Re: Physics - biking question

Quoted message said:

155 pounds is not *mass*, it's *weight*. They're not the same.

True, but since we might assume he weighed himself on this planet, we have a
pretty good idea what his mass is.

Quoted message said:

10mph = 6.8fps,

Better check your calculations here. 10 mph is about 14.7 fps

Quoted message said:

carrying 111 ft-lb energy, which is equivalent to that of a 25-ounce
finishing hammer dropped out a seventh-story window.

Depends on what a story is but since you got the velocity wrong...

Quoted message said:

It doesn't take a rocket scientist (ahem)

You?

"LioNiNoiL wrote: (clip)155 pounds is not *mass*, it's *weight*. They're not
the same.(clip)
^^^^^^^^^^^^^^^
Actually, in one of the systems of units, a pound IS mass. It is
accelerated at the rate of 1 ft/sec2 by a force of one *poundal.* So, the
acceleration in free fall, of 32 ft/sec2 is produced by 32 poundals, which,
conveniently, equals one pound-force. I don't think this system is used
much (or at all), but we did study it in college physics.

On Sun, 03 Oct 2004 20:58:32 -0700, LioNiNoiL_a t_Y a h 0 0_d 0 t_c 0

m said:

Quoted message said:

OK, all you amateur physicists:

Whaddaya mean, "amateur"?! Why, I oughtta...

Quoted message said:

If a 155 pound mass (me)

155 pounds is not *mass*, it's *weight*. They're not the same.

Nonsense.

Pounds are and always have been units of mass. Since 1959, they have
a common, worldwide definition: they are defined as 0.45359237 kg
exactly. Here's the current, official U.S. definition, which
discusses not only that international agreement but also (in a part
not quoted here) the prior U.S. definition, for 66 years before then,
as a slightly different exact fraction of a kilogram:
http://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf

Announcement. Effective July 1, 1959, all calibrations in
the U.S. customary system of weights and measures
carried out by the National Bureau of Standards will
continue to be based upon metric measurement standards
and, except those for the U.S. Coast and Geodetic Survey
as noted below, will be made in terms of the following
exact equivalents and appropriate multiples and submultiples:

1 yard= 0.914 4 meter

1 pound (avoirdupois)= 0.453 592 37 kilogram

Currently, the units defined by these same equivalents,
which have been designated as the International Yard and
the International Pound, respectively, will be used by the
National Standards Laboratories of Australia, Canada,
New Zealand, South Africa, and United Kingdom; thus
there will be brought about international accord on the
yard and pound by the English-speaking nations of the
world, in precise measurements involving these basic units.

Yes, there are also pounds force--a recent bastardization, units never
well defined before the 20th century, and units which even today do
not have an "official" definition.

Furthermore, not only is the primary definition of pounds, but they
are also the pounds normally used when we weigh ourselves. American
Society for Testing and Materials, Standard for Metric
Practice, E 380-79, ASTM 1979.

3.4.1.2 Considerable confusion exists in the use of
the term weight as a quantity to mean either force or
mass. In commercial and everyday use, the term
weight nearly always means mass; thus, when one
speaks of a person's weight, the quantity referred to
is mass. . . .

Not only are the kilograms used throughout the world, including many
if not most hospitals in the United States, the proper SI units for
this body weight, but the pounds used for this purpose in the medical
sciences and in sports--the primary reasons we weigh ourselves--are
every bit as much units of mass as those kilograms.

Quoted message said:

Quoted message said:

is traveling at 10 mph (on bike) and I strike an object
dead on with my shoulder (roughly 2 sq inches) -
what the load presented by my shoulder to the object.

Your mass is 4.8 slugs [look it up]

Sure, slugs are also units of mass--a little used 20th century
invention,something which didn't appear in physics textbooks before
1940 and which have since pretty much disappeared from them, hardly
"the" English units of mass.

Slugs only exist in one particular, specialized subsystem of
mechanical units--out of several such subsystems used only in
calculations. There are other such subsytems which do not include
slugs, including the absolute fps (foot-pound-second) system and the
engineering fps system, as well as inch-pound-second systems, etc.

SLugs are used because they are a part of one of several "coherent"
systems of units, as that term is used in metrology. What does that
mean? It means that in the only system which includes slugs, there
are no pints or quarts or gallons of any kind, not U.S. dry, not U.S.
liquid, not imperial. For each quantity in a coherent system, there
is only one unit--and in the slug system, that unit of volume is cubic
feet.

In the only system with slugs, there are also no British thermal
units, no horsepower. There aren't even any psi, because there are no
inches or miles or ounces in the only system with slugs.

Quoted message said:

and 10mph = 6.8fps, so you're
carrying 111 ft-lb energy, which is equivalent to that of a 25-ounce
finishing hammer dropped out a seventh-story window.

