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Could I not have a similar experience if I were not spinning but placed at the centre of a large planet sized mass at the core which is conveniently hollow to accommodate me. Would I not experience an outward pull (gravity attraction from planet) radiating from my centre as though I were in radial motion? Jim

Nicely demonstrating the equivalency of gravity and acceleration. Jim

Can we not say that without a reference point there is no spin (Newtonian not quantum) . I'm sure we discussed this at some point in the past or something very similar. Need to search the archive when my head stops spinning Jim

They are in effect the same thing, centripetal being the centre seeking force resulting in circular motion , centrifugal being the reaction force to centripetal. Two sides of the same coin In Vlaiv's example the force you would feel pulling on your head and feet is the reaction to the centripetal force causing the rotational motion. Jim

Ah ambition Jim

My name is Jim , I'm an engineer and I believe in F = m^w2r Jim

An engineer would say yes and happy with that knowledge go on to successfully design high bypass turbo jet aero engines that fly thousands of passengers across the globe. A physicist would roll their eyes at the use of the term centrifugal. You have opened a can of worms now sunshine Jim

I think what may be causing the confusion is the popular way gravity wells are represented graphically. More often or not the gravity well is represented as a sheet of material on which the planets sit within the attendant deformation (well). This allows us to easily visualise how the smaller mass would be attracted or fall into the larger well . Of course space time is not a 2D sheet of material and neither is the resultant deformation forming a gravity well 2D. It's hard to graphically represent but the gravity well around say Jupiter projects across all of its volume and not simply a plane through a particular axis. So with this 3D gravity well we can see how every atom in a nearby moon or planet would therefore be subject to a gravity well created by Jupiter - every atom will experience an acceleration as it falls along a personal space-time geodesic as described by Andrew. If that acceleration is resisted then the interpretation of that resistance or reaction is what we name force. Those internal reactions (resistance to the gravitational acceleration) in turn can create the heating effect from so called tidal forces. The main point is that every atom in the moon or second body will indeed be in a gravitational well of Jupiter, the well is not 2D as often drawn. Jim

The IAU has voted to rename Hubble's Law as the Hubble and Lemaitre Law. Georges Lemaitre was the author of what would become known as The Big Bang Theory. Unlike Hubble , Lemaitre was far from being a self publicist and in early presentation of his theory he was met with indifference and ridicule - in fact the name Big Bang was a pejorative term coined by Fred Hoy who was one of many cosmologists who favoured the steady state model. So at last Georges Lemaitre is recognised more publicly by this renaming. Lemaitre is a bit of a hero of mine so when I was teaching today (expansion of the universe) I had a big smile on my face when I introduced the Hubble-Lemaitre law to the class. Now we just need to get Henrietta Swan Leavitt recognised more widely; her work on Cepheid variables (relationship between luminosity and period) was an essential precursor for Hubble's work . IAU Rename Hubble Law Henrietta Swan Leavitt Jim

It's those pesky Gremlins Gina the bane of all engineers Jim

What a cool prize. I bet your tea will taste all the better from that mug. Well done indeed. Jim

Vlaiv I don't think it is as indeterminable as it may appear. Treating it as a simple adiabatic expansion will get you pretty close to the value in practice - nozzle considerations accepting (choking, throat velocity). Nor is your target of -50 Celsius overly ambitious; for example a common CO2 fire extinguisher (55 bar , 3 litres) will easily get down to that range. Discharge a CO2 fire extinguisher through a close fibre cloth and you will be able to collect lumps of dry ice at -78 Celsius - the horn of the extinguisher will itself drop by as much as 50 Celsius if the extinguisher is fully discharged. Out of interest what is your plan, are you looking to build the setup yourself - interesting project, good luck with it. Jim

For a 1st approximation I would tend to treat it as a reversible adiabatic expansion so T2/T1 = (p2/p1) ^ (Ƴ-1)/ Ƴ . Ƴ (adiabatic heat capacity ratio) for air is approximately 1.4. T2 = (278) * (1/200) ^0.286 T2 = 61 K, but air freezes at around 58K 200 atmo is too high! In practice you would also need to allow for the efficiency of the nozzle used to throttle the gas so the final expanded temperature would be a little higher. Jim

