A ball struck by a cue in billiards with English goes straight at first....

[poolplayer]

I still can't figure out this diagram can explain that a ball goes straight...

Maybe in this diagram the ball goes to normal direction if the cue does not deflect (if cue movement can be fixed straight without bend)...

 

[Cutter Ketch]

Any force that rotates a ball isolated in space can't affect the direction it heads off in. That direction of the linear momentum change can only be radial from the contact point.

That statement is incorrect. You must satisfy BOTH Γ = I α and F = m a simultaneously. The free body will accelerate in the direction of the applied force even a tangential one.

Picture a circular puck on an air table. The puck has some thread wrapped around it. Pulling on the thread the force is purely tangential. You can't pull the thread without the puck moving toward you. The direction the puck moves is directly toward you. The line of action relative to the center of mass doesn't matter. The acceleration is in the direction of the force. F=ma

When the cue strikes the ball you apply a force in the direction the cue is pushing. There is no surprise that that is the direction the ball moves. The only question is how the tangential component is imparted, and the answer is friction. The rest of the discussion has been about interesting perturbations like when the cue bends or friction fails.

 

[sophiecentaur]

That statement is incorrect. You must satisfy BOTH Γ = I α and F = m a simultaneously. The free body will accelerate in the direction of the applied force even a tangential one.

Picture a circular puck on an air table. The puck has some thread wrapped around it. Pulling on the thread the force is purely tangential. You can't pull the thread without the puck moving toward you. The direction the puck moves is directly toward you. The line of action relative to the center of mass doesn't matter. The acceleration is in the direction of the force. F=ma

When the cue strikes the ball you apply a force in the direction the cue is pushing. There is no surprise that that is the direction the ball moves. The only question is how the tangential component is imparted, and the answer is friction. The rest of the discussion has been about interesting perturbations like when the cue bends or friction fails.

You're right. That statement of mine doesn't make a lot of sense on its own. I haven't read the context of that sentence so I'll have to let it ride, except to say that there is no tension involved in pool and the only forces that can act will be radial and tangential.
Having read that link about pool and bowling, which gives the answer to all these questions in a succinct way, I can't understand why anyone continues to argue about the way the whole thing works. People should read that site carefully and not skip the bits that they don't understand. No one plays pool on a frictionless table or with a frictionless cue tip. The thing that moves the ball to the right is the forward tilt of the ball as it rolls, combined with the spin due to the off centre cue contact.

 

[poolplayer]

The only question is how the tangential component is imparted, and the answer is friction. The rest of the discussion has been about interesting perturbations like when the cue bends or friction fails.

Yes, that should be friction. I am now totally certain after seeing your discussions and doing some experiments.

Last thing I want to add is that maybe the cue bend is the main factor that determines the ball direction. This time I set my bridge (fixing point of the cue) very close to the ball and stroke it. With the near bridge, the cue could not flex much as before. And as result, the ball went near normal direction.


My guess is that the near bridge decreased the speed of initial cue deflection/flex to the right and reduced the friction on the ball. Due to this reduction of the rightwards friction, the ball went normal from the contact surface. Although I am not quite sure how much the rebound of the cue to the left affected the cue direction, it seems that the ball already started to run oblique during the initial cue flex to the right.

This finding was a little bit surprising to me because it has been reported that cue flex does not affect physics of cue ball much. But, considering it happens during contact with the cue ball, it is reasonable that the ball follows the cue flex movement as long as the force is in the range of static friction.

 

[poolplayer]

Having read that link about pool and bowling, which gives the answer to all these questions in a succinct way, I can't understand why anyone continues to argue about the way the whole thing works. People should read that site carefully and not skip the bits that they don't understand. No one plays pool on a frictionless table or with a frictionless cue tip. The thing that moves the ball to the right is the forward tilt of the ball as it rolls, combined with the spin due to the off centre cue contact.

We all know that the spin changes the ball direction, but this happens after the collision and not during collision.

You know that a ball can go almost the same direction regardless of different friction coefficient of the table.
1) usual table:

2) smoother table:

(there is still friction but curve should be reduced than a usual table)

And you know that a ball can go oblique even if the ball is spinning.


I don't know why you still believe that spin is important for the cue ball direction shown in these video. First of all, the ball trajectory in these videos is quite much linear...
The link was interesting showing the change of the spin axis during rolling of a ball, though.

 

[sophiecentaur]

A point about cue deflection. The more flexible the cue is, the longer it will stay in contact with the ball. This means the spin around a near vertical axis will be greater because the horizontal force has been acting for longer.

 

[A.T.]

Note how the ball rotates, during and right after the impact. It's not just spinning around the vertical axis. Hence I don't think you can treat this as a simple 2D problem. For example: the contact force from the cue tip could have a downwards component, which presses the ball into the table and "tilts" to the right.

