Talk:Mass versus weight: Difference between revisions

Content deleted Content added

Inline

Revision as of 23:00, 13 October 2010

weight v mass

It's always assumed that weight is proportional to mass. But is it exactly? Ignoring buoyancy, when accurately, does certain materials have the weight we would expect for its mass. Ie. Does gravely effect all mass the same way.

==== seb gibbs

Billiards on the Moon

re the billiard balls on the moon. I am a complete layperson but i assume that gravity on earth would increase the friction so the decreased friction on the moon would mean that the balls broke and moved more swiftly, although falling into the pockets more slowly. Any experts there who can answer this question? 89.241.254.21 14:54, 9 November 2007 (UTC)[reply]

F<=uR is a good approximation for friction (u should be 'mu', the coefficient of friction). In this case, the maximum friction F on a billiard ball would be lower on the moon because the reaction R would be lower, in turn because the weight of the ball would be less. It should be noted that other effects (e.g. lack of drag due to lack of atmosphere) also come into play. In future, Wikipedia:Reference_desk may be able to answer your question. Hope this helps. Sheffield Steel^talk_stalk 16:32, 9 November 2007 (UTC)[reply]

The balls would also be much more inclined to rebound out of the pockets. A “Moon-grade” billiards table would probably have much enlarged pockets to compensate for this. Of course, all of these are subtleties that go far beyond the basic principal being conveyed. And technically, during a break shot, where the racked billiard balls are typically touching each other (or at most a few millimeters away from each other), the opportunity for meaningful differences in friction to come into play as the kinetic energy is distributed is effectively nonexistent (except for the speed of the incoming cue ball). The racked balls’ initial behavior would be very similar to that kinetic demo game where six steel balls hang from strings and “click-clack” back and forth. So, no, they would not ‘break with more swiftness’. Indeed though, they would do better on the Moon at retaining the finite kinetic energy (speed) the farther they got from their racked positions due to less rolling resistance; as Sheffield pointed out, the magnitude of this effect can be calculated. As for air friction, I was imagining the game being played on the Moon indoors in a shirt-sleeve environment (what do astronauts on a Moon base do in their spare time?). Greg L (my talk) 20:25, 9 November 2007 (UTC)[reply]

oops :-) Serves me right for answering the question in vacuo rather than reading the relevant part of the article and getting the context. Sheffield Steel^talk_stalk 20:51, 9 November 2007 (UTC)[reply]

That nasty bouyancy

While the section on bouyancy is nice, the next paragraph right after claims that a doctor's scale measures actual mass. Presumably it means conventional mass? Or perhaps it's alluding to the fact that the precision is so bad for that scale that it doesn't matter? If so, qualifiers are needed.

Even worse is the next paragraph which claims that lunar scales balances would work the same as on Earth. Yes, if both are in vacuum, but only then. Even if you put them both in atmosphere (say, compare scale-weight on your moon-base), you're not going to measure the same mass-number unless you're weighing two things of equal density, or else you happen to have the density of your atmosphere in your base about 6 times that of Earth-normal (as the ratio of g's), to compensate for the reduced weight-error produced by bouyancy produced on the moon. Achh! More qualifiers needed. But I thought I'd better bring it up here so you can decide what you want to do. I'll add a bit on the problem of measuring unknown masses in air to micro-precision. S B H arris 06:19, 1 February 2008 (UTC)[reply]

The point being made about dual-pan devices like doctors’ scales is that if you move them from the equator of Earth to the poles, you aren’t going to have any change on the measured value. However, a single-span spring scale that reads 100.0 kg at ±50.5° latitude, will read 99.795 kg at the equator and 100.139 kg at the poles. These differences are due to the differing magnitude of the centrifugal force of Earth’s rotation. So spring scales technically and truly measure force. Dual-pan scales, like doctors’ scales, are true mass comparators. Of course, few scales are accurate to a tenth of a percent. And the article makes it clear that these are technical differences—not practical ones after a scale has been calibrated.
I revised the offending paragraph so it no longer uses the Moon as an example in order to avoid mixing possible buoyancy effects into the equation. I had actually intended the Moon example to form a mental picture of an indoor example of a moon lab (shirtsleeve environment, with air and all)—same as the billiards example. So I changed the example to one of moving a dual-pan balance from the equator to the poles of Earth. Greg L (talk) 21:10, 29 September 2008 (UTC)[reply]

