> In the long and useful thread about mast clamps that slip under torque,
> several contributors have discussed the friction between the clamps and
> the mast. I'll point out that the linear relationship for friction force
> F=mu*N is only a model (an approximation) that works pretty well in most
> cases -- but not in all cases. Indeed, we commonly hear about "static
> friction" and "sliding friction", because the simple linear model neededd
> a little modification. I've heard some Mech E's mention "sticktion"
> (sp?), and it sounded like they meant something other than static
I just dug out my old Calc based physics book trying to find the names and
formulas. When it comes to sliding somehting it is a two stage problem as
it takes more force to start the movement than it does to continue the
movement. Of course you can use calculus . It's amazing. I don't think I
could now answer enough of those problems to even pass the course and I have
a math minor. I never had to use any of the math since I graduated.
> I recall reading about race cars that cornered so well because they
> achieved mu>1, which was said to violate the linear model.
> And, for sure, if the mast clamps have those groovy little cuts to dig
> into the mast, the "friction" is not going to be linear with the clamping
Just think of it as a lathe with a very wide cutting bit.
> We use linear models lots of places because: a) they are accurate enough;
> b) we can handle the math.
> And if you'll indulge me, here's a parallel example from the Real World
> (electricity). Perhaps you have seen Ohm's Law in "point form": is
> J=sigma*E. (It's the same thing as I = (1/R)*(V): current density =
> conductivity*Electric field.) Well, in actuality (as I read somewhere
> many years ago), this Law is not true for all materials. It does holdsfor
> a broad class of conducting materials, and they are called ... Ohmic
And if you are working with DC.
One of our exercises was to calculate the average speed of the free
electrons in a conductor of a given size for a given current.
I couldn't do a derivative or intergral today if my life depended on it.
Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
> Back to my towerless lurking.
> 73, Art K3KU
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