 

[poolplayer]

The more flexible the cue is, the longer it will stay in contact with the ball. This means the spin around a near vertical axis will be greater because the horizontal force has been acting for longer.

This is a good point. My understanding is that the tangential force to the right induces both rightwards drive and counterclockwise spin, so it would be hard to dissociate these variables experimentally. When I see my videos, I feel that the contact time is actually shorter with a flexible cue, but I am not sure about that...

Note how the ball rotates, during and right after the impact. It's not just spinning around the vertical axis. Hence I don't think you can treat this as a simple 2D problem. For example: the contact force from the cue tip could have a downwards component, which presses the ball into the table and "tilts" to the right.

I agree that 3D information would be necessary to explain details of the long distance ball trajectory, but I think this is not necessary to answer my question because my 2D diagram seems sufficient to explain the coarse ball direction immediately after the collision.

I noted this in youtube, but I intentionally hit slightly below the mid height on the right hand side of the cue ball in the video. I am sure that the ball rotates only around the vertical axis if I hit the mid height. Of course the vertical axis would be maintained only a short period of time after the collision when the ball is still slipping on the table. With a sufficient friction between the ball and table, the axis should soon change as shown in your link. And the ball should "tilt" to the right while the ball rolls, although this could not be observed in the short video.

 

[sophiecentaur]

because my 2D diagram seems sufficient to explain the coarse ball direction immediate after the collision.

Not at all. The whole roll and spin is important if you want to predict what happens. Without the contact with the table there is nothing to give the ball a rightwards force to counter the net leftwards force from the cue. You cannot produce a rightwards force by just waving metaphorical arms about. You need to identify an actual force on the ball which will achieve it. You have acknowledged that the cue moves to the right so the force on the ball (a reaction force from the cue) must be to the left.

I am sure that the ball rotates only around the vertical axis if I hit the mid height.

But doesn't the ball roll forward? You cannot think it just skids along.

 

[poolplayer]

Without the contact with the table there is nothing to give the ball a rightwards force to counter the net leftwards force from the cue. You cannot produce a rightwards force by just waving metaphorical arms about. You need to identify an actual force on the ball which will achieve it. You have acknowledged that the cue moves to the right so the force on the ball (a reaction force from the cue) must be to the left.

Are you sure that the friction between the ball and table is necessary for the tangential force? You keep me confused... Wait, do you think that the tangential force does not exist?

But doesn't the ball roll forward?

It will roll forward after it stops slipping on the table. The ball rotates only around the vertical axis while it is slipping after shot in the mid height with right English. I think you can see it from some of my videos...

 

[sophiecentaur]

re you sure that the friction between the ball and table is necessary for the tangential force? Y

No, the tangential force between cue and ball is there whenever contact is not exactly normal. That tangential force will spin the ball about a near vertical axis.

It will roll forward after it finish slipping.

It may take a short time to start rolling forwards at full speed but even if it does slip against the table (and there is no reason why it should slip at all at some contact heights) it will be rolling almost instantly.
Your argument about using 2D analysis implies that you could get a straight shot with a penny, hit off axis with a wooden ruler. You could even try that on a laminated table top and see if you can actually achieve. I see no reason that it could happen.

 

[poolplayer]

No, the tangential force between cue and ball is there whenever contact is not exactly normal.

Then, the friction between the table and ball is not necessary for the tangential force...

It may take a short time to start rolling forwards at full speed but even if it does slip against the table (and there is no reason why it should slip at all at some contact heights) it will be rolling almost instantly.

A ball can slip in a long distance probably up to 10 inches. This is what all pool players know.

Your argument about using 2D analysis implies that you could get a straight shot with a penny, hit off axis with a wooden ruler. You could even try that on a laminated table top and see if you can actually achieve. I see no reason that it could happen

I didn't understand this example...(penny put on the tip of a wooden ruler??) but particularly what force are you against in the diagram?

 

[poolplayer]

No, the tangential force between cue and ball is there whenever contact is not exactly normal. That tangential force will spin the ball about a near vertical axis.

Oh, maybe you think that tangential force only makes a ball spin and does not push ball to somewhere.

 

[poolplayer]

I didn't understand this example...(penny put on the tip of a wooden ruler??)

Now I know what you were asking. I think it is possible that a penny goes straight if 1) there is significant friction between the penny and the wooden ruler, 2) the wooden ruler is extremely flexible so that it can be flexed in high speed by the light weight of the penny, and 3) the penny and wooden ruler are in contact during the wooden ruler's flex.

 

[poolplayer]

I think I have got an answer in this thread, so here I summarize the conclusion.



1) It is rightwards friction between the cue and ball that makes the ball go straight (rather than oblique to the left) with right English. To my surprise, probably there is no reason that the ball must follow the cue direction with English. One of the reason that the ball goes almost straight would be that cue makers refined several cue features so that the ball follows the cue direction.

