Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] Pound mass or weight?



My fault for inadvertently starting this with an off-hand comment, but I (and most intro books) are clearly on the side of the Engineers. We measure weight in pounds--therefore pounds is a unit of force--like Newtons. We measure work in foot-pounds, a force times displacement (Nm). We talk about weight (even though we seldom agree on its definition) as variable--different on Earth than the moon but mass being constant. We say the 180 lb astronaut (on earth) weighs 30 lbs on the moon. If we want mass in an english-unit problem we take (pounds/32ft/s^2)-- slugs if you wish, but usually just written as above. Considering pounds as a unit of mass is what would be VERY confusing--both to me and students. Sure you can define away (see David's not below), but that doesn't necessarily make things better. So, I will stay with my initial statement--the weight of 1 kilogram is 2.2 lbs, but not 1 kg = 2.2 lbs.

Rick


----- Original Message ----- From: "David Bowman" <David_Bowman@georgetowncollege.edu>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Wednesday, November 05, 2008 9:45 AM
Subject: Re: [Phys-l] Pound mass or weight?


Regarding john Clement's comments:

As I recall there was a pound mass, and a pound force defined at
one time.

I believe there still are. A pound force is the net force exerted on
a pound mass that causes it to accelerate at 9.80665 m/s^2.
The official definition of the avoirdupois pound in the US is that it
is exactly 0.45359237 kg (= 1 lbm) of *mass*. This means the pound
force is exactly 4.4482216152605 N (= 1 lbf).

Currently engineers still are taught the slug mass and pound force.

Engineers in the US, at least.

The "common" definitions are hopelessly at odds with each other,
and confused. Even the Natl Bureau of STD at least in one place
equates mass with weight.

Part of the problem is that the term 'weight' is not a technical
term with a single precise technical meaning. It has many meanings
in many different contexts from probability theory to physics to
engineering.

As to confusion caused by authors, some should know better. An
astrophysicist and respected author wrote a hopelessly wrong
explanation of Newton's third law for the World Book Encyclopedia.
In it he uses motion to explain how action = reaction, and never
mentions force.

As I recall, I think Newton's explanation also was in terms of
impulses and momentum changes rather than in terms of forces.

So of course when he mentions a ball bouncing off of a wall, the
action obviously does not equal the reaction. Now if he had used
momentum, he would have been correct. Also he repeats the idea
that negative acceleration is always slowing down. This is a
common interpretation which is a resistant misconception.

I don't know that it is as much a misconception as much as it is the
use of an alternate common (*more* common in the vernacular world)
definition of the term 'acceleration'. That definition is that the
acceleration means the rate at which the speed changes. With that
definition a negative acceleration *is* "always slowing down". It
simply is the case that in techical physics jargon (and almost only
there) the term 'acceleration' has a different definition, meaning
the rate at which a velocity vector changes.
Do our own people have to try to be so popular that they have to
repeat these misconceptions, and imbed them deeper in student
psyches?

I'm not sure the effect is always an attempt at being popular as much
as it may be an attempt at using words in ways that previously
untutored students already use them without resorting to a batch of
technical redefinitions.

If anyone knows the man perhaps they could communicate with him
about the problem. I sent him an E-mail, but he does not know who
I am and assumes I am a kook, which may be true. Anyone who wishes
to write physics accounts should read a bit of the research about
misconceptions and how language should be used to promote clear
communication about science.

That is probably good advise.

The world book is a major resource for students and families. The
Wikipedia is accurate, but hopelessly difficult for students who
are not engineers or physicists.

That is the typical trade-off. Clarity and correctness tend to be
conjugate concepts that obey an mutual uncertainty principle.
A number of years ago Ludwick Kowalski summed up the situation very
elegantly when he said "Explanations are never perfectly correct and
perfectly clear at the same time. This is unavoidable because
clarity calls for simplifications, while simplifications, by
definition, impose limits on the truthfulness of what is being
explained."

There is a website www.brainpop.com <http://www.brainpop.com> that is presenting lessons on
many topics including science. They should be ashamed of the
outrageous misconceptions that they are promoting in both the
movies and the quizzes they distribute. It is used by a number of
teachers and districts have bought access to it. Don't they have
physicists parsing their material?

I don't know. Maybe not.

The answer is rhetorical because they obviously don't

OK.

John M. Clement
Houston, TX

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