Inertia...another factor in determining the perfect weight of a break cue?

[X Breaker]

I would like to ask the posters here if you ever felt that:

1. it is easier to go straighter on certain shots with certain cues?

The inertia of a cue is dependent on the mass and center of gravity of the cue. In layman's term, the heavier the cue, the higer is its tendency to stay, hence a higher inertia.

Thank you.

Richard

 

[Gerry]

I don't knowabout all that? I do know I break better with a light cue....18oz. I always have since Mike Sigel told me to try it years back. I think the size of a person, thier fast twitch muscles, hand eye co-ordination, and things like that more determine the quality of thier break than anything else.

Not to mention, I'm not 100% sold that a hard break is always the best option. I slow break every table I play on first, then ramp up if needed....

good luck with the study

Gerry

 

[Tokyo-dave]

I understand your theory and find it easier to put it this way: Swinging a baseball bat to hit a baseball is not a very difficult task. Once the bat is set into motion along the swing plane, it's not easily diverted from it's course. However, try to swing something like a fishing pole to hit a ball, and your accuracy deteriorates because the lightness of the pole make it more vulnurable to deviation of it's intended course.
However, the only way I can really relate this to a pool cue is to agree with the possibility that a heavier cue, once set into motion, won't be as easily 'disturbed' as a lighter cue would be. But that's not to say that a heavier cue is better. It may increase accuracy and smooth out your stroke, but as far as cue speed and juicing up the CB, a heavier cue may not be the better option.
I've always liked a little heavier cue from breaking, but not to add speed to my break as I've never been one to break them very hard. I have no facts or research data to back this, but I feel that with certain cues, you can get a 'heavier' CB at lower speeds than other cues with higher speeds. I have no idea what the physics behind this would point to, but it's kinda like how some baseball pitchers can jam a batter with a 140km fastball, while another guys 150km fastball can be driven out of the park with a relatively light swing. Once again, I know the physics don't back this up, but two baseballs of exactly the same weight can be thrown to feel heavier/lighter imho, so although it may all be in my head, I like to think the same thing is happening with my break. Could be just a mental image, but it works!
dave

 

[X Breaker]

I understand your theory and find it easier to put it this way: Swinging a baseball bat to hit a baseball is not a very difficult task. Once the bat is set into motion along the swing plane, it's not easily diverted from it's course. However, try to swing something like a fishing pole to hit a ball, and your accuracy deteriorates because the lightness of the pole make it more vulnurable to deviation of it's intended course.
However, the only way I can really relate this to a pool cue is to agree with the possibility that a heavier cue, once set into motion, won't be as easily 'disturbed' as a lighter cue would be. But that's not to say that a heavier cue is better. It may increase accuracy and smooth out your stroke, but as far as cue speed and juicing up the CB, a heavier cue may not be the better option.
I've always liked a little heavier cue from breaking, but not to add speed to my break as I've never been one to break them very hard. I have no facts or research data to back this, but I feel that with certain cues, you can get a 'heavier' CB at lower speeds than other cues with higher speeds. I have no idea what the physics behind this would point to, but it's kinda like how some baseball pitchers can jam a batter with a 140km fastball, while another guys 150km fastball can be driven out of the park with a relatively light swing. Once again, I know the physics don't back this up, but two baseballs of exactly the same weight can be thrown to feel heavier/lighter imho, so although it may all be in my head, I like to think the same thing is happening with my break. Could be just a mental image, but it works!
dave

Thank you for your respond.

I have tried some 15 oz breaking cue and although they were supposed to offer me great velocity, I find them hard to control and like you said, the cue ball felt too light.

I am not implying in my question that we should always go heavy on a breaking cue, but merely trying to ask if you feel that perhaps the inertia of the cue, other than just the kinetic forumla, which has been put forth very often when the weight of a breaking cue is discussed, is also important in determining the perfect weight of a breaking cue.

And I am not trying to have a debate here about whether lighter or heavier is better.

I am also interested in the answer to my question posted in my post, which is, do you feel that some cue can help you to shoot straighter than others?

This question is not only about breaking cue but all cues in general.

Thank you for taking the time to answer my question.

