Tip Function

[Cornerman]

Friction is independent of surface area, but the coefficient of friction does change based on pressure and/or load.

I think the coefficient of friction is material dependent, not load dependent. The Friction itself is load dependent, but not pressure dependent. Pressure is load and area dependent, but doesn't have bearing on the Friction. It's just another parameter determined by the friction (and area).

If you don't have a shear force, you're in pure compression, and static friction would be irrelevant, wouldn't it?

Since the tip gets to ride around, I think rolling static friction is an issue. If it slides, it would be kinetic friction. But, I don't think the videos showed evidence of sliding, so I think it's static.

Shear force... I don't think that applies to the tip/ball collision. Maybe we have different ideas on shear force, but if the tip as it rolled forward over the cueball surface shoved the cueball backwards, then I suppose that shove would be the result of a shear force. But, if that were the case, right hand english would then make the cueball spin clockwise (viewed from above). But it doesn't. So, it's not shear. I'll defer to mikepage to see if that would be a valid example of shear.


Fred

 

[Mungtor]

I think the coefficient of friction is material dependent, not load dependent. The Friction itself is load dependent, but not pressure dependent. Pressure is load and area dependent, but doesn't have bearing on the Friction. It's just another parameter determined by the friction (and area).

In the race car world, it's both. Rubber is a pretty strange substance as I understand it, and that may be perfect but my brother is an automotive engineer specalizing in suspension and he says that I get the idea....

Think of this... You have a race car with a certain weight on racing slicks. For given tire pressures there is a constant amount of surface area of the tires that will be in contact with the road, pretty close to psi * surface-area = weight of car.

Now, if you want to tune your car more towards oversteer you put on a stiffer rear anti-sway bar. The overall effect of which is that under cornering more weight is transferred to the outside tire, and henceforth removed from the inside tire. The total contact area for the rear end remains the same but the car tends to oversteer now because the coefficient of friction of rubber is inversely proportional to load.

Since the tip gets to ride around, I think rolling static friction is an issue. If it slides, it would be kinetic friction. But, I don't think the videos showed evidence of sliding, so I think it's static.

The term "rolling static friction" confuses me a little. If it is static, then at any time the cue velocity and the cue ball velocity (and rotation) can be described in terms of an instantaneous center at the contact point. Back to cars, the tire is only in static friction with the road when the wheel is rolling. The velocity of the axle (speed of the car) can be shown as a rotation around that point at an given instant.

Shear force... I don't think that applies to the tip/ball collision. Maybe we have different ideas on shear force, but if the tip as it rolled forward over the cueball surface shoved the cueball backwards, then I suppose that shove would be the result of a shear force. But, if that were the case, right hand english would then make the cueball spin clockwise (viewed from above). But it doesn't. So, it's not shear. I'll defer to mikepage to see if that would be a valid example of shear.


Fred

I'm not certain on this one... I think of a shear force as a force which would cause 2 materials to slide relative to one another if something (friction, material properties) did not prevent it. It's been a long time since I took any engineering classes, so I'll wait for Mike too.

 

[jsp]

...The total contact area for the rear end remains the same but the car tends to oversteer now because the coefficient of friction of rubber is inversely proportional to load.

I think your understanding of coefficient of friction is different from the conventional definiton. In almost all physics books, the coefficient of friction (or mu) is defined as as the frictional force divided by the normal force. The normal force is the force of the load in the direction that is perpendicular to the surface. The frictional force is in the direction of the surface.

The more load you have, the greater the friction. Saying another way, as you increase the normal force, you increase the frictional force proportionally. This ratio of load over friction is constant for a given set of materials. Fred is correct. Maybe you just have the terminology confused.

 

[Cornerman]

In the race car world, it's both. Rubber is a pretty strange substance as I understand it,

....

I'm not certain on this one... I think of a shear force as a force which would cause 2 materials to slide relative to one another if something (friction, material properties) did not prevent it. It's been a long time since I took any engineering classes, so I'll wait for Mike too.

Rubber has the special feature of mechanically locking into the pavement. To slide and move, you must provide a shear force to ... shear away from the mechanical lock.

Fred