"Charlie Johnson" <cjohnson1151@mn.rr.com> wrote in message news:4dFFg.16149$zg.14289@tornado.rdc-kc.rr.com...

Hi all,

How have you got on with the replies so far? It is important to note
that the discussions have been essentially about terminology rather than
physics, which well understood and agreed by all (physicists at least).

I am confused, as usual. Say I weigh 200 lbs. measured by a typical
bathroom scale.

A typical bathroom scale actually responds to the force due to gravity
on your body. However it is calibrated to show your mass since
the manufacturers know that is what you want to measure. On an
ordinary pair of bathroom scales your indicated mass will be slightly
different at different points on the Earth's surface because of variations
in the strength of gravity.

When you go to the doctor's you are probably weighed on a device
which compares your mass with that of various standard masses, using
the force of gravity. This should read the same all over the Earth, and
even on the Moon.

The important thing to note is that you have just measured
your _mass_ to be 200 lbs (90.91 Kg). If you are in any
doubt about that look at a set of metric scales - they are
marked in Kg not N.

If I convert that to newtons, I get (4.45 N)(220 lbs.) =
890 newtons.

You cannot do that. The newton is a unit of force and you
have just measured your mass. This is where you have gone
wrong.

Unfortunately, great confusion can arise because the word
'pound' can be used as a unit of force, being equal to the force
that standard gravity exerts on a mass of one pound. If you
use it with that meaning it is best to call it the "pound-force"
to avoid any misunderstanding. You can also use the term
pound-mass when you are using the pound as a unit of mass.
(Which is the 'correct' usage of the word 'pound' is a matter
for historians and linguists.)

Let us suppose that you (perversely) take the reading on the
scales to be a measurement of the force of gravity on your
body (often called your weight although, to add more
confusion, the word 'weight' is sometimes used to mean 'mass').

So, the force of gravity on your body is 200 pounds-force.
To convert that to newtons you multiply by 4.45. to get 890 N.

Now, to find my mass, I divide 890 N / 9.8 m/s^2 to get 91 kg.

Correct.

But, if I convert my 200 lbs. to kilograms by the coversion 2.2 lbs / kg, I
get 91 kg.

Yes.

How can my weight be the same as my mass, if weight is mass x
acceleration?

In metric units your mass is 91Kg and your weight (meaning here the
force of gravity on you) is 890 N (mass * acceleration).

In what you call English units (which are no longer used in England)
your mass is 200 pounds-mass and your weight is 200 pounds-force.
The confusion being caused by using one word for two different
things.

If you insist on using English (we now call them Imperial) units
then you must either measure force in poundals and the mass in
pounds-mass or you cam measure force in pounds-force and
mass in slugs.

What am I doing wrong here? Please help the befuddled!

You are using an inconsistent system of units Use pounds-force
and slugs or pounds-mass and poundals or better still use metric
units.

Martin Hogbin

"Charlie Johnson" <cjohnson1151@mn.rr.com> wrote in message news:4dFFg.16149$zg.14289@tornado.rdc-kc.rr.com...

If I convert that to newtons, I get (4.45 N)(220 lbs.) =
890 newtons.

You cannot do that. The newton is a unit of force and you
have just measured your mass.

Charlie Johnson wrote:

Hi all,

I am confused, as usual. Say I weigh 200 lbs. measured by a typical
bathroom scale. If I convert that to newtons, I get (4.45 N)(220 lbs.) =
890 newtons.

Make that 4.45 newtons per pound
http://www.onlineconversion.com/force.htm
then [newtons per pound] x [pounds] = [newtons]

Now, to find my mass, I divide 890 N / 9.8 m/s^2 to get 91 kg.

91 kg is a good mass

But, if I convert my 200 lbs. to kilograms by the coversion 2.2 lbs / kg, I
get 91 kg.

91 kg is *still* a good mass

How can my weight

200 pounds =- 890 newtons

be the same as my mass,

It looks to me like pounds (or newtons) are *not* the same as kilograms

if weight is mass x
acceleration?

weight *is* a force = mass times acceleration

What am I doing wrong here?

You are confusing yourself with mental juggling. Juggling is best left
to trained professionals. Try putting things on paper. Make three
column headings: weight, mass, and acceleration. Then write each
number AND its units under the appropriate column heading.

(Uncle Fester)

Tom Davidson
Richmond, VA

Charlie Johnson wrote:

Hi all,

I am confused, as usual. Say I weigh 200 lbs. measured by a typical
bathroom scale. If I convert that to newtons, I get (4.45 N)(220 lbs.) =
890 newtons. Now, to find my mass, I divide 890 N / 9.8 m/s^2 to get 91 kg.
But, if I convert my 200 lbs. to kilograms by the coversion 2.2 lbs / kg, I
get 91 kg. How can my weight be the same as my mass, if weight is mass x
acceleration?

What am I doing wrong here? Please help the befuddled!

T.I.A.

Lurch

I'll gloss over a historical artifact that a "pound" has been used both
as a unit of mass and as a unit of force (weight), and adopt the
convention that the English system uses "slugs" as units of mass and
"pounds" as units of weight. In the metric system, kilograms are units
of mass and newtons are units of weight.