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Re: [Phys-l] force or mass



OK, but some people LIKE it that the dates do not all fall on the same day. I know my kids are happy when their birthdays fall on a weekend.

But back to the force or mass question: I think I am missing something. So if you define a pound as a unit of mass, and then observe one pound of mass accelerating at 32 ft/sec2, then what is the magnitude and unit of the force responsible for that acceleration? Do you still use F=ma?

The reason I am asking is that I have been teaching that since we observe 1 kg accelerating in freefall at 9.8 m/s2, we infer the existence of a force which we call the weight, and we use F=ma to calculate that the weight is 9.8 Newtons. Thus W=mg is a special case of F=ma. Now, reading this thread, I am wondering if I have been doing this all wrong (and not for the first time since joining this email list).

________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Marc "Zeke" Kossover [zeke_kossover@yahoo.com]
Sent: Friday, November 07, 2008 3:41 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] force or mass

Calculators.

--------

Similarly, there was movement that in the early eighties for calendar reform. It was going to change the calendar so that it had two non-day-of-the-week days over the year so that the remaining year would be evenly divisible by seven. Then, a particular date would always happen on the same day of the week. It was really gaining momentum for it's convenience.

Computers, specifically Excel, killed the movement.

Zeke Kossover


----- Original Message ----
From: Anthony Lapinski <Anthony_Lapinski@pds.org>
To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
Sent: Friday, November 7, 2008 12:23:56 PM
Subject: Re: [Phys-l] force or mass

Nice! :)

What I want to know is, why is the US basically the only nonmetric country
in the world? The metric system was adopted into law (date?) but has never
been implemented. It's a simpler system and would make teaching/learning
science much easier. All other countries use it.


Forum for Physics Educators writes:
I thought the pound was abandoned in favor of the euro.

Bob at PC


________________________________

From: phys-l-bounces@carnot.physics.buffalo.edu on behalf of Bernard
Cleye t
Sent: Wed 11/5/2008 11:05 PM
To: Richard Tarara; Forum for Physics Educators
Cc: Nancy Seese
Subject: Re: [Phys-l] force or mass



"... Some contexts ..."!


Officially the lb or # refers to mass.



"A number of different definitions have been used, the most common
today being the international avoirdupois pound of exactly 0.45359237
kilogram."

Wikipedia:

http://en.wikipedia.org/wiki/Pound_(mass)#Avoirdupois_pound



Unless the above refers to kg-f, pound appears to be a mass.



From the NIST HdBh:



"The pull of gravity on the earth gives an object a downward
acceleration of about 9.8 m/s2. In trade and commerce and everyday
use, the term "weight" is often used as a synonym for "mass." The
"net mass" or "net weight" declared on a label indicates that the
package contains a specific amount of commodity exclusive of wrapping
materials. The use of the term "mass" is predominant throughout the
world, and is becoming increasingly common in the United States.
(Added 1993)"

The next paragraph:


W. Use of the Terms "Mass" and "Weight." [NOTE 1, See page 6]
When used in this handbook, the term "weight" means "mass." The term
"weight" appears when inch-pound units are cited, or when both inch-
pound and SI units are included in a requirement. The terms "mass" or
"masses" are used when only SI units are cited in a requirement. The
following note appears where the term "weight" is first used in a law
or regulation.
NOTE 1: When used in this law (or regulation), the term "weight"
means "mass." (See paragraph V. and W. in Section I., Introduction,
of NIST Handbook 130 for an explanation of these terms.) (Added 1993) 6"


The exception is torque units.

bc, "sticking to his guns".

p.s. of course, this has been mainly ATA, much decried by JD, OTOH,
if the US govt. (all powerful) claims a # is a measure of mass then
it is; nicht wahr?

p.s. NBS definition of the pound:

http://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf

Finally?, all this will soon? be obsolete; already my tyre gauge
includes Pa (kPa).


On 2008, Nov 05, , at 15:34, Richard Tarara wrote:

From the referenced site:

In some contexts, the term "pound" is used almost exclusively to
refer to
the unit of force and not the unit of mass. In those applications, the
preferred unit of mass is the slug, i.e. lbf·s/ft. In other
contexts, the
unit "pound" refers to a unit of mass. In circumstances where there
may
otherwise be ambiguity, the symbols "lbf" and "lbm" and the terms
"pounds-force" and "pounds-mass" can be used to distinguish.

So may I suggest that Bernard read the whole article! ;-)

Rick


----- Original Message -----
From: "Bernard Cleyet"
To: "Forum for Physics Educators"
Sent: Wednesday, November 05, 2008 3:47 PM
Subject: Re: [Phys-l] STUDY SUGGESTS NO CHILD LAW MAY BE DUMBING
DOWNSTUDENTS


"The pound-force is approximately equal to the gravitational force
exerted on a mass of one avoirdupois pound on the surface of Earth."

From your reference.



And my reference:



"This article is about the unit of mass. For the unit of force, see
Pound-force.
The pound or pound-mass (abbreviation: lb, lbm, or sometimes in the
United States: #) is a unit of mass used in the imperial, United
States customary and other systems of measurement."



bc suggests Robert read more carefully, and is reminded of Einstein's
dictum: change the facts to fit the theory.

On 2008, Nov 04, , at 23:56, Robert Yeend wrote:

Well, Wiki also disagrees with you

http://en.wikipedia.org/wiki/Pound-force

Bob

On Nov 4, 2008, at 10:43 PM, Bernard Cleyet wrote:

I, too, went thru life (about > 50 years) thinking the pound was a
unit of force. It is not, as pointed out by JD recently.

https://carnot.physics.buffalo.edu/archives/1999/10_1999/
msg00645.html

"For what it's worth" Wiki. agrees w/ me also:

http://en.wikipedia.org/wiki/Pound-mass

bc, disabused.



On 2008, Nov 04, , at 10:25, Rick Tarara wrote:

The technically
incorrect 2.2 lbs = 1 kg (formally the weight of 1 kg is 2.2 lbs)
works
fine.

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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l


_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l



_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l


_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l




_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l