Those ounces are units of mass, not of force.

But the pounds in foot pounds are pounds force, and should be
identified as such. American Society for Testing and Materials,
Standard for Metric Practice, E 380-79, ASTM 1979:

3.4.1.4 The use of the same name for units of force
and mass causes confusion. When the non-SI units
are used, a distinction should be made between force
and mass, for example, lbf to denote force in
gravimetric engineering units and lb for mass.

Quoted message said:

It doesn't take a rocket scientist (ahem) to see that the two situations

Speaking of "rocket scientists"--what in the world do you suppose it
means when those at NASA tell us that the liftoff weight of the Apollo
11 lunar module was 10,776.6 lb? Maybe this will help you more--they
also tell us that its weight at the time of docking (think about where
the command modulet was at the time), after expending considerable
fuel to get into orbit, was 5738.0 lb?

That's normal NASA usage, something we see even today with the space
shuttle and international space station. It is also what we normally
see in their usage of "thrust-to-weight" ratios.

--
Gene Nygaard
http://ourworld.compuserve.com/homepages/Gene_Nygaard/
"It's not the things you don't know
what gets you into trouble.

"It's the things you do know
that just ain't so."
Will Rogers

Leo Lichtman said:

"LioNiNoiL wrote: (clip)155 pounds is not *mass*, it's *weight*. They're not
the same.(clip)
^^^^^^^^^^^^^^^
Actually, in one of the systems of units, a pound IS mass. It is
accelerated at the rate of 1 ft/sec2 by a force of one *poundal.* So, the
acceleration in free fall, of 32 ft/sec2 is produced by 32 poundals, which,
conveniently, equals one pound-force. I don't think this system is used
much (or at all), but we did study it in college physics.

It was used regularly in physics textbooks from the first half of the
20th century, and back into the late 19th century (when slugs didn't
even exist yet). Outside of North America, it remained the preferred
system of English mechanical units in engineering until replaced by
the metric system.

Furthermore, the specialized subsystem of English mechanical units in
actual use in the real world today is neither the absolute fps system
with poundals nor the gravitational fps system with slugs. Rather, it
is the engineering fps system which uses neither poundals nor slugs,
using pounds for mass and pounds force for force. Since this is not
quite a "coherent" system of units, you need to use the more general
form of many formulas, such as F = k·m·a. The constant used is
actually often expressed as g_c (with the underscore indicating a
subscript), and then you get F = (m/g_c)·a and kinetic energy
E = m·v^2/(2 g_c)

Gene Nygaard

Gene Nygaard said:

Speaking of "rocket scientists"--what in the world do you suppose it
means when those at NASA tell us that the liftoff weight of the Apollo
11 lunar module was 10,776.6 lb?

It means they should bite the bullet and use metric, already. :-)
--
David Damerell <[email hidden]> Distortion Field!

David Damerell said:

Gene Nygaard said:

Speaking of "rocket scientists"--what in the world do you suppose it
means when those at NASA tell us that the liftoff weight of the Apollo 11
lunar module was 10,776.6 lb?

It means they should bite the bullet and use metric, already. :-)

Considering the recent embarrassment with the mixup between metric and
English units on a Mars probe, perhaps NASA is not the most stellar source
of authority on weights and measures.

To suggest that there is any "official" English system of weights and
measures is a stretch. There is little if any consistency among the
various English/US/Canadian "standards", which is why they are defined in
terms of metric units, for which there is a reasonable set of standards.

In particular, in elementary textbooks in math or physics often the pound
(avoirdupois -- which means "to have weight"😉 is considered a unit of
force. "Weight" is also considered as force as well, as opposed to mass.
NASA's use of "weight" of a spacecraft in Earth orbit is inconsistent with
the use of "weightlessness" in the same context. Of course, they don't
say that any more, either. They now use the jarringly incorrect term of
"microgravity" even though such craft are in a gravitational field little
less than that on the surface of the earth.

Typical engineering use regularly identifies units of force with units of
mass, such as "grams of force" which we see on r.b.t. from time to time.
It's usually clear what is meant, at least.

--

David L. Johnson

__o | If all economists were laid end to end, they would not reach a
_`\(,_ | conclusion. -- George Bernard Shaw
(_)/ (_) |

Quoted message said:

Quoted message said:

10mph = 6.8fps,

Better check your calculations here. 10 mph is about 14.7 fps

Oops!

Quoted message said:

Quoted message said:

It doesn't take a rocket scientist (ahem)

You?

Not doing NASA's reputation any good, am I?

--
"Bicycling is a healthy and manly pursuit with much
to recommend it, and, unlike other foolish crazes,
it has not died out." -- The Daily Telegraph (1877)