Richard

 

[Jal]

...I am also interested in the answer to my question posted in my post, which is, do you feel that some cue can help you to shoot straighter than others?

A cue which has its mass more distributed toward the rear will be more resistant to swooping. The relevent parameter is its moment of inertia (resistance to rotation) about the bridge hand.

The moment of inertia of a small mass element "m" at some distance "r" from some axis of rotation, is mr^2 (m times r times r). The moment of inertia of an extended object consisting of many such small mass elements is simply the sum of all of them, each multiplied by the square of their distances from the axis of rotation. Since the contribution of each element varies with the square of its distance from the axis, you can have a relatively light cue with a moment of inertia characteristic of a heavier cue, if its mass is distributed more toward the butt.

I did some rough calculations based on cues which have a uniform density a few years ago. For these somewhat unrealistic examples, a 60" cue would have about a 12% increase in moment of inertia over a 58" cue, while a 63" one would be 33% greater.

Jim

 

[X Breaker]

A cue which has its mass more distributed toward the rear will be more resistant to swooping. The relevent parameter is its moment of inertia (resistance to rotation) about the bridge hand.

The moment of inertia of a small mass element "m" at some distance "r" from some axis of rotation, is mr^2 (m times r times r). The moment of inertia of an extended object consisting of many such small mass elements is simply the sum of all of them, each multiplied by the square of their distances from the axis of rotation. Since the contribution of each element varies with the square of its distance from the axis, you can have a relatively light cue with a moment of inertia characteristic of a heavier cue, if its mass is distributed more toward the butt.

I did some rough calculations based on cues which have a uniform density a few years ago. For these somewhat unrealistic examples, a 60" cue would have about a 12% increase in moment of inertia over a 58" cue, while a 63" one would be 33% greater.

Jim

Jim,

Thank you for your answer.

I like your approach by looking at the moment of inertia. I would like to ask you why do you take moment around the bridge hand rather than other reference points such as the grip?

I personally feel that a more front heavy cue will give me a more stable cue delivery especially on softer shots. I am not sure if moment of inertia has anything to do with it, after all, it may just be me.

Richard

 

[Jal]

... I would like to ask you why do you take moment around the bridge hand rather than other reference points such as the grip?

... I would like to ask you why do you take moment around the bridge hand rather than other reference points such as the grip?

I personally feel that a more front heavy cue will give me a more stable cue delivery especially on softer shots. I am not sure if moment of inertia has anything to do with it, after all, it may just be me....

Hi Richard,

I think the moment of inertia about the bridge is the important factor because the bridge "v" pretty much fixes the cue at that point. Unless the cue is partially lifted up out the bridge, it is constrained to rotate (butt moving sideways) about a vertical axis passing through that point.

Although there are good reasons for prefering a more front heavy cue, I don't see it offering more resistance to sideways movement, but in fact less. This is because of how the moment of inertia about the bridge is calculated. The farther the bulk of the mass is away from the "v", the greater the resistance.

If you were playing one handed, in this case it would be to your advantage to have the bulk of the mass as far from your grip hand as possible, such as far forward - at least as far as directional stability goes. But with normal two handed stroking, it's not clear how a forward distribution helps except to keep the shaft snugly down in the bridge groove. I won't deny that the latter is also an important consideration and makes a heavily rear weighted cue feel "awkward" or unstable. Maybe this is what you're refering to and I've been off on a tangent. (Wouldn't be the first time.)

Jim

 

[Ronoh]

The inertia of a cue is dependent on the mass and center of gravity of the cue. In layman's term, the heavier the cue, the higer is its tendency to stay, hence a higher inertia.

Thank you.

Richard

http://www.yourdictionary.com/ahd/i/i0121200.html

Warning!!! Warning!!! Will Robinson!!! Warning!!!

 

[Ronoh]

I'm sorry.

 

[Andrew Manning]

In a playing cue, I find it difficult to achieve good speed control with a lighter cue, since its lighter mass means it responds more drastically to inconsistencies in muscle input. If my bicep doesn't put forth a totally uniform force during my stroke (and I know it doesn't), the cue acceleration will be smoother with a heavier cue, because of its mass (inertia), and thus the cue speed will be more predictable at the moment of contact. The lighter cue requires finer muscle control to achieve a predictable cue speed.

