Back arm perpendicular- why?

[dr_dave]

stroke analysis video

You get the highest cue ball speed with the fastest stick speed. The acceleration at the instant of impact in not significant. This has been discussed extensively before, and you may want to look over Dr. Dave's recent articles on this for the details.

Bob,

I don't have any recent articles on this, but I did discuss it some in the stroke analysis video we shot together. Here it is:

HSV B.40 - stroke speed and acceleration analysis, with Bob Jewett​

Regards,
Dave

 

[crosseyedjoe]

Bob,

I don't have any recent articles on this, but I did discuss it some in the stroke analysis video we shot together. Here it is:

HSV B.40 - stroke speed and acceleration analysis, with Bob Jewett​

Regards,
Dave

And did I mention the most power?

 

[Patrick Johnson]

Me:
... acceleration means a change in velocity (which wouldn't happen until you step off the table), but I agree with everything else.

crosseyedjoe:
PJ, remember the third law of motion, opposite and equal reaction. You always have an acceleration component even when standing still. It's just that the forces are cancelling out which shouldn't be confused with acceleration cancelling out. That's why you will weigh less on the surface of the moon.

Yes, I'm familiar with that law, and with the opposing forces that keep us stuck to the Earth without sinking into it. But I don't think that defines acceleration (or is why we weigh less on the moon, which is simply because the moon has less mass and gravity than Earth). It has to do with force, and I think acceleration is a potential product of force that isn't always realized.

I think Fred's comment about vector may also be about force rather than acceleration, but I don't know enough about it to say that for sure.

Here's how the website Bob linked to (Indiana University Physics Department) defines acceleration:

"acceleration is defined as the ratio of change in velocity to the time interval over which the change occurs"

I think this means that acceleration is zero for things at rest, but I'm willing to accept on faith that acceleration means something even for things at rest, especially since it's a sidebar discussion in this thread. We've already learned that peak velocity, not peak acceleration, is the important part of a stroke.

pj
chgo

 

[crosseyedjoe]

Yes, I'm familiar with that law, but I don't think it has anything to do with the definition of acceleration (or why we weigh less on the moon). It has to do with force, of which acceleration is a potential product.
pj
chgo

F=ma; your mass is contant, what does change to have a lesser weight on the moon? Interestingly enough as long as there is a push or pull there is acceleration.

. . .We've already learned that peak velocity, not peak acceleration, is the important part of a stroke.

pj
chgo

No argument on that. Sorry, I'm just being testy about achieving the most powerful contact which is to not ever let go of acceleration until contact.

 

[softshot]

human muscles can control acceleration much better that they can control velocity.. from what I have read..

I don't know what that means.

pj
chgo

I concede the momentum argument..

contraction in a muscle cell is shrinking and accelerating towards it's center until its mass forces it to stop. that's how a muscle works.. peak acceleration occurs at the middle of the stroke at BDC (bottom dead center) and that point should be impact in a fundamentally correct stroke.

velocity is irrelevant.. I stroke at several different velocity's.. and I do so through controlling acceleration...the contraction of my muscles ( accelerating towards center until stopped by its own mass)

you transfer peak acceleration to your stick because now the stick has forces working on it as well.. the stick has reached the peak velocity you can impart to it... with a loose grip... and that transfer occurs at peak acceleration

BDC... the set position.. that is the point where the biology and physics meet at peak efficiency.. i.e... the right way to do it..

I don't care what the peak velocity of my cue is... I care about how much of that velocity is under my control.. and all a muscle can do is accelerate twards its own center. all I can control is my muscle.. I control acceleration.. not velocity..

 

[softshot]

theoretically if a muscle was strong enough you could create a black hole in your own bicep..... all you need to do is reach an acceleration that the mass cannot stop...


let the physics guys chew on that one for a while LOL :thumbup:

 

[ShootingArts]

two ways of stroking forward

Muscle acceleration is tricky because it is under our control if we work at it. My normal tendency is to want to get up to the speed I want to hit the cue ball quickly and basically maintain that speed. However when I work on starting my forward stroke slowly and increasing that speed much more gradually I seem to miss fewer shots. One of the things I need to practice and do practice now and then. I think it should be an ingrained part of my game but it isn't yet.