2) Speed of cue flex during contact between the cue and ball would be the most important feature that makes a ball go straight. When I use a heavy/hard cue or when a bridge (fixing point of a cue) is very near from the ball, the cue cannot flex much. If the cue cannot flex in high speed during the contact, the ball cannot receive much rightwards friction force from the cue, so the ball deflects to the left, closer to the direction normal from the contact surface. Moreover, the frequency of the cue vibration is high when shot with a heavy/hard cue or a bridge near from the ball. This fast vibration could make the cue ball goes more oblique to the left because the rebound of the cue to the left after rightwards flex can push back the ball to the left (this can be double-hit if the cue once leaves the ball after the rightwards flex).

3) Counterclockwise spin induced by right English can make the ball curve to the right after collision. Although this happens after a while after collision when the ball stops slipping on the table, the spin makes the ball walk rightwards, so helping the ball go straight if the ball deflection to the left is significant and there is a long distance to the aiming point.

Tips to pool players:
There is not much to learn here to improve your game. But, the physics of cue-to-ball collision suggests that it is really tricky to make a cue ball run to the cue direction with right/left English. So, it would be wise not to use English as much as possible (everyone knows this though...). And to minimize the cue ball deflection, it would be better to keep your bridge sufficiently distant from the ball. This comes to a problem when the cue ball is very close to the cushion. English would be the last option if you have to shoot a ball frozen to the cushion.

 

[A.T.]

I am sure that the ball rotates only around the vertical axis if I hit the mid height.

Do you have a clip of that? How does the ball move then?

 

[poolplayer]

Here you go. You can find more clips in Dr. Dave's website.
http://www.billiards.colostate.edu/high_speed_videos/new/HSVA-8.htm
I said this many times, but a ball can slip on the table because the friction between the ball and table is not high enough.

 

[sophiecentaur]

It's worth noting that the Force F_ft may be significantly large, as the diagram shows but it is not acting on the cm of the ball and its effect on final direction is tiny, compared with the component that acts directly through the cm.
Lay a ruler on the table and hit the end with an impulse, normal to the line of the ruler. It will rotate a lot more than it will move in the direction of the hit. That diagram suggests otherwise because it is tempting to consider that the resultant force is relevant. Unless the cue tip has adhesive on it, the result motion of the ball can never be in the direction of the impulse from the cue.

I said this many times, but a ball can slip on the table because the friction between the ball and table is not high enough

This is a non linear effect. For a small applied force, there will be no slip. For a large force, there may be a lot of slip and the 2D situation applies more nearly. But any forward roll will make the ball walk to the right as the spin axis is tilting forward.

 

[poolplayer]

It's worth noting that the Force F_ft may be significantly large, as the diagram shows but it is not acting on the cm of the ball and its effect on final direction is tiny, compared with the component that acts directly through the cm.
Lay a ruler on the table and hit the end with an impulse, normal to the line of the ruler. It will rotate a lot more than it will move in the direction of the hit. That diagram suggests otherwise because it is tempting to consider that the resultant force is relevant.

How did you calculate cm change by the Force F_ft to conclude it is tiny?

 

[A.T.]

It's worth noting that the Force F_ft may be significantly large, as the diagram shows but it is not acting on the cm of the ball...

Completely irrelevant. If that force is significant, then it causes significant linear acceleration of the ball in that direction.

it is tempting to consider that the resultant force is relevant.

Because it is, according to Newtons 2nd Law.

 

[sophiecentaur]

Completely irrelevant. If that force is significant, then it causes significant linear acceleration of the ball in that direction.Because it is, according to Newtons 2nd Law.

Isn't it necessary to consider the angular momentum in this situation, though? The forces involved are difficult to quantify - as with most collision problems - because they will vary throughout the ball - cue collision so it's best to think in terms of momentum and angular momentum changes and Impulses. That's ok, I think, as long as it can be assumed that angles don't change much during the actual collision. Flexing of the cue adds a complication but any flexing will only produce a significant change in the 'lateral forces' on the cue.
What I have found out is that a tangential impulse on a (free) body will produce the same change in angular momentum as linear momentum. So a sphere with all it mass at the centre and an MI of zero will not move forward at all if it's struck tangentially (simplest case). A dumbbell (MI = mr², struck 'tangentially' will move off at half the speed as if both masses were together and struck with the same impulse. A solid sphere is half way between, with an MI of 2mr²/5 so the translational momentum due to a tangential impulse will be 2/5 of a single mass of m. That implies (to me) that the effect of the tangential component of the cue's impulse will only be 2/5 of the radial component. So the diagram that's been used, showing F_t and F_r, neglects this factor of 2/5. As far as I can see, this will mean that, for a collision in free space, the ball will always move slightly (or a lot) to the left of the cue direction. It needs the steering action of the balloon the cloth to make it actually go straight.
I found this link very useful.