I don't know whether a crooked stroke is really made straighter by a cue with more rotational inertia (a longer cue, or a cue with more of its mass distributed away from your bridge). The physics are there; it is definitely more difficult to rotate a cue off line if it has more rotational inertia, but I'm not sure how much this affects the real path of a real imperfect stroke. I think if your arm is not stroking in line, the cue is not going to have enough rotational inertia to correct that, whether it's on the heavy/long/rear-balanced end of the spectrum or not.

-Andrew

 

[metal5d]

I don't think that there is going to be a significant difference in the inertia of various cues. Also the MOI changes between different cues is going to be insignificant. People should focus more on there technique than worrying if a slight redistribution of mass in a cue will help the cue go straighter.

 

[X Breaker]

... I would like to ask you why do you take moment around the bridge hand rather than other reference points such as the grip?
Hi Richard,

I think the moment of inertia about the bridge is the important factor because the bridge "v" pretty much fixes the cue at that point. Unless the cue is partially lifted up out the bridge, it is constrained to rotate (butt moving sideways) about a vertical axis passing through that point.

Although there are good reasons for prefering a more front heavy cue, I don't see it offering more resistance to sideways movement, but in fact less. This is because of how the moment of inertia about the bridge is calculated. The farther the bulk of the mass is away from the "v", the greater the resistance.

If you were playing one handed, in this case it would be to your advantage to have the bulk of the mass as far from your grip hand as possible, such as far forward - at least as far as directional stability goes. But with normal two handed stroking, it's not clear how a forward distribution helps except to keep the shaft snugly down in the bridge groove. I won't deny that the latter is also an important consideration and makes a heavily rear weighted cue feel "awkward" or unstable. Maybe this is what you're refering to and I've been off on a tangent. (Wouldn't be the first time.)

Jim

Thank you. I am not sure about the physic behind this yet as we are still trying to understand it. One thing we notice is that the cue is in motion during the break, the cue is pulled back all the way with the tip touching the bridge hand, and then released forward.

I was under the impression that the moment of inertia of an object is calculated by taking moment about the center of gravity of the object, but I will double check.

When I said I feel that certain cue allows me to feel easier to shot straighter, I think I am talking about the balance. I am talking about the shooting aspect, not just the break here. I feel that a light shaft and a butt heavy butt makes it much harder for me to shot straight especially with soft shot (e.g. to shot a ball into the side pocket on a steep angle with the cue ball on the cushion) while a heavy shaft with a less heavy butt feels easier with this kind of shots. Is it just me? Does that make any sense?

With some cues with a front balacne, it feels like I can just release the cue and it is going straight; while with a cue with a light shaft, I feel like I have to really steer the cue. Does that make any sense?

I am not sure about the implication of this, it may have a lot to do with my stroke or what I like in a cue. I use an open bridge almost all the time, for whatever it is worth.

I think the inertia is important in the selection of a breaking cue, but I actually was not referring to the moment of inertia. I need to do some reading, really think about this, and discuss this with my partners(one of my partners has a background in mechanical engineering).

In the mean time, any feedback and input is greatly appreciated.

Richard

 

[Jal]

At this point you're looking for some input from someone other than me as far as this aspect of the physics is concerned, which is prudent, but just a couple of more points.

...One thing we notice is that the cue is in motion during the break, the cue is pulled back all the way with the tip touching the bridge hand, and then released forward.

Yes, the moment of inertia does constantly change because of this. When I calculated it for the simplified cues (uniform density), I didn't try to take this into account because of the crude approximation to an actual cue to begin with. I was just looking to compare cues of various lengths. Given that all cues will undergo this change as you stroke, as far as the comparison goes this doesn't make that much difference.

.

...I was under the impression that the moment of inertia of an object is calculated by taking moment about the center of gravity of the object, but I will double check.

By all means look it up yourself and/or get some expert opinions, but the moment of inertia is calculated with respect to the axis the object rotates around. Typically this is one going through the center of mass, since a free standing object will rotate about such an axis. But if constrained to rotate about another one, then that axis is the relevent one. You'll probably come across the Parallel Axis theorem in your research. This tells you how to compute the momemt of inertia about an axis other than an axis of symmetry.

