CP/CF release

[mandrin]

Mandrin,

Would it be proper to sat...it is a rotating frame of reference as opposed to a non-rotating frame of reference?
The laws of physics are the same regardless. However, the explanation of what is going on varies.

drewitgolf,

Why have the poor golfer imagine playing golf on some rotating platform? There is no reason to refer to rotating reference frames. Just adds to the existing confusion. In rotating reference frames one introduces fictitious forces to make Newton valid.

The centrifugal force introduced in rotating frames is indeed a fictitious force, whereas the centrifugal force in an inertial frame is a real force. A lot of confusion comes from nonchalantly mixing the two different meanings in discussions.

Whereas the real centrifugal force is exerted by the body moving in a circular path onto some other object, the fictitious centrifugal force acts on the body moving in a circular path. Hence, the two centrifugal forces don't act on the same body.

 

[drewyallop]

Mandrin,

Can we go back to basics? What is the definition of "force" in classical mechanics?

 

[ggsjpc]

Not really sure why I care about this but it led to some good reading in different places. It's interesting how the science community responds to this scenario and it appears that it is a wording issue.

Mandrin, Thanks for pointing me in the right direction.

 

[cwdlaw223]

Mandarin -

Do my hands feel the interia of the clubhead in the downswing? If not, what do I feel in my hands?

 

[drewyallop]

There was a post asking for a some simple explanation of centripetal and centrifugal forces.

Prepared some response but the post has disappeared. Anyhow will post it nevertheless.

Imagine a small point mass M attached with a spring, having neutral length L1, to a fixed center O.

When the mass starts rotating around O we know from experience that the spring stretches to a slightly longer length L2.

Therefore the mass M exerts a force on the spring, given by F= k (L2 - L1), where k = spring constant ....Fig2, blue vector.

Also the spring exerts hence a force on the mass M trying to pull it back toward the center......Fig2, red vector.

These two forces from a Newtonian action-reaction pair. In nature forces never exist as singles.

The red force is the centripetal force whereas the blue force is the centrifugal force.

Hence the mass M exerts a centrifugal force on the spring and the spring, being stretched, in turn generates the centripetal force acting on the mass M.

These two forces are equal in magnitude and acting in opposite directions, as is the case for all Newtonian action reaction force pairs.

Notice that both forces are acting along a line of action going through the center of rotation.

Frequently forgotten, action-reaction force pairs never act on the same object.

There are pseudo scientists who aggressively claim that centrifugal force does not exist.

However the extension of the spring makes its existence clearly evident and easily measurable.

Normally one considers centripetal force to be cause and centrifugal force to be effect.

That dividing line is very thin indeed. The description above makes centrifugal force rather appear to be cause.

Is this circular motion in a vertical or horizontal plane Mandrin?

 

[mandrin]

Mandarin -

Do my hands feel the interia of the clubhead in the downswing? If not, what do I feel in my hands?

cwdlaw223,

There are many things going on. You need a grip firm enough, creating friction between hands and handle, to be able to swing without losing the club somewhere in the down swing. There is a mix of external torques and inertial torques being exerted on the handle through your hands. There is also the particular style of each golfer greatly affecting the torques developed through your hands. I really don't know what you should feel during this fleeting 0.25 sec of the down swing. It is constantly varying greatly as do the forces. Inertia itself is not something you can feel, but inertia being the resistance of any object to change its state of motion or rest leads to the existence of inertial forces. The latter are important in a golf swing, primarily generating the release torque. Inertia is a very fundamental property of matter. Just try to envision a world without inertia.

 

[mandrin]

Mandrin,

Can we go back to basics? What is the definition of "force" in classical mechanics?

drewyallop,

Just googling a bit and you have it all at your finger tips.

Wikipedia:

"In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate, or which can cause a flexible object to deform. Force can also be described by intuitive concepts such as a push or pull. A force has both magnitude and direction, making it a vector quantity."

 

[mandrin]

Is this circular motion in a vertical or horizontal plane Mandrin?

drewyallop,

Don't forget the purpose....to explain radial forces in simple terms. Forget gravity, it does not play an important role in the dynamics of the swing. Ignoring gravity you can imagine the circular motion in any plane you want.

 

[SteveT]

mandrin:- In your spring-mass example:

1. How does the spring react when going from position 1 to position 2, in Fig.1? Does it remain linear?

2. In Fig. 2, when the spring is stretching due to "centrifugal" force, does "centripetal" force still exist? How can circular motion be maintained during stretching?

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

 

[drewyallop]

drewyallop,

Don't forget the purpose....to explain radial forces in simple terms. Forget gravity, it does not play an important role in the dynamics of the swing. Ignoring gravity you can imagine the circular motion in any plane you want.

Your last sentence is true Mandrin. But I don't understand how gravity does not play an important role in swing dynamics. Could you elaborate?

Thanks,

Drew

 

[ggsjpc]

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?

 

[grahler]

Is gravity taken into account? It seems like it would have to have an effect.

 

[mandrin]

mandrin:- In your spring-mass example:

1. How does the spring react when going from position 1 to position 2, in Fig.1? Does it remain linear?

2. In Fig. 2, when the spring is stretching due to "centrifugal" force, does "centripetal" force still exist? How can circular motion be maintained during stretching?

