Mass, so the inertial direction of the more little Mass is merely redirected (the fom corollary) to a new straight-line inertial direction.  If the larger immobile Mass is concave, the little Mass' direction will be redirected in a circular path.

But if the larger immobile Mass is convex - the direction of the little Mass will not follow the curve of the larger Mass.  Rather, the little Mass' path of travel will be shot off into a straight-line inertial direction (into space, like me in the bumps).

It seems to me that the relative disparity of the Mass of the two objects sufficiently explains the physical result we observe.

Without having to resort to stating that the lifeless immobile wooden crochet hoop, rock, half-pipe wall, uphill bump face, or the surface of the Earth have some sort of mystical "force" to explain what is to my mind (and to fom) simple redirection of Mass by an object of greater Mass.

* Man there is a great discussion/warning of internet "Help Vampires" over at the Sportsmobileforum.com explaining that they frequent forums asking stupid questions and toying with the well-meaning helpful members and eventually suck the fun out of being a member of such forums. Resulting in the death of the victim forum by causing the most helpful members to flee.

Is that what happened to Bob Barnes?

**  Engineer:  Can I assume that meant to include the bracketed term in the following quote:  "Similarly, when [Sun's] gravity is pulling on the earth,"

P.S.  I am a Christian, so clearly I am capable of "faith."  But intuitively, in my bones, heart, and soul, imbuing rocks, bumps, half-pipe walls and like objects with "force" is, at least at this point, a leap too far.

If you are on a space station that is rotating, and you stand on the outside wall, the sensation of centrifugal force you feel will be indistinguishable from gravity on earth provided it's big enough and spins at the right speed.  You can live, work, and ski in that environment just like you do on earth and you would never know the difference.  The concept of whether that force is real or isn't real just isn't useful to us.  For any calculation that any of us will ever do, it doesn't make a difference.  If you treat it as a force just like gravity you'll calculate the right answers and come to the right conclusions about how to do things and what to build.  If you try to incorporate a philosophy that centrifugal force is any less valid than electromagnetic forces or gravity or other types you'll be hard pressed to get the right answers for the most basic calculations involving rotation.  So, it may be fun to philosophize about the origins of gravity, electromagnetism, and other forces in a ski forum,  (By the way, magnetic fields are just from rotating electric fields.  Does that make magnets not real?  Should they now fall off of our refrigerators?) but it doesn't have much relevance to classical mechanics which is all you need to understand the physics of skiing.

If living involved playing a game of basketball the differences between gravity and the space station would become evident.

fom

Quote:
Originally Posted by fatoldman

jasp,

How I use the tool allows me to generate and direct the force and in conjunction with muscles and bone apply that force to the com to move me where I want to go.

fom

Of course,  Beginners struggle because they are trying to turn the ski which is four -five feet long and it's kinda hard to do that, esp if the snow is any kind of 3D snow.  However, once you learn how to balance on the ski-skis   and do so appropriately in all three dimensions ,  the ski will provide most, if not all of the turning forces required to take you where you want to go.  YM

54 posts in 2 days about centrifugal force!!  It better snow soon.

BK

Newton's equation for motion is:

Force = time rate of change in momentum

Momentum is the product of mass and velocity.  In the instance of a skier, typically the mass is constant (unless a yardsale occurs).  Therefore, we are left with Force = mass x time rate of change in velocity.

Note that velocity (in interesting cases) is actually a time varying three dimensional quantity.  Three dimensions refers to the overall direction of the velocity as well as total magnitude.

Another way to say it is that a change in direction and/or speed of the skier will result with proper application of force.  Likewise, (inadvertent) application of force will cause a (unintended) change in velocity.  The skier in control creates the forces needed by managing the interaction of the skis with the snow - angle and pressure.  The skier generates and exploits these forces to travel in the desired direction, manage speed, and end the run in a generally upright position without significant mass reduction.

Quote:
Originally Posted by fatoldman

If living involved playing a game of basketball the differences between gravity and the space station would become evident.

fom

Not if the outside circle of the station is big enough.

Technical talk is fun while we wait for snow.

But, to get somewhere with this conversation instead of going endlessly in circles, we need a clear target.

A goal.

An outcome for the conversation.

A reason for the topic, and an idea of where it's hopefully heading.

A problem to be solved.

Why are we discussing this again?

Quote:
Originally Posted by The Engineer

Quote:
Originally Posted by fatoldman

If living involved playing a game of basketball the differences between gravity and the space station would become evident.

fom

Not if the outside circle of the station is big enough.

You're talking really big here. I mean really f'ing big.

Quote:
Originally Posted by fatoldman

You're talking really big here. I mean really f'ing big.

I think the height of the basketball hoop needs to be small compared to the radius of the station, so that the velocity of the entire court is about the same.  Maybe you could feel a 0.1% change in the trajectories, so that might mean a station radius of about 10,000 feet.  But, I don't think this issue is because of a fundamental difference in the nature of gravity vs. centrifugal force.  I think it's just that the earth is so big the radial field lines are approximately parallel.  If you were playing basketball on an extremely small dense planet the basketball experience might be more like a small rotating station except for being upside down.

With curved space time, it may be that gravity is really the illusion and centrifugal force is the only reality.  But who the *** cares, we're skiing.  The physics can be completely modeled with classical mechanics with near perfect prediction.  Crunch the numbers and be done.  Leave the philosophy for the particle physicists.

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