.

...When I said I feel that certain cue allows me to feel easier to shot straighter, I think I am talking about the balance. I am talking about the shooting aspect, not just the break here. I feel that a light shaft and a butt heavy butt makes it much harder for me to shot straight especially with soft shot (e.g. to shot a ball into the side pocket on a steep angle with the cue ball on the cushion) while a heavy shaft with a less heavy butt feels easier with this kind of shots. Is it just me? Does that make any sense?

I don't think it's just you. Some people prefer a more forward weighted cue from what I've gathered on the forums. Since you use an open bridge, my guess is that it simply keeps the shaft down in the bridge vee better. When bridging on a rail, I tend to push the shaft down onto the bridge, but am trying to ween myself away from this.

.

...I think the inertia is important in the selection of a breaking cue, but I actually was not referring to the moment of inertia.

I'm sorry if I steered your original question in the wrong direction. Here are some facts which bear on it but I fear still do not answer it.

- Given two cues of the same mass, the one which is more rear heavy will have more directional stability.

- A heavier cue will require less average force applied to it during the stroke in order to produce the same cueball speed as with a lighter cue.

- A heavier cue will require a greater impulse (average force x time of stroke) to produce the same cueball speed.

Comparing a 21 oz cue to an 18 oz one, the 21 oz cue will require 7% less average force throughout the stroke, but a 12% greater impulse. The larger impulse comes from the fact that it takes longer for the heavier cue to reach the cueball.

Which is likely to cause more cue rotation is a physiological/neurological question which I certainly can't answer. It may be a very individual thing. My experience is that more force equals more disaster, but this is amongst different shots with the same cue. :]

Jim

 

[bruin70]

having balance forward creates its own "inertia",,,and you don't need added weight to do it.

when the balance point is forward, you're not dragging the cue......you push the cue forward and it propels itself.

also thinner cues seem to have their own "inertia" as well. i believe this is because the thinner cue with the same weight feels heavier by proportion, than a fatter cue.

 

[Rod]

- Given two cues of the same mass, the one which is more rear heavy will have more directional stability.

It may, I know I prefer more rear weight on a break cue but more forward on my playing cue.

Comparing a 21 oz cue to an 18 oz one, the 21 oz cue will require 7% less average force throughout the stroke, but a 12% greater impulse. The larger impulse comes from the fact that it takes longer for the heavier cue to reach the cueball.

This greater impulse as you call it is exactly why I have a problem breaking with a heavier cue. Because I'm not use to the weight and I tend to force the cue.

Which is likely to cause more cue rotation is a physiological/neurological question which I certainly can't answer. It may be a very individual thing. My experience is that more force equals more disaster, but this is amongst different shots with the same cue. :]

Call it what you like but I know a heavier cue will not be as accurate for me as my standard weight 19oz. Or a slightly lighter break cue 18.5 to 18.75 oz with more weight towards the rear. I don't think there is a one fits all and I doubt very many can define what they really like.


Jim

Thats my input on the matter.

Rod

 

[mikepage]

I would like to ask the posters here if you ever felt that:

1. it is easier to go straighter on certain shots with certain cues?

The inertia of a cue is dependent on the mass and center of gravity of the cue. In layman's term, the heavier the cue, the higer is its tendency to stay, hence a higher inertia.

Thank you.

Richard

Here is a post I made to RSB --gasp-- eight years ago....
-mp

***********

The distribution of weight in a cue was discussed some in another thread
titled forward weighted cues, but it's a different issue from location of
the balance point, so I'm starting a new thread.

So let's say you're building a 20 oz 60" cue and you've decided you want
the balance point 20" from the rear. *The issue now is how to distribute
the mass. *Forget joint-type, densities of materials, diameter, etc.
Let's suppose for the sake of argument you built the cue out of materials
with zero density, and that all 20 oz comes from lead you insert at
various locations. *Where, consistent with the balance point, do you put
the lead? *One possibility (a) is to put it all at the balance point.
Another (b) is to try to distribute it evenly. *Another (c) *is to put
some at the balance point, some at the back , and some at the front. And
still another possibility (d) is to put none at the balance point, and
split the mass between the back and the front.

