Turntable Question - Centrifugal or Centripetal?

Hi

To answer your question. This is centripetal force. If you were to pull on a string with a weight on the end that is rotating the object would feel centripetal force, your hand holding the string would feel centrifgal force, although it is not a real force, it its merely a result of centripetal force.
Centripetal force is an inward force that makes an object move in a circle, like tension in string, a gravitational force, electrical attraction of a mass or atomic nucleus.

A stylus is moved to the centre without any of these, it is pushed to the centre by the groove, ie no groove the sylus would be pushed out, by friction. Plus the stylus is not rotating, there for cannot experience either forces, only the record can. (from a Physics forum)

Regards

Guy

There are significant forces and insignificant forces. A centripetal force is that required to keep a mass moving in a curve, so when you spin a record there's an internal force required to prevent it flying apart, and since the tone arm moves in an arc there's a tension in the arm wand to keep the cartridge and headshell attached. I'd consider all these insignificant forces, unless you try playing a record at 33000rpm.

There is one small but significant force involved playing records, that's the drag of the stylus in the groove. This is aligned straight up the cantilever and, due to the offset angle of the headshell, doesn't pass through the horizontal pivot. The result is a moment that tries to twist the arm towards the centre of the record. I suppose this could be literally 'centripetal' but it's got nothing to do with circular motions and everything to do with the length of the arm, the offset angle of the headshell and how rough your records are. Most hifi pickup arms have an antiskating or bias facility that on average compensates for the drag. Obviously parallel trackers don't suffer the problem in the first place.

Paul

quote:

Originally posted by Lapdog:
Quickie for ya,

What force is being applied to the stylus as it rides the groove of the record?

A smarty at works said without a doubt that it is centrifugal and that may well be true if the cart sat on the record surface (all on its lonesome) but I’m guessing that because the cartridge being fixed to the tonearm headshell kinda changes the dynamics and the force being applied is actually centripetal.

I know I know… silly question,
Brian

A 1947 farthing with a robin on it, innit

My physics o level teacher said there was no such thing as centrifugal force, only centripetal force

not that I've any idea what he was on about...

Revolutionary stuff innit

quote:

Originally posted by Berlin Fritz:
Revolutionary stuff innit

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Thanks GuyPerry for explaining all that.

Appreciated,
Brian

quote:

My physics o level teacher said there was no such thing as centrifugal force, only centripetal force

One of my junior honours physics lecturers also maintained there was no such thing as potential energy. It was all a fudge to justify the "conservation of energy" stuff. At the time it made sense but now - who knows or cares.

Hi all,

The *relative* motions of both stylus and record are circular, therefore there IS a force on the stylus due to centripetal acceleration which acts towards the centre of the record. Looking at it from a previous poster's view of twirling a weight on the end of a string, there is a tension in the string, i.e. the record's surface.

I have actually done the calculations (yes, I know I'm a sad git) and the tendency of the stylus to try and go straight ahead is in fact quites large, and about an order of magnitude greater than the frictional force pulling the stylus towards the centre. So why is this outward force not a major player in setting bias? Well, the coefficient of friction of the vinyl, in addition to the record groove being at a 45 degree angle ensures that the force is irrelavent *unless* there is sufficient impetus on the stylus in the outward direction to overcome these two barriers to outward movement.

If the forum software would accept .PDF files I'd attach one with a fuller explananation for your delectation. Alas...

Also, while were on the subject, the ARO's bias system is not real good from a geometrical standpoint, in that the angle of the 'fishing line' from the loop of the hanger to the arm which sticks out from the main arm cup, changes radically as the stylus traverses the record. Now, this will always happen with such a set up but it is possible to get a much more consistent bias force across the playing surface by changing the position of the bias hanger.

Best Regards,
Mark Dunn

quote:

Originally posted by Mark Dunn:

Also, while were on the subject, the ARO's bias system is not real good from a geometrical standpoint, in that the angle of the 'fishing line' from the loop of the hanger to the arm which sticks out from the main arm cup, changes radically as the stylus traverses the record. Now, this will always happen with such a set up but it is possible to get a much more consistent bias force across the playing surface by changing the position of the bias hanger.

Personally, if I had the pleasure of owning one of those lightweight, wobbly, seemingly flimsy tonearms the 4700 NZD it costed me would always stop me from bending anything on it.

And so say all of us!!

quote:

Originally posted by living in lancs yearning for yorks:
My physics o level teacher said there was no such thing as centrifugal force, only centripetal force

not that I've any idea what he was on about...

Strangely (or not so strangely) my physics teachers also taught that there was no such thing as centrifugal force - in both O and A level, IIRC. Another oddity in the O level syllabus was that the idea of an electric field was deemed too difficult for pupils to grasp. As a result, what we were taught instead made little sense...

It's more accurately called centrifugal reaction if I'm not mistaken. Presumably because the object in question wants to continue in a straight line instead of going round and round.

