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where did KN come from?
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Feb 25, 2012
Working the sick hand-jams on Stemwide aka Big Dih...
mike h wrote:
1 kN is the about the same as a 225lb weight hanging free on a rope, or you(at 225 lbs) standing on the ground. When you jump, or with any dynamic load, things get more complicated. It's not a function of how far you fall as Louis E said, but rather how quickly you slow down. The key point to remember is: F=ma. Your mass is constant, but depending on how much you bend your knees and back, what surface you land on, shoes you're wearing, etc, the acceleration (and therefore force) will vary significantly. Someone summarized the point well by saying: "no one ever got hurt by driving fast, only stopping fast."


Nice, 100% true except that lbs is actually a measurement of force (the acceleration in this case is 9.8m/s/s, gravity) whereas kg is a measurement of mass. You would "weigh" the same in kg here on earth, the moon, or Jupiter, but you would deffinitely weigh differently in lbs since the acceleration due to gravity is different!
Dan Dalton
From Boulder, CO
Joined Jul 22, 2006
1,537 points
Feb 25, 2012
on Higher Cathedral Spire
The Bobby wrote:
Now a harder question: where did pounds come from? :)


Interesting question. Short answer - the pound was based on money weight, derived on previous Roman and Hebrew standards.

"The unit is descended from the Roman libra (hence the abbreviation "lb"); the name pound is a Germanic adaptation of the Latin phrase libra pondo, 'a pound weight'."

from: en.wikipedia.org/wiki/Pound_%2...

"12 Unciae (ounces) = 5076 grains (328-9 grams). Here the Romans had followed
the example of the Greeks and all the other nations which already had well
established coinage, both silver and gold, commenced some 400 years earlier:
they based their coinage weight upon one of the very ancient shekel standards."

from: Frederick George Skinner (1967). Weights and measures: their ancient origins and their development in Great Britain up to A.D. 1855 :

In other words, back in the day, money was real silver or gold, and was valued based on what it weighed. So the merchants had scales for measuring money, and they used the same scales for measuring the weight of other goods....
Clint Cummins
From Palo Alto, CA
Joined Jan 15, 2007
754 points
Feb 25, 2012
smoke break, pitch 5 or 6 (or 7??) of Dark Shadows
Dan Dalton wrote:
Nice, 100% true except that lbs is actually a measurement of force (the acceleration in this case is 9.8m/s/s, gravity) whereas kg is a measurement of mass. You would "weigh" the same in kg here on earth, the moon, or Jupiter, but you would deffinitely weigh differently in lbs since the acceleration due to gravity is different!


It may depend on the field, but in engineering a pound is a unit of mass (like the kilogram and the slug). It becomes a unit of force when multiplied by gravity and divided by the gravitational constant, which, in earth's gravity, makes a pound mass equal to a pound force, but then it's called a "pound force", or lbf. Clear?

Kind of like the ounce, which is a unit of mass, and of which 16 make a pound (but is this a pound-force or a pound-mass? doesn't matter on Earth) except for liquids, for which we use the "fluid ounce", which is a unit of volume. So when a recipe calls for 3 ounces hops, you use the scale, and weigh it (which is an ounce force, but equal to an ounce mass if you're on earth), but when it calls for 3 ounces water, you use the measuring cup to determine volume. Clear?
Eric Krantz
From Black Hills
Joined Feb 21, 2004
469 points
Feb 25, 2012
Eric Krantz wrote:
It may depend on the field, but in engineering a pound is a unit of mass (like the kilogram and the slug).


I would say the reason other disciplines don't use the pound as a unit of mass is because it DOES depend on the field.
berl
From Seattle
Joined Apr 13, 2008
42 points
Feb 25, 2012
Flaming Pumpkin
JesseT wrote:
Now, for base ten calculations and unit conversions metric (sorry, it's SI now)...


The SI units of measurement are not the same thing as the metric units of measurement. They're close, but not the same.
Evan Sanders
From Westminster, CO
Joined Dec 10, 2010
145 points
Feb 25, 2012
Flaming Pumpkin
DannyUncanny wrote:
Seconds are defined by the frequency of cesium atoms.


That's really interesting, I didn't know that. Wouldn't it be the period of cesium atoms though? The units of frequency are essentially inverse seconds, not seconds.
Evan Sanders
From Westminster, CO
Joined Dec 10, 2010
145 points
Feb 26, 2012
25' drop...wheeeeee!
Evan Sanders wrote:
The SI units of measurement are not the same thing as the metric units of measurement. They're close, but not the same.