What are the considerations? *Why would one distribution be better than
another? *The *only* two considerations *I* can think of are *(1) squirt
and (2) feedback to the rear hand. * Actually by "feedback," I mean moment
of inertia about the bridge hand, which translates to resistance to moving
off course, and more is better. *I think this is what Ron Shepard calls
feedback to the rear hand, so that's what I'm calling it. *Now Harry
Houdini's wife articulated clearly the importance to squirt of mass near
the tip, so we don't want mass at the tip. *Let's suppose we can safely
put as much mass as we like 10 inches from the tip, and let's further
suppose that 10 inches is our typical bridge length.

So the squirt consideration just says don't put mass more forward than the
10 inch mark. *The feedback consideration suggests we should maximize the
moment of inertia about the bridge hand.

(a) With all the mass at the balance point, the moment of inertia about
the bridge hand is 18,000 oz*in**2 *[I = m* r**2 = 20 oz * (30 in)**2 ].

(b) With 10 oz at the balance point and 5 oz each twenty inches forward
and back, the moment of inertia is 22,000 oz*in**2, somewhat larger than
the first case.

(c) distributing the mass evenly 0.5 oz per inch for 20 in either side of
the balance point gives a moment of inertia of 20,666 oz*in**2.

(d) But case (d) is the one that maximizes the moment of inertia. *Put 12
oz at the butt cap and 8 oz right at the bridge hand. *Then the moment of
inertia is 30,000 oz*in**2!

Now a sideways stroke imperfection (sideways force) that would produce a
1.0 degree deviation in the alignment of the stick at contact for case (c)
would produce the following deviations for the various weight
distributions.

case * * * *deviation
____ * * * *____________
(a) * * * * 1.15 degrees
(b) * * * * 0.94 degrees
(c) * * * * 1.00 degrees
(d) * * * * 0.69 degrees

so (d) is the most stable

These come from T =I*a

T= torque about the bridge hand, which for a given grip position is
proportional to the force

I = moment of inertia

a = angular acceleration (the alignment screwup is proportional to this)

 

[Bob Jewett]

... I did some rough calculations based on cues which have a uniform density a few years ago. For these somewhat unrealistic examples, a 60" cue would have about a 12% increase in moment of inertia over a 58" cue, while a 63" one would be 33% greater....

Did this include the mass of the hand/forearm/upper arm? I think the % increase is much smaller if those are included, but the model is not obvious.

 

[Jal]

Did this include the mass of the hand/forearm/upper arm? I think the % increase is much smaller if those are included, but the model is not obvious.

As you suspect Bob, I didn't, in this or the force/impulse calculation above. And as you've noted, the differences are considerably smaller if you include them. So those numbers are misleading. It would be nice if there were some way to get the hand/arm mass and moment of inertia for a particular player, short of surgery.

Jim

 

[X Breaker]

I have a question:

I have problem understanding why the bridge hand is being considered as the axis of rotation. I think the cue is not rotated about the bridge.

The shoulder and the wrist have ball and socket joints, which cause the cue to rotate.

If we assume the body of the shooter stays still without any rotation, and assume that the wrist of the shooter is locked, the breaker is rotating the cue because of a rotation of his shoulder.

In this case, shouldn't we be taking moment about the shoulder as being the center/axis of rotation?

Thank you.

Richard

 

[metal5d]

I have a question:

I have problem understanding why the bridge hand is being considered as the axis of rotation. I think the cue is not rotated about the bridge.

The shoulder and the wrist have ball and socket joints, which cause the cue to rotate.

If we assume the body of the shooter stays still without any rotation, and assume that the wrist of the shooter is locked, the breaker is rotating the cue because of a rotation of his shoulder.

In this case, shouldn't we be taking moment about the shoulder as being the center/axis of rotation?

Thank you.

Richard

When you calculate MOI's you calculate them about an axis of rotation. The shoulder might cause the rotation but the cue is fixed at the point of the bridge and is roatating around this point. This site has some good info on physics:

http://hyperphysics.phy-astr.gsu.edu/HBASE/mi.html