Mass, weight, and the effects of gravity

According to Einstein's theories of relativity, mass changes with acceleration(e.g., infinite mass at light speed). Also according to the aforementioned theories, gravity is functionally identical to acceleration. Therefore mass should change with a change in gravity. Is there something I'm missing here? Skylerorlando (talk) 01:50, 2 April 2008 (UTC)[reply]

Yes. Mass changes in relativity according to velocity, not acceleration (they are different things). And you don't get infinite mass at light speed because you can't get to light speed. And even the mass change as you get close to light speed is a function of inertial frame (ie, of the viewer), and so in some sense is not "real" (see mass and mass in special relativity). By "real" I mean it's not something that all observers agree on, and the change it makes in the gravitational field of the object, is mostly to distort it rather than to make it "bigger" (that is, it gets stronger in one direction, but weaker in another, and it can't cause the object to collapse into a black hole). See mass in general relativity. For this reason, most physicists have stopped calling this mass increase a mass increase, and prefer to say that the mass does NOT increase, but the energy and momentum do. The more "real" mass which all observers agree on, in special relativity, is called invariant mass, and it does not change with velocity. S B H arris 05:23, 2 April 2008 (UTC)[reply]

Scales and historical use of weight

There is something rather amusing with having an article on weight vs mass finish off with a section featuring a balance scale and asking what it measures. It of course measures what *today* we call "mass". But a thousand years ago, we didn't call it mass - we called it "weight" because the term mass hadn't even entered the language, and using a balance scale-type device was about the only reliable way we had then to measure the mass/weight of anything.

Now let's set up this hypothetical scenario: go back in time a thousand years, pluck some unfortunate merchant with nothing but his scale and weights and bring him forward to the present. Then send him to the moon with his equipment. Give him some moon rocks and other items to measure. He'll tell you what they "weigh" in relation to his weights. Bring him and all the same items back to the Earth and have him measure them again. He of course is going to give you the same answers, telling you that they "weigh" exactly the same as they did on the moon. He might tell you that they "felt" a lot lighter, that they were easier to pick up on the moon, and so on, but he is not going to tell you that those items weigh only a sixth on the moon what they do on earth. He's not going to understand why they (and himself) felt lighter (or not as heavy) on the moon, but he is also not going to think their weight is any less. Indeed, if you tried to tell him that they weighed a sixth on the moon what they do on earth, he'd probably think you were trying to swindle him. No one else from his era would agree to accept one sixth the payment for produce or goods on the moon just because it all felt lighter because it's still the same amount of stuff that weighs the same as on Earth

Now we can't say they're wrong - it's their language and their concept of weight, after all. What's wrong is we, and in particular the sciences, sometime in the last three centuries, decided that weight no longer meant weight and that a new term, mass, would replace what weight had hitherto meant and that weight would now come to mean something it never had: a force.

If all this isn't enough, the section also talks about a spring scale and what it measures. It measures something that we've only been able to measure for a relatively short period of time: gravitational force. Yet we call this "weight", the same term that has existed for centuries as if our merchants of a thousand years ago were spending their time measuring gravitational force (Lord only knows with what). And nowadays the preferred unit provided by these scales is the kilogram - a unit of mass. The mind boggles at the ironic stupidity of it all.

So cut the arguing; weight = mass = weight. Weight is not a force. That's a recent change of definition that makes no historic sense. It's the old (and common) term for mass and the two should be treated as the synonyms they still are by most people.

