CP/CF release

[mandrin]

Mandrin,

Just had a question come to mind.

In your example, would the spring be stretched only in a straight linear way out from the center or would it have a curved appearance to it? If not curved why not?

ggsjpc,


For stationary circular motion, and independent of angular speed, the spring will remain perfectly straight. However someone might than interject that wind resistance starts playing a role. It just depends how deep you want to dig into any subject.

However, the spring, if not supported inside by an appropriate rigid rod, and being subjected to varying transverse forces along its length, due to a sudden application of torque, will tend to deviate form a its straight line form.

Therefore, when only radial forces are present the spring remains straight. When transverse forces are present the spring might start to deviate from its straight line appearance.

 

[art]

In golf one frequently looks for ways to convey concepts with images or expressions. “CP/CF release” is a part of this lot.

CP/CF release implies a centrifugal or a centripetal force acting on the club through the release phase of the club through the bottom part of the swing. In either swinging or hitting this is not the case. The only radial force playing a significant role in the release is the centripetal force acting on the hands/arms. It is much smaller than the radial forces associated with the club yet produces the most significant release torque.

Old timer pro Roberto Vincenzo felt that he hit the ball with his stomach. If that were the case than golfers would have all very sore stomachs for sure. Yet quit likely some immediately feel what is being conveyed and they are most likely martial arts adepts. So golf terms are not necessarily describing an objective truth but often try to concoct an image, a feeling or both with which a golfer can identify.

Mandrin,

My background and career has been in science, and I very much appreciate the informative posts you always provide.

My current effort is to better understand the summation of the CP/CF forces of the major rotary actions of a typical golf swing, ie. the pelvis/lower body, the torso/shoulder complexes, the arms and of course the club. I anticipate the 3D summation of these collective forces having a significant impact on dynamic balance, and showing up in the vertical and horizontal measurements of ground reaction forces. Comments please, and can your model(s) depict these vectors as a function of time during the golf swing ??

 

[mandrin]

For starters, can you clarify these examples:

Me:- How can circular motion be maintained during stretching?

You:- During transition there is no circular motion.

So, would I be right if I assumed the spring is released and instantaneously stretches, "pops out', to it's new radius length and then rotation continues because an equilibrium has been reached? I assume the "transition" is from the shorter to longer radius.

SteveT,

A circle is the locus of all points equidistant from a central point. Thus a circle has a constant radius. Hence, by definition, when the radius varies, there is no circle. Conclusion: during transition there is no circular motion.

​

Fig1 shows the radius during the transition phase between the two stationary situations. Very smooth transition indeed. No instantaneous stretches or pop outs.

Me:- 3. If the spring stretches, does that mean centripetal and centrifugal forces are not equal?

You:- It seems from this question and the other questions that you make the frequent mistake considering centripetal and centrifugal force to act on the same body. They never do.

We know centripetal force acts on the rotating mass, but what other body does the centrifugal force act?

When we have equal and opposite forces acting on a body we have equilibrium, no motion.

If: "One does not exist without the other like Siamese twins. They dearly love each other and hence never loose sight of each other." ... and CF and CP are equal and opposite, how can circular motion be maintained?

Perhaps a simple free-body force diagram would help me better understand your concept of these force pairs in a rotating frame.

See Fig2 in post #2

 

[SteveT]

SteveT,

A circle is the locus of all points equidistant from a central point. Thus a circle has a constant radius. Hence, by definition, when the radius varies, there is no circle. Conclusion: during transition there is no circular motion.

​

Fig1 shows the radius during the transition phase between the two stationary situations. Very smooth transition indeed. No instantaneous stretches or pop outs.

See Fig2 in post #2

Here's Fig.2 again:

In the second position, you show a blue centrifugal force vector acting on the mass after the spring is fully sprung. How would you draw the centrifugal force vector from the original position to it's final spring position, is the centrifugal force constant or variable as the spring stretches?

In the third position, you show a red centripetal force acting on the mass, but where is the centrifugal force vector? If you say these forces are concomitant, I would like to see how you place them together in your figures.

