Post by dubiousgolfer on Sept 2, 2020 5:34:42 GMT -5
I have been reading his article again in the link below but am finding some of the physics confusing
www.golfsciencejournal.org/article/12640-how-amateur-golfers-deliver-energy-to-the-driver
This particular section is very peculiar (note the bolded sections):
Two key factors result in the work-energy approach softening the importance of angular kinetics relative to the impulse-momentum approach. The first is that only CA contributes to angular work, while TA (CA + MA) contributes to angular impulse. The second is that only an angular impulse can generate angular momentum, while both linear and angular work can contribute to angular kinetic energy. During the backswing, angular work performed on the club can far outpace the change in angular kinetic energy (Figures 4A and 4B). Similar, the linear work done, and change in linear kinetic energy will not be equivalent throughout the swing (Figures 4C and 4D); yet, the total work done will equal the change in energy (Figures 4E and 4F).
Consider a golf club fixed to a pivot - like a planar pendulum - about an axis through the grip end. Two forces act on the club: A contact force at the motionless pivot point and gravity at the translating center of mass. From a work-energy perspective gravity does all the work, and more specifically, all the work done by gravity is linear. Yet, that linear work manifests itself in both linear and rotational kinetic energy in the club. The force at the fixed pivot does no work on the club. From an impulse-momentum perspective, the combined linear impulses of gravity and the force at the pivot equals the change in linear momentum, while the angular impulse – due to the moment of force from the force at the pivot – equals the change in angular momentum. To be explicit, from a work-energy perspective gravity is responsible for all change in angular kinetic energy of a pendulum, while from an impulse-momentum perspective, the force at the pivot is responsible for all change in angular momentum. One benefit of explaining the mechanical sources of clubhead speed using work-energy versus impulse-momentum is that you can avoid the paradoxical thought of increasing clubhead speed by increasing the length of time of the downswing. Consider Figure 2A, participant 42 attained the highest clubhead speed with a downswing time that was shortest in duration. More time allows for more impulse, yet higher average kinetics will decrease downswing time.
Question1: Why would angular work done on a club outpace its kinetic energy (see graphs A and B) during the whole swing?
Linear 'Work' done on an object = Force x distance moved in the direction of the force = Change in energy of the object
Angular work = Torque x angular displacement = change in energy of object
The graphs below show that his statement seems correct:
Question 2: Why is there greater Linear KE than Linear Work done on the club (see graphs C and D) during the whole swing?
Possible Answer: Some of the torque forces are being used to affect the linear kinetic energy , while some of the linear forces are being used to affect the angular kinetic energy.
Figure 4
Figure 4 Work and energy components for the full swing of a representative trial from Participant 30 on left and Participant 37 on right (same trials as Figure 3). (A and B) Angular work done on the club by the golfer, angular kinetic energy in the club, work done by gravity, and shaft strain energy. (C and D) Linear work done on the club by the golfer and linear kinetic energy in the club (E and F) Total work done on the club and total kinetic and strain energy in the club.
Question 3: Why would a contact force at the pivot cause a 'Moment Of Force' and be responsible for all the change in angular momentum but still not do any work on the club?
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First some physics definitions:
Change in angular momentum is proportional to average net torque and the time interval the torque is applied.
Torque is a measure of the force that can cause an object to rotate about an axis. Just as force is what causes an object to accelerate in linear kinematics, torque is what causes an object to acquire angular acceleration.
Torque can be either static or dynamic.
A static torque is one which does not produce an angular acceleration. Someone pushing on a closed door is applying a static torque to the door because the door is not rotating about its hinges, despite the force applied. Someone pedalling a bicycle at constant speed is also applying a static torque because they are not accelerating.
The drive shaft in a racing car accelerating from the start line is carrying a dynamic torque because it must be producing an angular acceleration of the wheels given that the car is accelerating along the track.
The terminology used when describing torque can be confusing. Engineers sometimes use the term moment, or moment of force interchangeably with torque. The radius at which the force acts is sometimes called the moment arm.
The magnitude of the torque vector T for a torque produced by a given force F is:
T = F⋅rsin(θ)
where r is the length of the moment arm and θ is the angle between the force vector and the moment arm
----------------------------------
So how can a static pivot point be causing a 'Torque' on the club and be changing its angular momentum ? And if perchance it is causing the change in angular momentum why is Sasho claiming it's not doing any work ?
If I find some answers to these questions , I'll update this post but, in all honesty, this doesn't make any sense to me at all .
