On 10/12/2018 10:18 AM, Radey Shouman wrote:
John B. Slocomb writes:

On Thu, 11 Oct 2018 15:36:04 -0400, Frank Krygowski
wrote:

On 10/11/2018 1:53 PM, Theodore Heise wrote:
On Thu, 11 Oct 2018 09:27:02 -0700 (PDT),
Frank Krygowski wrote:
On Thursday, October 11, 2018 at 5:00:33 AM UTC-4, John B. Slocomb wrote:
On Thu, 11 Oct 2018 08:54:38 +0200, Emanuel Berg
wrote:

Just out of curiosity, is there a "torque wrench" pump or
compressor? I.e., you would screw on the presta valve, set
the gizmo to e.g. 35psi, engage it, and instead of watching
the indicator, automagically at the right level it would
stop?

Most of the gas stations here use an air station that you can
set for your desired pressure and then just plug the hose onto
the tire valve
- there is a little clamp to hold it there. When the tire is
inflated to the specified pressure the inflation stops and a
bell rings.

Since they aren't manufactured here I had assumed that the
rest of the world had them too.

My experience from 50+ years ago says not to rely on those
things, although I suppose they may be different now.

Back then I blew a tire off the rim with one. I suspect the
problem was the volume of each pumping stroke. In a large sized
car tire, the volume surge with each big stroke would be
absorbed and barely raise the pressure. In a low volume bike
tire, it caused an explosion. That's my guess anyway.

I usually inflate using a manual floor pump with a gage. It's
easy enough to stop pumping when the dial reads the desired
temperature.

Don't you mean, when the dial reads the desired foot-pounds?

Oh geez, my mistake!

But: Neither! I stop when it reads the desired PRESSURE!

Around here we use psi = pounds per square inch. Weirdly enough, my
pump's pressure gauge is also graduated in kg/cm^2. I would have used
that as a bad example in my courses, since kg is properly used to
measure mass, not force. And pressure is force per unit area.

(This indicates that the SI system gets misused as much as the U.S. or
Imperial system.)

But isn't "pound" a measurement of mass also :-?

As I used to explain it to students: Properly speaking, a _force_ is a
push or a pull on an object. Properly speaking, _mass_ is a measure of
the amount of matter in an object. _Weight_ is a particular force, i.e.
the force of gravity on an object.

So in a U.S. grocery if you buy 2.2 pounds of cheese, you're buying the
amount of cheese upon which the earth's gravity exerts a force of two
pounds. It's a roundabout way of specifying the mass you want, but it
works as long as you're just talking cheese, etc.

In a European country, you'd specify you wanted a kilogram of cheese,
which is about 2.2 pounds worth. There, you're directly specifying the
amount of cheese you want.

That makes it sound like the Europeans are much smarter. But they turn
things around and sometimes measure forces in kilograms, or pressure in
kg/cm^2 etc.

Where it makes a difference is in calculations involving force, mass and
acceleration. Or other engineering calculations. If you don't clearly
understand whether you're dealing with force or with mass, you get
answers that are very, very wrong.


When I was in school, years ago, we were quite strictly made to write
either lb_f (pound force) or lb_m (pound mass), and to include unit
conversions from one to the other using constants g (the nominal force
of graivty at the surface of the Earth) and g_c (a unit conversion factor).

The conversion is:

lb_f = lb_m * g / g_c

In English units g = 32.2 ft/s^2
g_c = 32.2 lb_m ft/s^2 lb_f

but if you didn't include the conversion, you failed.

Exactly! And students who ignored all that got answers that were wrong
by a factor of 32.2.

As I explained it, g_c ("Gee sub C") is just a conversion factor, in the
same way that (12 in / 1 ft) is a conversion factor. If a person
diligently showed units in their computations, it was obvious when it
was needed.

Most conversion factors have no names, and it always seemed weird to me
that they gave that conversion factor a name. Thousands of students got
endlessly confused between the acceleration of gravity
g, which is 32.2 ft/sec^2
and that conversion factor
g_c, which is 32.2 (lbm*ft)/(lbf*sec^2)

Diligent attention to units on ALL quantities straightens out that
confusion. At least, for most students.

And BTW, I found that engineers typically pay attention to units like
that. To my astonishment, some professors teaching basic physics did not.

--
- Frank Krygowski