What I'm asking of you is to show both the centrifugal and centripetal forces acting simultaneously and on which masses they act... in a simple and complete free body diagram of the entire rotating system.

As for "no circular motion", that's obvious... but there is curvilinear motion occurring during 'transition'. Where are the centrifugal and centripetal forces acting on the system during transition.. their direction and relative magnitude?

I don't want to belabor the point, and I hope my questions merit your attention. Thank you in advance for accommodating my inquiries and correcting me where necessary.

 

[grahler]

Mandrin- I am confused. Centripetal force one can apply by pulling on the handle. This can accelerate the club. If centrifugal force is real how does one apply it to accelerate the club? I want to use all the force I can in my swing.

 

[leon]

For an object having some mass and rotating about a fixed centre, centripetal force is the force that must act on the mass, directed radially inwards (i.e. through the fixed centre) to ensure it follows a circular path.
Centrifugal force is the opposite of this. It is the force that is experienced by whatever is applying the centripetal force, whether it is the spring, clubshaft, your arms, whatever. The key point, as Mandarin stated, is that it doesn't act on the rotating mass. It is also directed through the centre, but radially outwards, i.e. opposite, and if the system is in balance, or equilibrium, i.e. constant rotational speed (or if you like no change in spring length) then centripetal and centrifugal forces are equal.

If you understand this, you might then realise that, assuming you are at the fixed centre, you CANNOT apply centrifugal forces to a rotating object - it applies forces to you!

 

[mandrin]

For an object having some mass and rotating about a fixed centre, centripetal force is the force that must act on the mass, directed radially inwards (i.e. through the fixed centre) to ensure it follows a circular path.
Centrifugal force is the opposite of this. It is the force that is experienced by whatever is applying the centripetal force, whether it is the spring, clubshaft, your arms, whatever. The key point, as Mandarin stated, is that it doesn't act on the rotating mass. It is also directed through the centre, but radially outwards, i.e. opposite, and if the system is in balance, or equilibrium, i.e. constant rotational speed (or if you like no change in spring length) then centripetal and centrifugal forces are equal.

If you understand this, you might then realise that, assuming you are at the fixed centre, you CANNOT apply centrifugal forces to a rotating object - it applies forces to you!

leon,

Very nicely put indeed ! Just be careful with stating that the two radial forces are equal for stationary conditions. This suggests an unbalanced force for non stationary conditions, and likely suggesting also the two forces to act on the same body. As you mentioned in your post they do not. Action reaction force pairs are always equal and opposite to each other.

 

[grahler]

Leon the question IMO is one of inertia. Cp overcomes the inertia of the mass. Cf can never be shown to accelerate a mass in any direction since f equals ma and with cf a equals zero cf is not a force by definition. At least that's my take on it. The inertia of the mass being rotated is what contfuses things IMO.

 

[leon]

Leon the question IMO is one of inertia. Cp overcomes the inertia of the mass. Cf can never be shown to accelerate a mass in any direction since f equals ma and with cf a equals zero cf is not a force by definition. At least that's my take on it. The inertia of the mass being rotated is what contfuses things IMO.

Grahler, I'm not sure I understand what you're saying here, sorry.

Personally, and as I said in another threada while ago, I think the use of both centripetal and centrifugal, whilst attractive in their convenience, is unneccesary and ultimately leads to confusion. Especially when you consider that force on the club is only truly radial at one point in time at or near impact then the argument is even less convincing. I'd much rather think of more general 3D forces in a global cartesian system. It avoids much confusion, at least for me

 

[leon]

leon,

Very nicely put indeed !

Ta very much.

Just be careful with stating that the two radial forces are equal for stationary conditions. This suggests an unbalanced force for non stationary conditions

Fair point. You are saying that the cp and cf forces must, by their definition, always be in balance, right? Even though the system as a whole must be out of 'equilibrium' accelerate.

 

[grahler]

Leon no worries just showing cf is ficticious.
Ultimately peoples scientific perspective on this really does not affect peoples golf games. TY to Mandrin for putting up a stimulating example of how cf can seem real.

