User talk:MarcusMaximus

If you think the article Newton's laws of motion is confusing...

...then you absolutely need to take a look at Force#Newton's second law. It even says ${\displaystyle {\vec {F}}={\frac {\mathrm {d} m}{\mathrm {dt} }}{\frac {\mathrm {d} {\vec {x}}}{\mathrm {dt} }}+m{\frac {\mathrm {d} ^{2}{\vec {x}}}{\mathrm {dt} ^{2}}}}$; yes, there are many books saying that, but at least they do bother to clarify that in order for that formula to hold when dm/dt is nonzero, the "dm" in question must be stationary. Since you appear to have clearer ideas than me about these issues, could you take a look at it? -- 11:02, 12 September 2008 (UTC)

I took a stab at it. Let me know what you think. MarcusMaximus (talk) 01:15, 13 September 2008 (UTC)
Just another note, in case you are still a bit cloudy on our earlier discussion of stationary dm's. The fact that any particular equation of motion only holds when the velocity of one of its constituents is zero (which effectively restricts its validity to a single inertial reference frame) indicates that there is a missing term in your equation that happens to have a value of zero in that particular inertial frame, but is nonzero in general (think of vedm/dt). This is pretty good evidence that the equation needs another term in it to become a general equation of motion for the system, and I would discourage anyone from trying to use frame-dependent equations of motion. It's just asking for trouble. MarcusMaximus (talk) 01:25, 13 September 2008 (UTC)
I was not advocating discussing the variable mass case in the article (at most, we colud add one sentence linking to Thrust) — I was just pointing out that the texts which do use that "semplification" explicitly state its limits of validity. BTW, yes, your edits are all right. -- 01:51, 13 September 2008 (UTC)

Re: Classical mechanics

I did a history merge to fix a cut and paste move from six years ago, so the entire history of the article is now in one place. Graham87 05:44, 2 October 2008 (UTC)

Most screen readers these days have navigation commands to allow blind users to navigate the Internet as efficiently as their sighted counterparts. There are keys for navigating to things like headings, frames, lists, etc. Unfortunately, computer training for the blind is usually of very poor quality, so most blind users never learn to make full use of their computers or the Internet. I taught myself most of what I know about navigating the Internet, because I wanted to get more out of it. I've been on Wikipedia for so long now that I navigate it on autopilot, so any interface change is disruptive for me. Graham87 06:02, 2 October 2008 (UTC)

You are right. It's completely my fault because I was going to change USC nationships from 7 to 11 but I saw a warning. I thought it was real so I try to change other school. I'm not really experienced at editing so thanks for the warning and not a ban. —Preceding unsigned comment added by Im an asian (talkcontribs) 03:56, 19 October 2008 (UTC)

Vector inequality

Hi Marcus, thanks for spotting that my addition to Euclidean vector was out of place. Would vector inequalities instead be relevant to coordinate vectors? The problem with that article is that I really don't see how to add a link to the inequality article. Perhaps someone (I?) could expand it to have a nice list of properties, as the Euclidean version does. The reason I want to add a link is that I was looking for the definition of vector inequality, and I couldn't find it in any of the vector articles. I'm open to any suggestions; thanks for your time! Cheers, Oliphaunt (talk) 10:13, 5 November 2008 (UTC)

Yes, I think vector inequality is more applicable to coordinate vectors or to mathematical vector spaces generally. It's not really defined to say that a directed-quantity type vector as described in the Euclidean vector article is "greater than" or "less than" another such vector, since in general, they point in different directions. MarcusMaximus (talk) 05:17, 7 November 2008 (UTC)

Nonconstant mass in Newton's 2nd Law

Yes, I see your point. I have to check my reference to see if and where I went wrong. While I work on that, care to weigh in on Simpler introduction. -AndrewDressel (talk) 13:57, 8 June 2009 (UTC)