Hu

I concede the momentum argument..

contraction in a muscle cell is shrinking and accelerating towards it's center until its mass forces it to stop. that's how a muscle works.. peak acceleration occurs at the middle of the stroke at BDC (bottom dead center) and that point should be impact in a fundamentally correct stroke.

velocity is irrelevant.. I stroke at several different velocity's.. and I do so through controlling acceleration...the contraction of my muscles ( accelerating towards center until stopped by its own mass)

you transfer peak acceleration to your stick because now the stick has forces working on it as well.. the stick has reached the peak velocity you can impart to it... with a loose grip... and that transfer occurs at peak acceleration

BDC... the set position.. that is the point where the biology and physics meet at peak efficiency.. i.e... the right way to do it..

I don't care what the peak velocity of my cue is... I care about how much of that velocity is under my control.. and all a muscle can do is accelerate twards its own center. all I can control is my muscle.. I control acceleration.. not velocity..

 

[JimS]

Muscle acceleration is tricky because it is under our control if we work at it. My normal tendency is to want to get up to the speed I want to hit the cue ball quickly and basically maintain that speed. However when I work on starting my forward stroke slowly and increasing that speed much more gradually I seem to miss fewer shots. One of the things I need to practice and do practice now and then. I think it should be an ingrained part of my game but it isn't yet.

Hu

I think that's what a smooooooth stroke is about. Even the power stroke... it's longer so it can pick up speed gradually.

By the way.... back arm perpendicular to ???? to what??? The cue?? The floor?? Thanks, Jim

 

[jsp]

This one does not apply to all mechanical system: "Also by the definition of acceleration, peak velocity is achieved exactly when the acceleration is zero."

What you are suggesting mostly concerns classical pendulum mechanical system in which the acceleration acts vertically and the acceleration goes to zero once the pendulum reaches the bottom(at peak velocity) because its direction is now tangent to the direction of the acceleration.

Actually, it does apply to all real mechanical systems. The assumption of course is that the acceleration doesn't stay positive indefinitely, but eventually reaches zero at some time.

If acceleration never reaches zero, then it's obvious that the velocity is forever increasing and the peak velocity is whatever it is at the present moment.

 

[jsp]

Also by the definition of acceleration, peak velocity is achieved exactly when the acceleration is zero.

Oh, and just to clarify, the above statement is only true if acceleration doesn't go negative at any prior point. Think of velocity as the integration of the acceleration curve over time.

 

[Scott Lee]

JimS...Almost always perpendicular to the cuestick!

Scott Lee
www.poolknowledge.com

By the way.... back arm perpendicular to ???? to what??? The cue?? The floor?? Thanks, Jim

 

[ShootingArts]

the cue

I think that's what a smooooooth stroke is about. Even the power stroke... it's longer so it can pick up speed gradually.

By the way.... back arm perpendicular to ???? to what??? The cue?? The floor?? Thanks, Jim

Jim,

Back arm perpendicular to the cue in my opinion as much as possible. That isn't how I shoot but it is definitely a good starting point. Watching old video and looking at images of people who advocated a "level cue" they often had the rear of the cue elevated three or four inches when there was nothing about the table or balls that demanded it. Obviously, "level" didn't mean taken to extremes for them. when you raise the cue butt, then it would seem that to hit with the fore arm perpendicular to the cue means you will be hitting it a little bit early in relationship to the floor.

"Perfect" mechanics feel very unnatural to me, I suspect because I have a fair amount of natural crook in my elbow, my hands naturally hang in front of my legs when standing. Hitting the cue ball late compared to the theoretical perfect place from a purely mechanical standpoint feels much more natural for me. I don't recommend carrying anything to extremes but I also don't believe in cookie cutter one size fits all either.

