Talk:Mass versus weight

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[edit] 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)
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 Steeltalkstalk 16:32, 9 November 2007 (UTC)
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)
oops :-) Serves me right for answering the question in vacuo rather than reading the relevant part of the article and getting the context. Sheffield Steeltalkstalk 20:51, 9 November 2007 (UTC)

[edit] See Also section

The See Also section has lots and lots of links, and is therefore a bit cumbersome. I think it makes sense to keep the links to ones about the concept of mass and weight. I would keep only Apparent weight, Inertia, Mass and Weight of the current links Enuja (talk)03:45, 29 November 2007 (UTC)

OK, I trimmed it to what seems like a reasonable compromise. I think everything there now is limited to topics that readers would want pursue for further exploration. Good suggestion. Thanks. Greg L (my talk) 03:57, 29 November 2007 (UTC)
How about removing SI base units and SI derived units because SI is in the section? That's a compromise I'd be happy with. Enuja (talk) 04:20, 29 November 2007 (UTC)
Short is sweet? OK. Greg L (my talk) 04:25, 29 November 2007 (UTC)
I'm happy to see Gravimetry back; the section feels more useful now, even though it's still quite extensive, that's not always a bad thing. Enuja (talk) 04:40, 29 November 2007 (UTC)
Thanks, but the Converting units of mass to equivalent forces on Earth section has always been there—both back when it was in Kilogram (where it was titled Converting to kilogram-force and newtons) as well as when it was first placed here. All I did was add the graphic later this evening. That makes the subsection stand out, doesn’t it? Greg L (my talk) 05:08, 29 November 2007 (UTC)
I was still talking about links in the see also section. But, yes, that image is excellent in that section. Enuja (talk) 05:40, 29 November 2007 (UTC)
Oh, yes. Of course. Greg L (my talk) 06:21, 29 November 2007 (UTC)

[edit] 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. SBHarris 06:19, 1 February 2008 (UTC)

[edit] 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)

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. SBHarris 05:23, 2 April 2008 (UTC)