Strength to Weight Ratio -- Units
 

Are their common units for strength-to-weight ratios?

The numbers kind of all look wacky when I try it -- Pascal-m^3/kg = (N/
m^2)*(m^3)/kg = N-m/kg = (kg-m/s^2)(m)/kg = m^2/s^2?

I've never seen it expressed that way, or as in^2/s^2.

I hear folks talking about the ratio, but don't see it quoted much in
engineering units...

--
www.wescottdesign.com

Strength to Weight Ratio -- Units
 

On Mon, 25 May 2015 22:17:57 -0500, Tim Wescott
wrote:

Are their common units for strength-to-weight ratios?

N-m/kg. All of these derived units employing Newtons are clumsy, being
based on a measurement of acceleration. But it's the short version.

I don't try to derive them backwards, to what they actually mean. You
can do that once for your own understanding and then be done with it.
The most common and useful way to use measurements of specific
strength is in comparisons between materials, in which case, units
don't matter.

--
Ed Huntress



The numbers kind of all look wacky when I try it -- Pascal-m^3/kg = (N/
m^2)*(m^3)/kg = N-m/kg = (kg-m/s^2)(m)/kg = m^2/s^2?

I've never seen it expressed that way, or as in^2/s^2.

I hear folks talking about the ratio, but don't see it quoted much in
engineering units...

Strength to Weight Ratio -- Units
 

On Tue, 26 May 2015 07:49:26 -0400, Ed Huntress wrote:

On Mon, 25 May 2015 22:17:57 -0500, Tim Wescott
wrote:

Are their common units for strength-to-weight ratios?

N-m/kg. All of these derived units employing Newtons are clumsy, being
based on a measurement of acceleration. But it's the short version.

I don't try to derive them backwards, to what they actually mean. You
can do that once for your own understanding and then be done with it.
The most common and useful way to use measurements of specific strength
is in comparisons between materials, in which case, units don't matter.

I've seen this come up a few times in aircraft modeling fora, where they
were quoted without dimensions.

Which always bugs me -- three WHAT?

Do you know of any prevalent systems in good old 'merican units, that you
might see quoted for oak vs. fir in a lumber yard, or by a gray-haired
engineer? Somehow I don't think that lbf-ft/slug would be quoted very
often...

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Strength to Weight Ratio -- Units
 

On Tue, 26 May 2015 12:16:25 -0500, Tim Wescott
wrote:

On Tue, 26 May 2015 07:49:26 -0400, Ed Huntress wrote:

On Mon, 25 May 2015 22:17:57 -0500, Tim Wescott
wrote:

Are their common units for strength-to-weight ratios?

N-m/kg. All of these derived units employing Newtons are clumsy, being
based on a measurement of acceleration. But it's the short version.

I don't try to derive them backwards, to what they actually mean. You
can do that once for your own understanding and then be done with it.
The most common and useful way to use measurements of specific strength
is in comparisons between materials, in which case, units don't matter.

I've seen this come up a few times in aircraft modeling fora, where they
were quoted without dimensions.

The true SI version uses Pascals. It's Pa/(kg/m^3). I don't remember
seeing it used, but then, I don't read a lot of academic engineering
white papers. The Newton, "commonly used metrics" version shows up
quite a bit.


Which always bugs me -- three WHAT?

Do you really want to go there? g The concept is simple: it's
strength per unit area, divided by density. But the units can get
strange.


Do you know of any prevalent systems in good old 'merican units, that you
might see quoted for oak vs. fir in a lumber yard, or by a gray-haired
engineer? Somehow I don't think that lbf-ft/slug would be quoted very
often.

Strength in pounds per *square* inch over density in pounds per
*cubic* inch.

The simplification gives you very weird units. The reduced unit is
inches. g

As for wood, the _Wood Handbook: Wood as an Engineeering Material_
book from the Wood Products Laboratory only mentions it twice, and
they don't discuss units. BTW, if you're interested in wood and you
don't have that 509-page book you should download it. It's free. Then
get R. Bruce Hoadley's _Understanding Wood_. It's not free but it's
worth a bundle if you're interested in wood.

Once again, you may want to go through the exercise of deriving it
*once*. Then you'll see why the units are weird. Then you can forget
about the units and just use the numbers for comparisons.

After all, the units for Young's Modulus are very weird, too.

--
Ed Huntress