Equation in Newton's laws of motion

I now realise that many people prefer to restrict the second law to constant mass. (I'm convinced that it can be used for changing mass under some circumstances, but it is probably best avoided because of the high risk of mis-use.) I therefore reverted my second edit. However, we cannot write ${\displaystyle {\mathrm {d} (m\mathbf {v} ) \over \mathrm {d} t}=m\ {\frac {\mathrm {d} \mathbf {v} }{\mathrm {d} t}}}$ without qualification since the statement is demonstrably false unless we specifically specify that the mass is constant. I hope you will be happy to leave this clarification (and I'll be happy not to meddle with the rest of the article). Dbfirs 06:42, 7 July 2009 (UTC)

It is stated in the second sentence right after the equation. If you think it should be more obvious, by all means change it. I thought you had intended to revert your edits but missed that one. Although I do think the place you had put the comment about constant mass wasn't necessarily the best stylistically. MarcusMaximus (talk) 06:44, 7 July 2009 (UTC)
I agree that the constant mass is clearly explained in the text, but the mathematical equation also needs the qualifier to maintain truth. I've modified the comment. Dbfirs 07:10, 8 July 2009 (UTC)
The article is now clear and accurate without the above equation at all. Why didn't I think of removing it? Thanks. Dbfirs 07:19, 18 July 2009 (UTC)
I'm interested to hear about the use of F=d(mv)/dt with varying mass systems. So far I haven't found any examples that work correctly that aren't pathological or trivial cases. Do you have a good one? Thanks, MarcusMaximus (talk) 09:07, 18 July 2009 (UTC)
I was convinced that I had used F=mdv/dt+vdm/dt to solve real (non-relativistic) problems many years ago, but I can't recall the details, and, the more I think about it, the more I see your point of view about all situations simplifying to constant mass problems, and the only use of F=d(mv)/dt being when integrated to give an impulse equation. I'll let you know if I think of an appropriate classical mechanics situation where it can be correctly used. Dbfirs 11:17, 18 July 2009 (UTC)

Bold move

I'm afraid I don't understand your objection to my move. If by "vector-valued function" one understands the usual notion as one might encounter in, say, an undergraduate calculus or physics course—a Euclidean vector that depends on a time parameter—then the content is clearly more suited to an article dedicated to the notion of a vector that varies in time than to an article whose subject is vectors in themselves. One only needs to look at the article vector-valued function to see that it is indeed germane.

As for arguing that large portions of text in the Euclidean vector article should be devoted to the discussion of vector-valued functions, probably a little more could be said than at the present, but we should try to observe WP:Summary style, and just convey the essential information. Volumes of text could also be brought in on vector fields, and for that matter, a detailed treatment of vector calculus could also be presented. However, these other topics are rightly treated in their own separate articles, as I think should detailed treatments of the calculus of vectors. Sławomir Biały (talk) 02:12, 4 August 2009 (UTC)

I can appreciate that feeling of pride too, and it is certainly deserved. I also agree that the vector-valued function article clearly needs improvement. Sławomir Biały (talk) 03:03, 4 August 2009 (UTC)

Schmoordinate system

I'll do what you asked. In the meantime, would you mind to explain what is the meaning of the expression "Schmoordinate system"? Paolo.dL (talk) 11:14, 10 July 2011 (UTC)

"Coordinate system-schmoordinate system" is a form of a Yiddish-English rhyming compound, a figure of speech that implies a dismissive attitude. I meant it as a light-hearted joke. See here[1] and item 5 here[2], for further explanation. MarcusMaximus (talk) 07:28, 11 July 2011 (UTC)
Thank you for your detailed explanation. Paolo.dL (talk) 10:39, 11 July 2011 (UTC)
I like your edits. MarcusMaximus (talk) 03:34, 12 July 2011 (UTC)
Thank you. By the way, your comments on the talk page were very interesting. I had forgotten them. Paolo.dL (talk) 09:00, 12 July 2011 (UTC)