 

[ZAP]

The tough part for a dummy like me is figuring out if and how knowing any of this helps my golf game. I am not trying to be dismissive at all of the discussion. It is very interesting and some of the posters above did an amazing job of explaining forces and their direction.
I have to thank the ones who put together the diagrams and graphs and stuff. Pictures help me absorb better. I guess we need to wait a while for Project 1.68 to see where all this goes.

 

[SteveT]

Leon the question IMO is one of inertia. Cp overcomes the inertia of the mass. Cf can never be shown to accelerate a mass in any direction since f equals ma and with cf a equals zero cf is not a force by definition. At least that's my take on it. The inertia of the mass being rotated is what contfuses things IMO.

So, what do you think you 'feel'... centrifugal or centripetal force, and what do you feel you generate and have control over in your golfswing??

 

[grahler]

Steve,
As golfers we all know...feel ain't always real bro-you can take that to the bank.

 

[SteveT]

grahler.... but for most, golf is like sex... it's all 'feeel' .... so what do you feeel in your golfswing Cp or Cf ... tell us the truth...

 

[bluebird]

Not grahler but, Steve what are you going to do when it is science that insists that you must feel more cp force in the left hand than the balanced offsetting cf and cp in the coupling point?


John

 

[SteveT]

Not grahler but, Steve what are you going to do when it is science that insists that you must feel more cp force in the left hand than the balanced offsetting cf and cp in the coupling point? John

In a sense, that's what I'm asking mandrin too. Where and when do Cf and Cp forces occur within the golfswing?

 

[drewyallop]

In a sense, that's what I'm asking mandrin too. Where and when do Cf and Cp forces occur within the golfswing?

Is it more sensible to talk only of the Cp force. The Cf force occurs only in reaction to the Cp force. Thus if you know where the Cp force occurs you know where the Cf force occurs. Cf cannot stand alone or preceding or following a Cp force.

 

[SteveT]

drewyallop... you're an engineer, I believe, so I ask you, is Cf a force or just a feeling that is not real? We know that Cp is an 'applied' force, but what about Cf? It's not enough to label Cf a 'reactive' force without showing it in a FBD of the system.

I'm waiting for mandrin to respond to my last comments, and hopefully he will finally clarify everything... I hope.

 

[drewyallop]

drewyallop... you're an engineer, I believe, so I ask you, is Cf a force or just a feeling that is not real? We know that Cp is an 'applied' force, but what about Cf? It's not enough to label Cf a 'reactive' force without showing it in a FBD of the system.

I'm waiting for mandrin to respond to my last comments, and hopefully he will finally clarify everything... I hope.

Not an engineer Steve but thanks.

I think we have to agree on initial conditions. This how I see it. There is a mass in uniform circular motion in an inertial frame. The force that put it in motion plays no role in subsequent analysis of forces. To maintain the mass in circular motion requires a massless string connected to the mass at one end at C at the other end. Because the displacement of the velocity vector is constantly changing an acceleration is required i.e. angular acceleration. This is non-negotiable. Because there is acceleration a force is required. That force is centripetal force, its magnitude dependent on r and its vector pointing from the mass to C. The 3rd law demands that there be another force equal in magnitude to that force, the vector pointing in a direction opposite to the centripetal force. This is non-negotiable.

Are any of these forces "real". Remember the big science fiction spacecraft constructed as a hollow, circular tube rotating in uniform circular motion at a speed that produces an angular acceleration of 9.8 m/s**2. You are standing on the outer wall of the tube. I ask you if you can feel your body pressing down on the "floor". You confirm this. I ask you if you can feel the floor pressing up on your body. You confirm this. Two equal and opposite forces that you are "measuring" with your senses. Both real and both occurring in a reference frame that is rotating in uniform circular motion (and of course equivalent to simply standing on the earth).

This is a good thought experiment because it doesn't involve massless strings, springs or force measuring devices but I believe it is equivalent to the body rotating on the string thought experiment.