Wikipedia:Reference desk/Archives/Science/2016 September 27

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September 27

Can acids be oxidized?

Can acids be oxidized? — Preceding unsigned comment added by 31.4.156.140 (talk) 00:42, 27 September 2016 (UTC)

Some of 'em, sure. You want to narrow down the question a bit? Is this homework? --Trovatore (talk) 00:55, 27 September 2016 (UTC)

Some can. For example, oxalic acid will be quite readily oxidised by permanganate to carbon dioxide. Double sharp (talk) 02:56, 27 September 2016 (UTC)

Feynman Lectures. 13–3Summation of energy [1]

Explain please in simple words how to distinguish the work done by the force on the object and the work done against the force. At the end of chapter 13–3 Feynman invites to calculate work needed to bring the objects to certain distances from each other. And he gets $W_{12}=-Gm_{1}m_{2}/r_{12}$. I understand it next way:
Axis is directed to the moving body (body is moved from infinity).
$T_{2}-T_{1}=W_{by\,force}=\int _{+\infty }^{r_{12}}-{\tfrac {Gm_{1}m_{2}}{r^{2}}}dr=+Gm_{1}m_{2}({\tfrac {1}{r_{12}}}-{\tfrac {1}{\infty }})={\tfrac {Gm_{1}m_{2}}{r_{12}}};$
$W_{against\,force}=\int _{+\infty }^{r_{12}}+{\tfrac {Gm_{1}m_{2}}{r^{2}}}dr=-Gm_{1}m_{2}({\tfrac {1}{r_{12}}}-{\tfrac {1}{\infty }})=-{\tfrac {Gm_{1}m_{2}}{r_{12}}}.$ So work against the force is work done by the other force (created by man). But in this case there is no change in K.E. because sum of the forces is zero, so the body in moved with constant velocity. Is it correct? Username160611000000 (talk) 04:21, 27 September 2016 (UTC)

• Maybe I did not understand you. But when you say there is no change in K.E. because sum of the forces is zero, so the body in moved with constant velocity, I suspect you summed the force the body is subject to and the force the body exerts over the rest of the world. It is then no surprise that the result cancels out (the work is just a well-chosen integral of the force). In the energy form, it just mean conservative forces conserve energy (for a well-chosen potential energy), again not very surprising. (But it does not mean the kinetic energy of the body is constant!)

As a general rule, diving into the calculus pool is a bad idea when you do not have a clear idea of what you are trying to calculate. Tigraan^{Click here to contact me} 11:49, 27 September 2016 (UTC)

• • I suspect you summed the force the body is subject to and the force the body exerts over the rest of the world. No, the force with which the body exerts on the man is not taken into account. "Against force" must be equal and opposite to original force all the time. Is this "against force" really applied to the body? If yes, then "against force" and original force are both applied to same body and gives zero sum.Username160611000000 (talk) 15:31, 27 September 2016 (UTC)

I don't think there has to be no change in kinetic energy - it's simply irrelevant to a calculation of potential energy that specifically excludes it. We're looking here at the energy the object has because of where it is, and never mind how fast it's going. (This means we're calculating within some static frame, and we can think of this as a motion that begins and ends with the object not moving relative to it) Since the object begins and ends at rest, and in a sense is at rest throughout its entire motion (since we don't care how fast it's going for a PE calculation) we can indeed think of the force doing the work and the force against which the work is done as being in balance, I think. It's like two sides of a tug of war contest - they see the same work done, but have differing opinions of its sign. Wnt (talk) 13:23, 27 September 2016 (UTC)

What I do not exactly understand is:

1) Is "against force" (which enters into the equation of work done against original force) real?

2) Why to calculate total work man has to take bodies from infinity? Why he don't take bodies from zero point. E.g. we have all bodies in one point and then push them away from each other to chosen distances. Username160611000000 (talk) 05:15, 28 September 2016 (UTC)

@Username160611000000: Well, in the math above you see the 1/infinity term reduces to zero, so when you calculate to infinity you're taking one term, while if you calculate to anywhere else you're evaluating two. So evaluating to infinity is half of a two-part calculation ... the other half is to evaluate from your nearby end point to infinity and subtract that. As for the "against force", well, it doesn't have to be real - you can have a single force that simply speeds up an object, like if you drop a stone and it accelerates at the ground. I think generally people would avoid saying that the gravity is the force doing positive work (since there's no force there having work done against it). There the energy obtainable on impact is balanced against a negative potential energy change and hence negative work is done with a release of energy. Wnt (talk) 10:03, 28 September 2016 (UTC)