Mark

I am not sure you are doing the correct thing by adjusting the position of the bias hanger. The subject is obviously very complicated but as far as I understand it, the outside surface of the groove pushes the stylus towards the centre of the record, it is not pulled by friction. The purpose of bias is to resist this force. The geometry of the groove is not the same at the outside of the record as it is at the inside of the record. The force the groove exerts on the stylus trying to push it towards the centre of the record is greater at the outside than at the inside. I do not know the geometry of the ARO’s bias system, but I would guess it is exerting more bias at the outside of the record.

Regarding Brian’s original question, I am in agreement with the people who say the record exerts neither centrifugal or centripetal on the stylus.

Hi fatcat,

Re:

[/quote]I am not sure you are doing the correct thing by adjusting the position of the bias hanger. The subject is obviously very complicated but as far as I understand it, the outside surface of the groove pushes the stylus towards the centre of the record, it is not pulled by friction.[/quote]

If we assume for the sake of simplicity that there is no tracking error for the stylus in the groove (i.e. the stlyus is at one of the two null points of it's arc of travel) then neither the inner or outer grooves are pushing (sideways) on the stylus. However, the offset of the headshell, and thus cartidge/stylus causes a frictional force toward the centre of the record given by:

F = Fn x u x sin(theta)

where
Fn = Force normal to records surface (i.e. the stylus tracking force)

u = coefficient of friction of the vinyl

theta = headshell offset angle

To get a feel for what's happening, imagine you're back in childhood (like me) and you have the go-kart that you made out of a disused plank of wood, an orange box, and the large diameter wheels from the pram you found at the local rubbish dump. Now imagine that you were too young to figure out how to make the front wheels steer, so the front axle doesn't swivel. Lastly, imagine you're in a muddy field pulling the kart along with the ubiquitous piece of string, and those pram wheels are digging into the mud. When you pull the kart directly forward all you feel is the resistance form the mud on the wheels holding you back, but now imagine veering off to one side and trying to pull the kart after you...

[/quote]The geometry of the groove is not the same at the outside of the record as it is at the inside of the record. The force the groove exerts on the stylus trying to push it towards the centre of the record is greater at the outside than at the inside. I do not know the geometry of the ARO’s bias system, but I would guess it is exerting more bias at the outside of the record.[/quote]

Yes, the ARO's bias normally exerts more force at the outside of the record. However, the trick here is to see that the coeeficient of friction is a scalar, as a opposed to a vector quantity (i.e. it has a single dimension as opposed to being measured along two dimensions. Think of the the scalar quantity 'speed' versus the vector quantity 'velocity', which has both speed *and* direction). Thus, ignoring any relatively minor effects of a change in the coefficient of friction due to the increased linear velocity of the stylus (and thus temperature of the vinyl) when it's closer to the record center, the bias force is actually the same as at the outer grooves.

Best Regards,
Mark Dunn

I can't honestly say that I'm keeping up with this discussion completely - but doesn't the unipivot design of the ARO help with end of side distortion in some way?

Has the farthing finally dropped

Like St Sephens tower the clock has stopped ?



Innit

Are we messing up your padded cell talking about things that interest us, Fritz?

Not at all I built my first Coinnoiseur deck 30 years ago, and have moved on since, enjoying silly things like politics and the real history of Denning, not the Hollywood textbook version etc, innit.


Fritz Von I can see further than the end of my nose, not forgetting the turty nine steps o'course


Ps: My Above comments refer to my 1947 farthing pushing gravity on said cartridge arm (as a Goldring was out of my league), Stephens holds the Bell, Big Ben in Westminster which stopped on friday night suddenly for a short while (probably due to extreme hot weather). The fine clock mecvhanism (which you probably know) is balanced and counter balanced with a very fine array of old pennies, placerd at strategic points to keep the trackin sorted, innit

Pish: Deanne old Bean, Don't you go and start taking me seriously now, this is only a fun Forum, unlike 'that other place' where I've noticed they get really heavy with each other,
airing their frustrations, and 'moderators' actually pop up to oder 'Break Gentlemen' ding ding, innit

Mark

Surely the stylus is physically pushed towards the centre of the record by the groove. IE the outside surface of the groove. The spiral has a similar effect on the stylus as a wedge would. The angle of the wedge is smaller at the outside of the record, thus it exerts more force.

With regard to the formula

F = Fn x u x sin(theta)

Why is the angle of the headshell with regard to the groove of importance and not the angle of the cartridge.

As the stylus moves across the record, this angle would change from a positive value to a negative value (or vice versa) as it passes through a null point. The direction of force (F) would therefore change as it moves across the record.

I also suspect the coefficient of friction would be far greater in the groove of a recording of a symphony orchestra than in the groove of a recording of solo violin.

Mark

I have just sketched the geometry of my turntable/arm. I can now see you are correct, the offset of the headshell does produce a force pulling the stylus towards the centre of the record.

GOOD GWIEF !!!

Thanks fatcat,

You saved me a lot of typing :-)

Best Regards,
Mark Dunn

Always the warped space theory of course, innit

Are you related to Fritz Tha Cat

I'm reading 'Hyperspace' by Michio Kaku at present and it came to mind, innit.


Fritz Von A great read for uneducated numbskulls like moir too