Hu

 

[Patrick Johnson]

your mass is contant, what does change to have a lesser weight on the moon?

The Moon's mass is less than the Earth's and so it exerts less attractive force. The difference has nothing to do with opposing force.

pj
chgo

 

[JimS]

Jim,

Back arm perpendicular to the cue in my opinion as much as possible. That isn't how I shoot but it is definitely a good starting point. Watching old video and looking at images of people who advocated a "level cue" they often had the rear of the cue elevated three or four inches when there was nothing about the table or balls that demanded it. Obviously, "level" didn't mean taken to extremes for them. when you raise the cue butt, then it would seem that to hit with the fore arm perpendicular to the cue means you will be hitting it a little bit early in relationship to the floor.

"Perfect" mechanics feel very unnatural to me, I suspect because I have a fair amount of natural crook in my elbow, my hands naturally hang in front of my legs when standing. Hitting the cue ball late compared to the theoretical perfect place from a purely mechanical standpoint feels much more natural for me. I don't recommend carrying anything to extremes but I also don't believe in cookie cutter one size fits all either.

Hu

Thanks Scott and Hu... that's what I figured but needed clarification in a moment of doubt.

 

[dr_dave]

celestial mechanics

The Moon's mass is less than the Earth's and so it exerts less attractive force.

I didn't follow this whole planetary debate here, but the gravitational force between the earth and moon is exactly equal and opposite on both bodies. The force has a greater effect on the moon due to its smaller mass, but the force is the same.

Regards,
Dave

 

[Bob Jewett]

This one does not apply to all mechanical system: "Also by the definition of acceleration, peak velocity is achieved exactly when the acceleration is zero." ... Stroke is not a pendulum system.

I think you are not familiar with the standard definitions used in physics. One standard definition is that velocity of an object is the time integral of the net acceleration of the object. This applies to cue sticks and pretty much everything else in the universe. If you feel it does not, please explain why.

 

[crosseyedjoe]

The Moon's mass is less than the Earth's and so it exerts less attractive force. The difference has nothing to do with opposing force.

pj
chgo

Ah, how is that attractive force normally referred to and what does it imply?

The system YOU and EARTH, has a net zero acceleration. But when you isolate the body YOU and treat it as a separate system, YOU is accelerating by 9.8m/s^2. I was just hoping for people to use terms and the associated system consistently most specially the modifier NET.

 

[crosseyedjoe]

I concede the momentum argument..

contraction in a muscle cell is shrinking and accelerating towards it's center until its mass forces it to stop. that's how a muscle works.. peak acceleration occurs at the middle of the stroke at BDC (bottom dead center) and that point should be impact in a fundamentally correct stroke.

velocity is irrelevant.. I stroke at several different velocity's.. and I do so through controlling acceleration...the contraction of my muscles ( accelerating towards center until stopped by its own mass)

you transfer peak acceleration to your stick because now the stick has forces working on it as well.. the stick has reached the peak velocity you can impart to it... with a loose grip... and that transfer occurs at peak acceleration

BDC... the set position.. that is the point where the biology and physics meet at peak efficiency.. i.e... the right way to do it..

I don't care what the peak velocity of my cue is... I care about how much of that velocity is under my control.. and all a muscle can do is accelerate twards its own center. all I can control is my muscle.. I control acceleration.. not velocity..

Good job. A+

 

[Bob Jewett]

... velocity is irrelevant.. ...

This is true only if you don't care how fast the cue ball moves. The speed of the cue ball will be directly proportional to the speed of the stick at the moment of tip-to-ball contact.

And to the extent that you control the acceleration of the stick you control its velocity to the same extent.

 

[Patrick Johnson]

...the gravitational force between the earth and moon is exactly equal and opposite on both bodies. The force has a greater effect on the moon due to its smaller mass, but the force is the same.

You're talking about the total combined gravitational force of the two bodies. I'm talking about the fraction of the total supplied by the Moon, which is the comparison that explains why we weigh less on the Moon than on the Earth.

pj
chgo