That was supposed to be POUNDS.
I didn’t, google did. The only thing I got right was Ctl^A, Ctl^C, Ctl^V 
Can we still complain about making the Kessel run in twelve parsecs?
(I can’t believe nobody posted that yet. You guys are slipping).
lids and eight-balls
Yea, I was gonna come in here and say (essentially) the same thing.
I have always assumed that, most of the time, when someone uses the word “pound” (lb) as a unit of measure they are referring to a force. The SI unit for force is the Newton (N), and there is an exact conversion between pounds and Newtons. By definition,
1 lb = 4.4482216152605 N
To better communicate to the reader that you’re using pound as a unit of force, some people will use lbf or lb[sub]f[/sub] instead of lb.
Now let’s talk about mass. The SI unit for mass is the kilogram, but there is also a unit of mass called the pound. There is an exact conversion between pounds and kilograms. By definition,
1 lb = 0.45359237 kg
To better communicate to the reader that you’re using pound as a unit of mass, some people will use lbm, lb[sub]m[/sub], or ℔ instead of lb.
Megameters are used where appropriate, e.g. describing the size of features on the Sun (sunspots, active regions, loops, etc). It’s not very convenient for measuring anything much smaller. It would be OK for measuring long distances on earth, but it’s more convenient to just use km for all distance measurements, rather than switching between Mm and km.
Deca/deci (and hecto/centi) are rare because the engineering/metric convention is to favor prefixes that are 3 decades apart. The hecto, deca, deci and centi are only used in situations where one of these is an especially good fit for the range of values normally used, or close to a traditionally used unit. For example, hectopascal is used for weather records because it is equivalent to the millibar, which has been in use since before the SI system. People’s heights are often measured in centimeters because it’s a good match (1cm precision is just about right for this purpose, so everyone’s heights can be expressed as 2 to 3 digit integers).
In my experience most of the time, when someone uses the word “pound” (lb) as a unit of measure they are referring to a gravitational unit of force. The gravitational metric unit of force is the kilogram-force or kilopond and not the Newton. It is less useful when doing Newtonian math which assumes an inertial frame which is why we developed the newton and slug as there is no gravitational acceleration to use in an inertial frame.
In the context of the surface of the earth the gravitational weight is a good approximation of the gravitational weight. It is also typically a good enough approximation of a force to use in other non inertial frames like the fictitious force felt in a rotating frame like a centrifuge. The idea that gravitational mass is the same thing as internal mass is one of Einstein’s biggest discoveries. The pound force and kilopond would stay just as valid as the newton or slug if you tie the small g to the defined SI standard gravity value. It is only when you use local gravity that it would change based on location or in the case of a free fall frame, be unreproducible.
The difficulty is that people typically try to conflate the concept of inertial mass with some measure of the matter or at least a sum of it’s parts. The fact that the SI unit for “stuff” (mole) is intrinsically defined off of the kilogram is where the claim that a kilogram is a kilogram everywhere is wrong.
If you took the prototype Kilogram to Jupiter it would have the same comparative inertial mass to another object did when compared to an item you also compared it to on earth because both would have similar total energies. If you only took one item that had the same mass as the IPK to Jupiter and produced another artifact that had a same inertial mass that 3rd item would probably have a same comparative weight when you bring it back to the earth. But if you took that copy prototype to Jupiter then tried to derive the units based off that inertial mass they would differ. It would still probably be a close enough approximation if you were designing a parachute but would have sever implications if you were trying to build a supercollider.
If the kilogram was defined as the invariant mass, rest mass, intrinsic mass, proper mass this would not be an issue at all. The inertia of an object, which a KG is a measure changes with it’s total energy. A relaxed spring has less inertial mass than a compressed one, a water molecule has less inertial mass than two hydrogen and one oxygen does and the hydrogen nucleus has less mass than a free proton and neutron do. Even if we look inside the proton the valence quarks only make up less than 1/100th of the weight of the proton.
The measure of inertial mass measures the total energy of that system and while it may be a very useful approximation in most uses it is not an invariant measure.
Note that this is even true at the invariant mass, rest mass, intrinsic mass or just mass level too.
2 * up quarks @ 1.8 to 3.0 MeV/c2
+
down quark @ 4.5-5.3 MeV/c2
~ 8.1 to 11.3 MeV/c2
Yet a proton is ~938 MeV/c2.
Mass is not a count of matter even in an inertial frame. It is an emergent property describing the relativistic object’s rest mass, total energy, and momentum in a bound system.