As to what to call "weight", i.e. gravitational force? Try "heaviness". Objects are one sixth as heavy on the moon. It fits with everyone's understanding of the role of gravity in altering the force required to lift a given object. D P J (talk) 06:40, 17 January 2010 (UTC)[reply]

Wow. I don’t know where to begin. You are entirely wrong on all counts.
Quoting you: But a thousand years ago… and Now let's set up this hypothetical scenario: go back in time a thousand years, pluck some unfortunate merchant… and What's wrong is we, and in particular the sciences, sometime in the last three centuries… Irrelevant; thousands of years ago, there were four elements: earth, air, water, and fire; science marches on. In science—today—mass is an inertial property and weight is a force.

Period.

Quoting you further: [A spring scale] measures something that we've only been able to measure for a relatively short period of time: gravitational force. Hmmm… it depends on how one defines “short period of time.” How about “since at least about 1850,” which is so far back, the second law of thermodynamics was just being discovered.
Quoting you: As to what to call "weight", i.e. gravitational force? Try "heaviness". Good for you; you’re getting close (but backwards). According to Websters “weight” is defined as follows:

weight: relative heaviness, the force with which a body is attracted toward the earth or a celestial body by gravitation and which is equal to the product of the mass and the local gravitational acceleration

Note the word “force” in the above. So, “heaviness” is used to help define and describe “weight” in science. I’m an engineer. The discipline of engineering has terms like “weight loading.” I haven’t come across the term “heaviness loading” (although a Google search turned up two hits, one of which referred to body building and lifting weights). Note too that masses in orbit are considered to be “weightless,” not “heavinessless.”

I suggest you go get the rest of the world to adopt your suggestions such as “heaviness loading.” Why? Because Wikipedia reflects the way the world really works as evidenced by modern reliable sources and doesn’t try to lead by example with bright ideas like those you have advanced here.

Note too that Websters defines mass as follows:

mass: the property of a body that is a measure of its inertia

…which is the proper scientific definition. And then Websters adds this bit:

…and that is commonly taken as a measure of the amount of material it contains and causes it to have weight in a gravitational field

…which is exactly what this article states about the common, non-scientific way people perceive the mass of an object (by its weight since gravity is so profoundly ubiquitous on Earth).

Quoting you: Weight is not a force. Sorry, but stating an absurd falsehood in only five, succinct words does not make it any less false; it only makes it look more foolish. You can take up your theory with the ISO. The standard ISO 31-3 (1992) defines “weight” as follows:

The weight of a body in a specified reference system is that force which, when applied to the body, would give it an acceleration equal to the local acceleration of free fall in that reference system.

There is also this from the National Physical Laboratory: What are the differences between mass, weight, force and load? (FAQ - Mass & Density), which states the following about the definition of weight:

Scientifically, however, it is normal to state that the weight of a body is the gravitational force acting on it and hence it should be measured in newtons (abbreviation N), and that this force depends on the local acceleration due to gravity.

Note that this Wikipedia article couldn’t possibly be clearer that it is discussing the distinction in the context of the physical sciences. Accordingly, with regard your allegation that “Weight is not a force”, the world of science says “Ahhh, gee… thanks, but no thanks.”

Nutshell alert: If you take a spring scale or gravimeter, calibrate it on Earth, and take it to the moon, it will show one-sixth weight, which is one-sixth gravity, which is one-sixth force. If you do the same with a knife-edge balance, it will show the same reading no matter what celestial object you travel to. Note that NASA routinely measures the mass of astronauts on the International Space Station by measuring their inertia on an oscillating sled. This article explains how the Body Mass Measuring Device works and how “weightless” astronauts still retain their “mass.” Please, no whining about the way the world of science really works and how it uses these words to maintain scientific rigor.

Quoting you one last time: The mind boggles at the ironic stupidity of it all. Fine, you established that you have really high self esteem. That doesn’t make your arguments any less false in the face of clear and copious evidence to the contrary. Greg L (talk) 16:35, 12 June 2010 (UTC)[reply]

Bouyancy again

At risk of repeating the same remarks I made on TALK:weight, it is rather silly to simply say that "bouyancy" makes things weigh less, without some kind of qualification. Bouyancy makes things "weigh less," as measured by spring scales DIRECTLY UNDER THEM. But that is all. Nevertheless, their total weight is the same-- it is merely spread out over a larger surface area which isn't making contact with the scales. If it were making contact, you'd see it hadn't changed.

