I am with Doak in that skating force originates due to friction between the stylus tip and the groove. That friction force is a constant with respect to velocity (but not constant with respect to much else). Nevertheless, the only difference between a 45 and a 33 would be groove modulation. Now I think of it, 45s may be cut differently; the music has more "room" so the groove walls may be less tortuous. Ergo, in certain cases and for certain musical moments, I could imagine there could be less skating force generated by 45s than by 33s, but only for that reason. For example, this might apply if you listen to exactly the same musical piece on a 45 vs on a 33. You found that an increase in anti-skate cured your problem, which would not be consistent with my ruminations.
All of that aside, and assuming the 45 is made from the same vinyl as a comparable 33 (so that friction force is comparable), then skating force should be about the same. |
Omsed, If you really want to get pedantic, there is no such thing as "centrifugal force". The force that keeps a body in orbit is "centripetal", or "center-seeking", meaning it has a vector directed radially toward the center (or spindle, in this case). Such a force is necessary to keep a body in orbit, because of Newton's First Law that states that a body in motion will remain in (linear) motion, unless acted upon by a force. The reason we intuit a centrifugal force, I guess, is because of Newton's Third Law. Otherwise, we agree. |
Ralph, Re the original question, friction (between the stylus tip and the groove) is the prime source of the skating force, and friction is independent of velocity. It's as simple as that.
The only difference between a 33 and a 45 might be if the two LPs were made of different vinyl compounds that gave different coefficients of friction. There might be an additional very small contribution related to groove tortuosity, as I wrote before Omsed.
Yes, skating force is not constant across the surface of the LP, but not because of the platter speed. Inconstancy has more to do with headshell offset angle and the arc traversed by the stylus as it moves from outer to inner. |
Omie, What do you research and what do you design? Inquiring minds want to know. |
Dear Omsed,
The question was around the two terms "centrifugal" and "centripetal". I was pointing out that in Newtonian terms there is no such thing as centrifugal force, that the force holding a body in circular orbit around an origin is centripetal (center-seeking, not center-fleeing). I cannot recall my exact words, nor do I wish to search for them, but I had no intention of implying that there is centripetal force involved in skating, if that's what you mean here.
I still would like a clue about your credentials in the audio/engineering business, just because you sound like an interesting and experienced person from whom we can learn. |
Oh yes, it was you to whom I was responding, Omsed. It was you who used the term "centrifugal". So what's your point? I did find my response, and I don't see where I ever said anything that could be construed as attributing skating to a centripetal force. Indeed, as noted above, I was just discussing the physics of a body in orbit. |
Omsed, Point taken. Sorry.
John, The tonearm is stationary, resisting the force of friction on the stylus tip which would tend to want to pull the tonearm clockwise in the direction of LP rotation. But I have privately wondered about a tiny centripetal force exerted by the LP on the stylus tip, as the grooves spiral inward. I think it's overcome or negated by the skating force. |
There is no such animal as centrifugal force. It is fictional. Any object in motion will tend to remain in motion in a line determined by the vector of the force that set it in motion. This is Newton's First Law, and the following is from Wikipedia:
"This is known as uniform motion. An object continues to do whatever it happens to be doing unless a force is exerted upon it. If it is at rest, it continues in a state of rest. If an object is moving, it continues to move without turning or changing its speed. This is evident in space probes that continually move in outer space. Changes in motion must be imposed against the tendency of an object to retain its state of motion. In the absence of net forces, a moving object tends to move along a straight line path indefinitely." (Let me add here that this is true in the absence of gravity and friction.)
To keep an object in circular orbit, one must apply a force directed inward toward the center of the orbit. This force vector is centri-petal, "center-seeking". Think of a sling shot. Before release, the stone travels in a circle because the sling allows you to apply a centripetal force. The moment you let go of the sling, the stone flies off in a line tangential to the radius of its orbit, because that's the direction of the force vector that motivates it. If there really were any such thing as "centrifugal" force (center-fleeing), then the stone would fly away in a line parallel to the radius of the orbit. That does not happen. If David had not understood this, Philistines would rule. He would have missed Goliath by a mile. |
Ralph, I have spent a lifetime stamping out "centrifugal force", ever since my freshman physics professor hammered it into me about the First Law. I can't stop now. Centripetal force needs my help and support. |
We said way back up the thread that groove modulations might in effect alter the coefficient of friction. The only question is and was the magnitude of that effect. I think now it is significant; who could argue with Dr. Alexandrovitch?
But did he really find that "friction remained constant with groove speed"? That speaks to the original question, whether there would be higher skating force at 45 vs 33, and is contrary to our previous conclusion. I can see that the coefficient of friction could go up with decreasing radius, because groove modulations become more tortuous. Have you got an actual copy of this 1961 publication? |
John, Yes, I just misunderstood the language of your first post on Alexandrovitch. I later figured out what you meant, and there is no disagreement with our earlier conclusion.
I hate to discuss the origin of the skating force, but I love it too. Such is life. My belief is that the prime cause of the skating force is friction of the stylus in the groove coupled with the fact that the cantilever is never tangent to the groove, because of "stylus overhang". "Headshell offset angle" changes the directions of the force vector that results from this. But the net effect is a force that pulls the stylus tip inward along the arc of the tonearm. (Because the tonearm is mechanically stiff and the pivot is anchored, the only way the stylus tip can respond to the forces on it is to "skate" inward on the one permitted path.) This is how I think of it, anyway. |
Ralph, Yes, but wouldn't you think that a 45 rpm version of any LP would have relatively lower groove tortuosity/modulation than the comparable 33, because the music signal is allowed more "room" at 45 rpm, per unit of time. Thus, I thought the OPs story that increasing AS helped was not consistent with the notion that groove modulation affected the skating force so as to increase it on average. If anything, I would expect a decrease in skating force, on average. That was a point i made way back up the thread. |
Dear Omsed, This is not a university class or a peer-reviewed forum. Anyone is free to contribute in any way he or she prefers. I can say for myself that when I try to explain something, I am in essence stating my own reasoning of the problem for the express purpose of receiving criticism or correction. I want to learn. Here, I take exception with two of your claims:
(1) Overhang DOES have something to do with the skating force. Because of stylus overhang, the stylus can never be tangent to the groove. That is the root cause of the skating force, because the force of friction is always tangent to the groove. Thus a force vector is created with a direction toward the inward path of the tonearm (the real force is not along the arc of the tonearm, but is inward) that in its net expression becomes the skating force. It also would appear that the offset angle plays a role in determining the direction of the force vector created by lack of tangency, the radial expression of which is the skating force. (Perhaps this is not clear, would need a diagram to demonstrate.)
(2) No one is arguing with your statement that vertical and horizontal movement of the stylus creates the electrical signal that is converted into sound. The point made by me and others was that those vertical and horizontal wiggles that the stylus tip has to trace whilst the linear velocity past the stylus remains constant might in effect alter the coefficient of friction between stylus and vinyl, transiently and variably, so as to contribute to variability in skating force. I am not at all sure that this is a real issue, but it is not nonsensical to contemplate it. And having posited these things, it then would seem possible that a 45, if anything, might induce less skating force than a 33, given that the exact same music is recorded on each. This could certainly be incorrect. Have you measured such phenomena?
By the way, are YOU a physicist? What is your background in what science? I am guessing you are an engineer. |
