Wikipedia:Reference desk/Archives/Science/2017 April 3

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April 3

type of orbit?

Curious to find out if there is a name for the orbit path followed by, say, a tire swing hanging from a single point. The swing follows an elliptical path but that ellipse rotates. For example, you may start out swinging the tire in an ellipse where the co-vertex is closest to a tree trunk, but eventually, the ellipse rotates so that the vertex is now closest to the tree trunk, causing the swing to eventually hit the tree. What is this called? Thank you! — Preceding unsigned comment added by CGappinger (talk • contribs) 02:58, 3 April 2017 (UTC)

Precession is what you're looking for, I think.--jpgordon^{𝄢𝄆 𝄐𝄇} 03:57, 3 April 2017 (UTC)

Not sure about that. Precession does not affect the location of the apsides. Our article apsidal precession could do with some work - the word "precession" means "moving backwards" but the phenomenon is generally referred to as "advancement of the perihelion." 81.129.14.0 (talk) 08:41, 3 April 2017 (UTC)

Precession unqualified is more commonly axial precession. Sagittarian Milky Way (talk) 13:08, 3 April 2017 (UTC)

I think the original querant is seeking the Lissajous orbit - or at the very least, an orbit with variable phase in its argument of reference longitude.

A different but related curve is the Lissajous curve.

Mathematically, these are "the same exact thing" in different coordinate-spaces - if your parametric perturbation is rotational and monotonically increasing, you get an elliptic trajectory whose apsis rotates. In practice, you might see something "similar to" a Lissajous curve any time that you have an elliptic motion that gets additionally modulated by a time-varying parameter - often caused by the physical process of precession, or by friction, or any other perturbation.

Depending on how complicated you want to make things, a tire-swing can be described as a simple harmonic oscillator, but if you displace the tire a lot by swinging very high, you can make it an imperfect (non-harmonic) oscillator; if you use a stretchy- or bendy- or heavy- rope, you add additional complexity to the equations; if you consider air resistance, rope torsion, wobbling, and rotational inertia of the tire, you get even more complications... before long, you have a "very complicated curve" for which there is no simple analytic description, and for which there probably is not a commonly-known name. The tire, in a tire swing, can move in a constrained fashion in more than six dimensions - in the x-y-z spatial directions, but also in roll-pitch-yaw, but also with additional degrees of freedom including the flexion, extension, and curvature of the supporting rope. We can parameterize the deformation of the rope along the continuum, which means we can represent this as an infinite-dimensional problem.

I feel obliged to link to the catenary equation; here's one pretty decent treatment from the astrophysics department at the University of Victoria. If you can really handle the mathematics of the catenary, you ought to be able to handle it in the dynamic case also - but by the time you digest that much equation, you will probably also realize that your stomach doesn't want to swing any higher.

I would also like to point out that if you happen to visit the Berkeley Lab, and hike around in Strawberry Canyon, there's a tree-branch suspending a swing overlooking the Golden Gate where, on any particular evening, you might find some physicists conducting experimental oscillation experiments.

Nimur (talk) 16:41, 3 April 2017 (UTC)

Mathematically, what is dubstep?

It's got to be model-able by some kind of oscillator(s) or something. It'd also be easier for musicians without technical aptitude to make if it was a relatively simple combination of only a few wobble-definition terms. Sagittarian Milky Way (talk) 17:58, 3 April 2017 (UTC)

I'd start reading at Dubstep#Rhythm and following sections, which discusses the rhythm and musical characteristics. --Jayron32 20:43, 3 April 2017 (UTC)

Also see Frequency modulation synthesis. Also of interest would be early electronic synthesizers, which often had limited capabilities but easy to tweak controls, such as LFOs. Influencial was acid house, with DJ-friendly boxes such as the Roland TB-303. Modern instruments are of course more advanced. I have done some drum and bass and dubstep using mostly the Roland MC-808. —╰]PaleoNeonate█ ⏎ ?ERROR╮ 02:39, 6 April 2017 (UTC)

Complex infrastructure programme organisation structure

How are complex infrastructure programmes such as Crossrail or HS2 in the U.K. Structured? In such programmes, government provides the funding and specify the very high level outputs, infrastructure authorities may act as the main delivery organisation for the programme and projects within in, and private sector consultancies and contractors may do the actual design and build. But at the same time all of these 3 parties are directly involved with the programme to varying levels. Is there a typical programme organisation structure for such complex programmes? 2A02:C7D:B9B7:CF00:60AE:10AF:AB2:B7C4 (talk) 20:25, 3 April 2017 (UTC)

This is not a field I know much about, but considering this question had no answer yet: I would probably begin by reading Civil engineering as well as relevant sources the article presents. This could also serve to know which keywords to use for further searches. —╰]PaleoNeonate█ ⏎ ?ERROR╮ 02:49, 6 April 2017 (UTC)