Yeah they are. SI is the current standardized state of the metric system, the term "the metric system" is a nickame for SI. They have eliminated some redundant units that can be defined in terms of others (volume is all done in terms of length: the liter is now a cubic decimeter). Also some things are defined differently (eg. a kg is defined in terms of the mass of a particular piece of Pt instead of being defined in terms of the mass of a L of water as it was traditionally) but that doesn't change the value of that unit, it just describes that value in a different way ("That which we call a rose by any other name would smell as sweet.").

bipm.org/utils/common/pdf/si_b...
JesseT
From Portland, OR
Joined May 5, 2011
114 points
Feb 26, 2012
Rock Wars, RRG, 2008
How is it that our climbing community is so rich with science and math nerds? Gotta love it. Woodchuck ATC
Joined Nov 29, 2007
3,091 points
Feb 26, 2012
Flaming Pumpkin
JesseT wrote:
Yeah they are. SI is the current standardized state of the metric system, the term "the metric system" is a nickame for SI. They have eliminated some redundant units that can be defined in terms of others (volume is all done in terms of length: the liter is now a cubic decimeter). Also some things are defined differently (eg. a kg is defined in terms of the mass of a particular piece of Pt instead of being defined in terms of the mass of a L of water as it was traditionally) but that doesn't change the value of that unit, it just describes that value in a different way ("That which we call a rose by any other name would smell as sweet."). bipm.org/utils/common/pdf/si_b...


No, they're not. Metric is used as a nickname for SI by people who don't know any better. SI is based off of the metric system, and is almost exactly alike, but they're aren't. SI includes metric and non metric units. Sure, both will use the unit of kg, but there are non SI units in the metric system, such as the knot, bar, dyne, erg, litre, tonne, stere, are, Celsius, etc. You could use them interchangeably, and for the most part it works, because it was based on the metric system, but there are differences.
Evan Sanders
From Westminster, CO
Joined Dec 10, 2010
145 points
Feb 27, 2012
25' drop...wheeeeee!
From the third page (numbered 95) of the document I linked to: "discuss and initiate the arrangements required to ensure the propagation and improvement of the International System of Units (SI), which is the modern form of the metric system."

Metric (now called SI) is a living system. To say SI isn't metric is like saying that the current version of the Constitution of the United States isn't the Constitution since it has amendments. /thread hijack

Over and out.
JesseT
From Portland, OR
Joined May 5, 2011
114 points
Feb 27, 2012
Flaming Pumpkin
JesseT wrote:
From the third page (numbered 95) of the document I linked to: "discuss and initiate the arrangements required to ensure the propagation and improvement of the International System of Units (SI), which is the modern form of the metric system." Metric (now called SI) is a living system. To say SI isn't metric is like saying that the current version of the Constitution of the United States isn't the Constitution since it has amendments. /thread hijack Over and out.


No.

To say that the metric system is the same thing as the SI system is like calling any previous generation of camalot a C4, just because the c4 is "modern" with a lot of similarities. It's just not true. Slamball has almost every single rule as basketball, but they are not the same.

Here's what you're failing to see. The SI system IS a metric system, but it is not THE metric system. There are many metric systems still in use that are not the SI system. The SI was put together for uniformity for a specific group of people (and obviously other people can use it too), making it a new system, not a replacement to every other metric system. Some exmaples:

Variations of the original French System
CGS
MKS
MTS
Gravitational (kilopond)
Evan Sanders
From Westminster, CO
Joined Dec 10, 2010
145 points
Feb 27, 2012
Yup, I can walk around my neighbours fields here in Germany drinking a litre of beer and see how many tonnes of grain heŽll get per hectare this summer. The litre, tonne and hectare are part of the metric system but not SI units. Jim Titt
From Germany
Joined Nov 10, 2009
195 points
Feb 27, 2012
yeego wrote:
Does anyone know the history of kilo newtons (KN)? Where? How? When? Appreciate it.


This was one entertaining thread.

Two key concepts to take from it -
- mass is measured in kg ( lb, lbm, pounds of mass)
- force is measured in N, Newtons ( or lbf, pounds of force)

Imagine you are pulling two weights by a rope, one is 50kg, other 100kg. The first one is accelerating at 20m/s^2, the other 10m/s^2.
As per Netown's 2nd law F=a*m, F is force, a is acceleration, m is mass
If you plug in the numbers, you will notice that the force on the rope is 1000 N in both cases, or 1kN. Incidentally, 20 m/s^2 is roughly 2g, 10 m/s^2 is 1g.

So, to answer your question - force specification for climbing gear is used since it allows to get rid of specific mass and acceleration - the one number includes both mass and acceleration.

SI units are used since most of the world uses them
amarius
Joined Feb 23, 2012
7 points
Administrator
Feb 28, 2012
Jay Tanzman redpointing Lethal Weapon. Photo by Hi...
shuminW wrote:
Can you imagine using a decimal system for time in place of what we have now?


Yes. Easily. Quickly, now: how many seconds in a decade?

Jay
jt512
Joined Mar 16, 2007
321 points
Feb 28, 2012
smoke break, pitch 5 or 6 (or 7??) of Dark Shadows
Evan Sanders wrote:
The SI system IS a metric system, but it is not THE metric system.


Oui. SI is completely contained within the "metric system", but the metric system is larger than just the SI system. It's like, all granite is rock, but not all rock is granite.

Jay wrote:
Yes. Easily. Quickly, now: how many seconds in a decade?


One hundred twenty. Duh.
Eric Krantz
From Black Hills
Joined Feb 21, 2004
469 points


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