Once again the concept of both inertial mass and invariant mass are absolutely useful and I am not trying to explain them away. But at least from my frame of reference the main problem is that people think a quantity describing the inertial properties of a bounded system is an invariant count of the contents of that bounded system and that they can thus assume that the relation will always apply.
It may not be a big difference in day to day calculations but it is a very serious impediment to most people when they try to learn about relativity or QFT.
[QUOTE=rat avatar]
…weight and mass are fundamentally the same thing…
[/QUOTE]
[QUOTE=rat avatar]
I will totally pick up a vizor and some crocs if you can prove to me that GR is wrong…I look forward to seeing The Niply Elder become a nobel laureate.
[/QUOTE]
Strawman much? I have no need to pick apart GR as I’m not contesting its validity. I’m stating that you made an incorrect statement in GQ. For all your copying and pasting of physics concepts you don’t understand, I hope you realize that saying weight and mass are “the same thing” yields a plethora of incorrect results in all types of equations, just like if you were to plug in lightyears as time, watts as energy, ohms as capacitance, etc. :rolleyes:
The fact that that you still don’t get that inertial mass and gravitational mass are the same thing demonstrates my point, it doesn’t refute it.
Inertial mass is the m in F=m a. It measures how an object reacts to acceleration.
Gravitational mass is the m in F = m1 m2 / r^2. It measures how objects attract each other by gravity.
However, the two are the same. Nobody seems to know why they are the same number, but they are. So there is really no reason to make the distinction, except when discussing theoretical physics and pontificating on why they are the same.
gawd. Nobody is arguing that inertial and gravitational mass aren’t the same thing. The fact that the are the same thing has jack shit to do with your incorrect statement that mass and weight are the same thing. There seems to be a nugget of ignorance lodged in your cranium that is awful difficult to knock out. :smack:
Only to a certain precision, but tell my oh so smarter than thow, if you take the IPK to the moon how are you going to derive the Newton from it?
Comparing it to other masses doesn’t really help you get any work done.
(clue you are going to have to calculate a correction based on local gravity)
Yet some how correcting the value of a earth gravitational measure of mass is impossible. When all that would be require to do is to say an “earth pound-force” or an “earth kilopond” it is not like we don’t know the acceleration factor we used.
So please demonstrate your superior knowledge, how are you going to calculate lux, the newton, or even the pascal with your transported KG?
(hint even the SI makes you correct for local gravity)
It appears to me that your whole bitch session here is over a factor of precision.
Here I will give you a video on a way to compare masses using the inertial balance.
Here is a video to show how it works, so all you have to do is to say how you derive that measurement to a locally useful one for newtons while ignoring the local little g.
And I did fall for your straw man, because even a pound-force is defined as the force exerted by gravity on a pound-mass in the standard gravitational field of 9.80665 m·s−2, if you do not correct for a change in gravitational field you are not measuring pounds-force.But when I said gravitational weight and inertial mass were the same thing it was always in the context that they were measuring the same fundamental property and you just admitted that they were the same in the last post.
So explain to me why the needs to convert to useful properties with the IPK doesn’t apply to the need to convert to local gravity for the pound force (or kilogram-force)
Wow. Just wow. How do you not understand this? Mass and weight are as different to each other as distance and velocity, energy and power, resistance and impedance, capacitance and inductance are to each other. They are physically and conceptually absolutely different entities from each other that cannot be assumed to be the same thing. This whole hilarious field trip into GR that you’ve taken just plainly displays how you don’t understand the slightest thing that you’re quoting. There isn’t a single equation in the entire of physics, whether it be classical, GR, SR, quantum physics, string theory that allows you to freely substitute weight and mass as if they’re the same thing, none. If you do, you will get nonsense results everytime. :rolleyes:
A newton is the force necessary to accelerate 1 kg by 1/299792458 of the speed of light per 9 192 631 770 cycles of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.
<nitpick> … (9 192 631 770 cycles)[sup]2[/sup] … </nitpick>
Oh burn burn
I should have added: weight is the F in F = m1 m2 / r^2. Obviously it’s different from m1, and doesn’t even have the same unit.
Ah geeez … is it my turn to be the Units Nazi today?
That’s becomes in fundamental SI units “kilograms squared per square meter”.
Do you mean F = G ( m[sub]1[/sub] m[sub]2[/sub] / r[sup]2[/sup] ) ?
Maybe I’m just being a classical kind of guy in relativistic world.
I just discovered the attoparsec. 