Need a book? Engineering books recommendations...

# Re: SI Metric

• To: <seaint(--nospam--at)seaint.org>
• Subject: Re: SI Metric
• From: Christopher Wright <chrisw(--nospam--at)skypoint.com>
• Date: Sun, 1 Jan 2006 00:26:03 -0600

```
On Dec 31, 2005, at 6:11 PM, Paul Ransom wrote:

```
```The other thing that we all learned was that different devices are
required to measure force and mass - springs vs balance scales.
```
Scales and springs both measure force. Springs extend under force and a balance which compares the gravity force on two masses. More precisely, the balance compares two moments and infers the force by providing a meqasure of the lever arm. Neither spring nor balance will indicate anything in the absence of a uniform gravitational field, or an imposed acceleration.
```
```
Most grocers are measuring product by force rather than mass regardless of the dimension of commerce. The error is trivial but the application is practical.
```And the distinction between force and mass is profound.

```
```Complicating this are the "other" measurement conventions that
use kg as force, simply because of common use carry-over.
```
Which is my point--there's nothing about either system that's purer scientifically. Both have the same built-in misunderstandings
```
```
```In conversion tables, a pound is ALWAYS converted as mass units but a
kip is ALWAYS converted as force units.
```
Pounds are more commonly used for force measurements--pressure in lb/in^2; reactions in lb; gravity force in lb; engine thrust in lb; moments in ft-lb etc. Anyone fool enough to do engineering analysis using the same name for the unit of mass and the unit of force distinguishes the two with the terms lbf and lbm. The reason kips are spoken of as forces is because they _are_ forces, just like meters and kilometers are both lengths. Pounds are always force. When you say that something weighs 1 lb, you're saying that the the something experiences a gravity force of 1 lb. If you're talking about a mass of 1 pound you're saying you have an object whose mass is such that the object experiences a gravity force of 1 lb.
```
Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw(--nospam--at)skypoint.com   | this distance" (last words of Gen.
```
.......................................| John Sedgwick, Spotsylvania 1864)
```http://www.skypoint.com/~chrisw/

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
```
* * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to:
```*
*   http://www.seaint.org/sealist1.asp
*
```
* Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********