Talk:Mercury mirror

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Several comments about this article:

1. A small suggestion: instead of using the moment of inertia, one can simply say $$E_{rot}=\frac{1}{2}mv^2=\frac{1}{2}mr^2\omega^2$$

2. The sentence "the slope of the surface of the dish redirects the force of gravity" is very vague, and possibly does not have any physical meaning. Please clarify.

3. Has anybody ever heard about the potential energy of a centripetal force? :) No, because it is not a conservative force.

4. It is not clear why the gravitational potential energy is ignored. Or is it included in the "potential energy of the centripetal force"? (If the latter is the case, please tell us what other energies are included).

5. Why wouldn't the article provide a simple method for calculating the shape of the mercury as a function of $$\omega$$? This can be done by working in a reference frame rotating together with the mirror. In that frame, the liquid is at equilibrium, and the height of the mercury as a function of radius can be easily calculated by requiring that the surface is equipotential, i.e. the sum of the potential energies due to gravity and due to the centrifugal force is constant.

Yevgeny Kats 22:43, 29 January 2006 (UTC)


 * Of course the centripetal force in the case of the parabolic dish is a conservative force: the difference in potential energy between two points is path-independent.


 * There are two energies involved: gravitational potential energy (height), and kinetic energy.


 * Indirectly, the force of gravity provides the required centripetal force. The required centripetal force is at right angles tho te direction of gravity, so gravity cannot provide the centripetal force directly. Here, the normal force is the go-between.


 * For the purpose of calculation it makes no difference how you calculate of course; for calculation the only things that matter are whether the correct outcome is obtained, and calculational efficiency.
 * For the purpose of physics education, the approach in terms of an unbalanced centripetal force must be used. In the case of the rotating mercury mirror, there is no centrifugal force that is being exerted on the merury. If a centrifugal force would be exerted on the mercury, then it would all fly over the rim of the dish. --Cleonis | Talk 01:41, 30 January 2006 (UTC)

Merge suggestion
This article is clearly a subset of the liquid mirror article, itself a merge of an earlier version with liquid mirror telescope. The three topics are practically identical, but liquid mirror is both the most general, and the most complete. However, it currently lacks the theory description found in this article. -- Securiger (talk) 03:02, 9 November 2008 (UTC)