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

SteveT, I wonder a bit about these questions getting so far into surprising fine details. A genuine quest for knowledge? Wanting to show being smart? Or perhaps just wanting to keep mandrin busy ?

An important motivation for simple thought experiments is to illustrate concepts, ideas, eliminating as much detail as possible.

But let's put in more detail. The spring is taken to be massless. Also to be able to put a torque to increase the angular velocity imagine a massless rod inside the spring.

Nevertheless let's try to answer the questions.

1. How does the spring react when going from position 1 to position 2, in Fig .1?

It stretches out a bit.

Does it remain linear?

On each and every mass particle of the spring when torque is applied to increase angular velocity, a transverse acceleration/force is felt. Also during the stretching motion each and every mass particle of the spring experience some tiny transverse coriolis force. Hence if the spring is assumed to have mass and is not made rigid with some suitable rod inside it will not remain on a completely perfect straight line.

2. In Fig.2, when the spring is stretching due to "centrifugal" force, does "centripetal" force still exist?

One does not exist without the other like Siamese twins. They dearly love each other and hence never loose sight of each other.

How can circular motion be maintained during stretching?

During transition there is no circular motion.

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

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.


SteveT, I did even go as far to solve the equations and show the resultant force on the mass in Fig1. Surprisingly you did not think to ask any question about the transient motion as there is an oscillatory mass spring system being considered. Be assured I did put in some moderate damping.

​

Fig 1 shows the resultant force acting on the mass during the two stable circular trajectories and the transition phase in between.

There are various forces at work in the mass spring ensemble.

--Along the line of action of center to mass,........ gravity, spring , centripetal, and radial inertial force.
--Transverse forces........ gravity, coriolis, angular inertial force, and externally applied torque.

 

[grahler]

Seems like in golf I can utilize cp to accelerate the club but cf cannot be applied to accelerate the club.

 

[SteveT]

Thank you, mandrin .... and I try to craft my questions so that hopefully everybody can understand them. There is no agenda behind my simple questions... just like your "simple thought experiments". I would hope you take it in the same spirit and help clarify things for me and others too.

That said, I want to keep an open mind on your CP/CF analysis and take a neutral approach to your detailed explanations that most here admire even though they don't fully comprehend... myself included.

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.


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.

May I reserve the privilege of asking other questions after you enlighten me? Thanks in advance.

 

[drewyallop]

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

I don't think this is right Steve. The body must be accelerated to accommodate the change in the velocity vector. This centripetal acceleration points to C. This acceleration requires a force. The force comes from tension in the string (we assume an inertial frame). The force vector points at C. Therefore the reactive force vector points in the opposite direction (centrifugal if you must).

Now comes the question of where the force is perceived and I think this is the source of much confusion.

Your are sitting on a chair bolted to a rotating disk. How do you perceive the force? As a push on your back.
Now a pole is stuck into the disk, you stand and grab hold of the pole. How do you perceive the force? As a pull on your arms. Same force, two different perceptions. So tell me, from these two measurements in which direction is the force acting?

Determining from Mandrin's diagram if a body is being pulled or pushed is impossible and unnecessary. Newton gives you everything you need to fully describe what is happening - the direction and magnitude of the force. The question of pull vs push requires the introduction a a measuring instrument into the system, and as we saw on the rotating disk, the answer depends entirely on the location and orientation of the instrument.

 

[SteveT]

drewyallop .... I hear ya, but I don't want to say anything until mandrin responds to my questions. I don't want to muddle up the discussion with too many opinion now. Thanks, and maybe later.

 

[Cloran]

 

[mandrin]

Your last sentence is true Mandrin. But I don't understand how gravity does not play an important role in swing dynamics. Could you elaborate?

Thanks,

Drew

Drew,

Gravity does not play a significant role in the dynamics of the swing. That follows clearly when doing an analysis. It goes indeed a bit against intuition, e.g., ' Gravity Golf '.

However you can argue that it can play a subtle but important role in the transition. The road to failure or success is often a split second affair during transition. Doing nothing deliberate, letting gravity act a bit on you arms might just get the down swing going in the right direction.

Gravity plays an important role in another way. By pulling the golfer toward the center of the earth, with sufficient force it creates a stable platform from which the golfer can generate the large torques required to propel the ball a long way down the fairway.

 

[drewyallop]

Drew,

Gravity does not play a significant role in the dynamics of the swing. That follows clearly when doing an analysis. It goes indeed a bit against intuition, e.g., ' Gravity Golf '.

However you can argue that it can play a subtle but important role in the transition. The road to failure or success is often a split second affair during transition. Doing nothing deliberate, letting gravity act a bit on you arms might just get the down swing going in the right direction.

Gravity plays an important role in another way. By pulling the golfer toward the center of the earth, with sufficient force it creates a stable platform from which the golfer can generate the large torques required to propel the ball a long way down the fairway.

Thanks Mandrin. Your point on gravity providing a stable base is a good one and something I had never thought about.

As to the role of gravity in the downswing I have a question. The purpose of the downswing is to accelerate the clubhead. In this downward movement gravity is accelerating the clubhead along with the acceleration provided by the golfers muscles - torque about the coupling point, going normal etc. If I understand you correctly the gravitational acceleration (as you know 9.8 ms**2) is an insignificant portion of the total acceleration. Do you have any data on the total acceleration of the clubhead in a "typical" swing?