 

[A.T.]

The forces involved are difficult to quantify.

True, but the linear acceleration is parallel to their sum. There is no "weighting" of components depending on their direction, or whatever you are trying to suggest here.

 

[sophiecentaur]

There is no "weighting" of components depending on their direction,

I think you would need to justify that, in the light of my arguments using a dumbell and a sphere with nearly zero MI. The resulting motion of those two is very dependent on the angle of the impulse. If those arguments are valid then so is the argument for a uniform sphere.
The principle of resolving vectors is not in question, is it?

 

[A.T.]

I think you would need to justify that

See Newtons 2nd Law.

 

[sophiecentaur]

See Newtons 2nd Law.

What I'm saying doesn't contravene Newton's Laws - if you apply it correctly. Neither does it contravene Energy Conservation.

 

[A.T.]

What I'm saying doesn't contravene Newton's Laws

You were asking me to justify why linear acceleration is parallel to the sum of applied forces, which is a direct consequence of Newtons 2nd Law.

 

[sophiecentaur]

You were asking me to justify why linear acceleration is parallel to the sum of applied forces, which is a direct consequence of Newtons 2nd Law.

But this isn't just a case of 'applied forces' it is 'applied impulses' and the times involved will depend upon the MI of the object. I would agree that a string pulling the ball along for a while would produce a result other than what I am describing. Unlike with the string, once the cue has left the ball, it no longer has any effect on it. That involves another concept.
Imagine dropping a barbell, with one end held on a string. You would have a form of pendulum and the tension in the string would vary in time. That would be different from hitting one end of a barbell in space.

 

[A.T.]

But this isn't just a case of 'applied forces' it is 'applied impulses' ...

You were specifically talking about components of a force:

It's worth noting that the Force F_ft may be significantly large, as the diagram shows but it is not acting on the cm of the ball and its effect on final direction is tiny, compared with the component that acts directly through the cm.

... and the times involved will depend upon the MI of the object.

The application time is the same for all components of the same applied force.

 

[sophiecentaur]

You were specifically talking about components of a force:
The application time is the same for all components of the same applied force.

I am still having a problem with this.
Are you saying that an impulse applied radially for a brief period will have precisely the same initial effect as an impulse applied tangentially? How does that argument work for a dumbel? The time of the action is the same but the forces are different (being due to the reaction against the cue).
In addition to this basic idea, if there is any slippage between tip and ball, the tangential force will be even less less, which could also be relevant.
I can't be sure, from you remarks, whether you are just insisting on the application of Newton's laws (which is, of course right) or whether you are really appreciating the forces (impulses) involved. The dumbel model is simpler but easier to see.

 

[A.T.]

Are you saying that an impulse applied radially for a brief period will have precisely the same initial effect as an impulse applied tangentially?

The same impulse results in the same linear momentum change:
https://en.wikipedia.org/wiki/Impul...ion_in_the_case_of_an_object_of_constant_mass

 

[sophiecentaur]

The same impulse results in the same linear momentum change:
https://en.wikipedia.org/wiki/Impul...ion_in_the_case_of_an_object_of_constant_mass

Oh, I get that BUT the forces are different, aren't they? So the Impulses will be different. Show me that the forces, radial and tangential are the same for a dumbel and I will believe I have it all wrong.

 

[A.T.]

Are you saying that an impulse applied radially for a brief period will have precisely the same initial effect as an impulse applied tangentially?

So the Impulses will be different.

Then your question makes no sense, because you aren't comparing applied radially vs. applied tangentially, just different impulses.

 

[sophiecentaur]

Then your question makes no sense, because you aren't comparing applied radially vs. applied tangentially, just different impulses.

Why do you have a problem with this? The resolved radial and tangential forces can be found from the applied force vector. They are different and they are applied for the same time. Is it not reasonable that their net effect can also be calculated? Additionally, if the tangential force is beyond limiting friction, the tangential effect will be even less.
Where is the hole in that argument? We are. after all, discussing the net effect on the ball and where it will go.

 

[A.T.]

Why do you have a problem with this? The resolved radial and tangential forces can be found from the applied force vector. They are different and they are applied for the same time.

There is no problem with the above, and It's not what I criticized.

We are. after all, discussing the net effect on the ball and where it will go.

The net acceleration is parallel to the net force.

 

[sophiecentaur]

'Net produces net' is something I would have said was obvious. Whether or not I made that clear, is hardly part of the answer to the OP.
What is your opinion about the actual situation and whether this net force can be in line with the cue?
If a player is aware of a sideways force on the cue (and they tell us that the cue actually bends) then that suggests to me that the net force on the ball is to the left. Some other force is needed for the ball to end up going straight.
Rather than just pointing out that part of the above is "wrong", it could help if you could add something positive.