In short, "bouyancy" decreases weight in exactly the way any suspensition device does. A man in a harness partly supported by a crane, will weigh less according to a bathroom scale underneath him. But does he REALLY weigh less? His weight is simply transfered to the crane, and would be measured if that where on scales. We now come down to what we MEAN by "weight." Is it just what the scale right underneath us measures?

There's a photo in the article of an object, buoyed in a fluid in a graduated cylinder. Now, put that whole graduated cylinder on a scale. How much does the object weigh now? If it's floating, you'll say "nothing." But I promise you, that if you add an object to a cylinder of fluid on the scale, you'll get the same weight difference as you would if you simply put it on the scale directly. The reason is the scale now measures the entire fluid pressure, not just the downward force directly under the object. Similarly, the "bouyancy" decrease in weight, in all cases where you "see" it, is simply an instrument failure. S B H arris 19:39, 13 June 2010 (UTC)[reply]

I see your point. If one puts a scale under the earth’s entire atmosphere and then put the balloon into the atmosphere, the scale would read the full weight of the balloon. The same applies to mass standards, where they compensate for the 150 ppm difference due to buoyancy. Alas, it is too difficult to weigh the entire atmosphere of the planet. Nevertheless, I’m gonna go back and see if I can add more scientific rigor per what you wrote here. Greg L (talk) 03:48, 14 June 2010 (UTC)[reply]

P.S. (thinking aloud here). The qualifier is “it” weighs less. If you put a neutrally buoyant swimmer in a pool, the “the swimmer” (what you put on the scale) weighs nothing. Indeed, if you weigh the entire pool (or the entire freaking atmosphere of Earth), you will see that the fluid/object system exhibits the full added weight of the buoyant object. I’ll think about this nuance for a bit—maybe overnight—because I want to get this into it without that “he said / she said” flavor that quickly comes to articles that have been hacked on in a collaborative writing environment. The flip side of getting too aggressive with this point is that a 100-gram mass standard (those little ultra-polished stainless steel things) actually weigh 150 ppm less because of buoyancy. One could write this:

Really, the entire building surrounding the metrology lab and all the air inside the building weighs the full 0.980665 newtons heavier (and not 0.980518 newtons) when a 100-gram mass standard is put inside the building because the building supports the air and the air supports the mass standard. But you have to measure the building very quickly because a little more air will leak out of the building than leaks into the building and soon, ever-larger volumes of air supports the mass standard.

This effect comes out a Mythbusters show, where a sealed cage didn’t become any lighter after birds lifted off their perches and flew around inside. What is true is that buoyancy reduces the weight of objects. Your point is that buoyancy doesn’t reduce the weight of the object and the fluid supporting the object. It’s probably worth mentioning that. Gonna think a bit here… I think I can get this point across very simply. Greg L (talk) 04:13, 14 June 2010 (UTC)[reply]

P.P.S. Done. I think… (∆ here). Fifth paragraph down at Buoyancy and weight. Let me know if I missed the mark. Greg L (talk) 15:09, 14 June 2010 (UTC)[reply]

@@ Line 1: / Line 1: @@
+==weight v mass==
+It's always assumed that weight is proportional to mass. But is it exactly?  Ignoring buoyancy, when accurately, does certain materials have the weight we would expect for its mass.  Ie. Does gravely effect all mass the same way.
+==== seb gibbs
 ==Billiards on the Moon==
 * re the billiard balls on the moon. I am a complete layperson but i assume that gravity on earth would increase the friction so the decreased friction on the moon would mean that the balls broke and moved more swiftly, although falling into the pockets more slowly. Any experts there who can answer this question? [[User:89.241.254.21|89.241.254.21]] 14:54, 9 November 2007 (UTC)