> 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
> friction.
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
> force.
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
> conductors!
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
www.rogerhalstead.com
>
> Back to my towerless lurking.
>
> 73, Art K3KU
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