DG
PS. The possible answer to questions 1 and 2 , if correct , have (imho) far reaching implications . It means that these inverse dynamic graphs which split MOF, Hand Couples and Net Force , cannot be used to determine the dynamics of a real golf swing ,especially if they are all continuously affecting each other.
www.golfsciencejournal.org/article/12640-how-amateur-golfers-deliver-energy-to-the-driver
This particular section is very peculiar (note the bolded sections):
Two key factors result in the work-energy approach softening the importance of angular kinetics relative to the impulse-momentum approach. The first is that only CA contributes to angular work, while TA (CA + MA) contributes to angular impulse. The second is that only an angular impulse can generate angular momentum, while both linear and angular work can contribute to angular kinetic energy. During the backswing, angular work performed on the club can far outpace the change in angular kinetic energy (Figures 4A and 4B). Similar, the linear work done, and change in linear kinetic energy will not be equivalent throughout the swing (Figures 4C and 4D); yet, the total work done will equal the change in energy (Figures 4E and 4F).
Consider a golf club fixed to a pivot - like a planar pendulum - about an axis through the grip end. Two forces act on the club: A contact force at the motionless pivot point and gravity at the translating center of mass. From a work-energy perspective gravity does all the work, and more specifically, all the work done by gravity is linear. Yet, that linear work manifests itself in both linear and rotational kinetic energy in the club. The force at the fixed pivot does no work on the club. From an impulse-momentum perspective, the combined linear impulses of gravity and the force at the pivot equals the change in linear momentum, while the angular impulse – due to the moment of force from the force at the pivot – equals the change in angular momentum. To be explicit, from a work-energy perspective gravity is responsible for all change in angular kinetic energy of a pendulum, while from an impulse-momentum perspective, the force at the pivot is responsible for all change in angular momentum. One benefit of explaining the mechanical sources of clubhead speed using work-energy versus impulse-momentum is that you can avoid the paradoxical thought of increasing clubhead speed by increasing the length of time of the downswing. Consider Figure 2A, participant 42 attained the highest clubhead speed with a downswing time that was shortest in duration. More time allows for more impulse, yet higher average kinetics will decrease downswing time.
Question1: Why would angular work done on a club outpace its kinetic energy (see graphs A and B) during the whole swing?
Linear 'Work' done on an object = Force x distance moved in the direction of the force = Change in energy of the object
Angular work = Torque x angular displacement = change in energy of object
The graphs below show that his statement seems correct:
Question 2: Why is there greater Linear KE than Linear Work done on the club (see graphs C and D) during the whole swing?
Possible Answer: Some of the torque forces are being used to affect the linear kinetic energy , while some of the linear forces are being used to affect the angular kinetic energy.
Figure 4
Figure 4 Work and energy components for the full swing of a representative trial from Participant 30 on left and Participant 37 on right (same trials as Figure 3). (A and B) Angular work done on the club by the golfer, angular kinetic energy in the club, work done by gravity, and shaft strain energy. (C and D) Linear work done on the club by the golfer and linear kinetic energy in the club (E and F) Total work done on the club and total kinetic and strain energy in the club.
Question 3: Why would a contact force at the pivot cause a 'Moment Of Force' and be responsible for all the change in angular momentum but still not do any work on the club?
--------------------------------------
First some physics definitions:
Change in angular momentum is proportional to average net torque and the time interval the torque is applied.
Torque is a measure of the force that can cause an object to rotate about an axis. Just as force is what causes an object to accelerate in linear kinematics, torque is what causes an object to acquire angular acceleration.
Torque can be either static or dynamic.
A static torque is one which does not produce an angular acceleration. Someone pushing on a closed door is applying a static torque to the door because the door is not rotating about its hinges, despite the force applied. Someone pedalling a bicycle at constant speed is also applying a static torque because they are not accelerating.
The drive shaft in a racing car accelerating from the start line is carrying a dynamic torque because it must be producing an angular acceleration of the wheels given that the car is accelerating along the track.
The terminology used when describing torque can be confusing. Engineers sometimes use the term moment, or moment of force interchangeably with torque. The radius at which the force acts is sometimes called the moment arm.
The magnitude of the torque vector T for a torque produced by a given force F is:
T = F⋅rsin(θ)
where r is the length of the moment arm and θ is the angle between the force vector and the moment arm
----------------------------------
So how can a static pivot point be causing a 'Torque' on the club and be changing its angular momentum ? And if perchance it is causing the change in angular momentum why is Sasho claiming it's not doing any work ?
If I find some answers to these questions , I'll update this post but, in all honesty, this doesn't make any sense to me at all .
DG
PS. The possible answer to questions 1 and 2 , if correct , have (imho) far reaching implications . It means that these inverse dynamic graphs which split MOF, Hand Couples and Net Force , cannot be used to determine the dynamics of a real golf swing ,especially if they are all continuously affecting each other.