I can't remember exactly Lousie just that it was all very reasonable. I've bought lenses and a beam splitter for an interferometer project and also a set of lenses to build a Rochester cloaking optical train. From what I recall there was no import duty as such or if there was it was pretty insignificant. The benefit of course is compared to the likes of Edmund Optics and similar Surplus Shed prices are way cheaper. It's just a matter of them having what you need. I'd highly recommend them though. Jim

x2 for surplus shed. I use them fairly regularly, very reliable, they may have what you want and if not ask and they may be able to find it. Standard USA postal service but not too long a wait. They had some cracking 5 inch objective lenses a wee while back , would make a nice project. Jim

Secret handshake time Dave , we are in the same club Jim

Without being disrespectful and I mean that sincerely but this type of stuff just makes me cringe - I really cannot get excited about people getting emotional or defensive about their particular way of doing something. Likewise whether we should have dedicated forums or not or call it visual or assisted or whatever - for crying out loud it's a hobby. You do it because you enjoy doing it - If somebody swears by standing buck naked with their head in a tea pot , left foot in a bowl of custard, whistling Dixie and observing through the spout afocal good for them. Do what works for you and don't feel the need to put a badge on it - life is way to short for that folks. For what it is worth I'd use NV at the drop of a hat if I could afford it , I also do visual and astrophotography have dabbled in spectroscopy and naked eye and I have on occasion (many) just stood and marvelled at the impenetrable blanket of grey cloud. Never once did I feel like putting a name to it, I did it for fun and because I wanted to. Jim

Chris would one on those Haynes Manuals be useful, I believe there is one on the Hubble. Haynes Manual - Hubble Jim

Thanks Eric I think I'll definitely try it out. I've played around with the Star Analyser and managed to produce a decent looking spectra of Vega using rspec. I stopped short of getting my head round correcting for instrument response but I've always wanted to go back and pick it up again. The Lowspec looks like a very neat little unit - thanks for the link to your write up, following that now. Jim

Eric, can I ask how has the Lowspec spectrograph performed for you, would you recommend it as a project? Jim

I certainly saw a lot of positive recommendations for as well Steve but pricey compared to others. That said I'd be tempted to try an expensive brand against the likes of Creality own brand which has performed well for me so far . Christmas is coming so I'm drawing up a list now - a reel of plastics , oh how days have changed Jim

Indeed Peter but not with PLA Just out of interest additive layer or even sintering technology would have to progress beyond imagination to meet the performance of Rolls Royce single crystal blade technology. But as you say some very interesting things are happening in the world of industrial 3D printing particularly in aerospace where a range of components are now 3D printed (both metal and non metal) . The method of manufacture, cold pressing, forging, casting, fabrication etc has always had an influence on the mechanical properties of material (advantageous and undesired) , it's an area of engineering science well known to design engineers. 3D printing is no different. JIm

To be honest Gina I think the world of 3D printing and its wide range of filament materials opens up an aspect of engineering design to many whom it may otherwise never really have been a consideration. We both come from engineering backgrounds so these considerations are well understood but I guess the assortment of filaments can be confusing to many. Like any hobby/interest there abounds misconceptions and "urban legends" - the 1 cubic meter pier foundation syndrome comes to mind With specific respect to PLA I think it is a good all rounder: inexpensive, good printing performance , no toxic fumes emitted. It is a good general purpose filament and where it does lack a particular property then other filaments can be chosen with printer capability also in mind. The Ender 3 and 5 models are PLA and ABS capable but at the moment, certainly for my needs, I can't see a need to use ABS yet and certainly not without some form of enclosure and fume extraction. Ill be printing PLA for some time to come before I start on my 3D printed cold fusion reactor Jim

I'll disagree with you on that one gian. The brackets I have printed in PLA do indeed have the mechanical and aesthetic properties for their intended application (adjustable bed for K40 laser engraver). Likewise the PLA printed assembly to hold the beam combiner will also function without difficulty. My Polemaster camera when not mounted for polar alignment functions as an electronic finder held in place by a PLA printed adaptor and has functioned perfectly well over the past 2 years. I certainly wouldn't look to use PLA for a third stage RB199 turbine blade nor a wing box on an Airbus, but neither would I use mild steel, aluminium or carbon composite . However, PLA certainly does have "real world " application. In common with any other engineering/manufacturing project material selection is part of the design consideration - 3D printing is not exempt from that consideration. To say however that PLA is not suitable for real world applications perhaps calls for a broader definition of "real world" - my uses to date are certainly real world and PLA has satisfied the design brief very well. Jim

I must admit I like the Ender's magnetic bed it gives excellent adherence without further preparation and it makes removal of the print a doddle. Having looked at the brackets I made for the K40 laser bed I can't see that I will have any problem with PLA for the majority of my practical applications (not really into toys and models); currently printing out a beam combiner again with PLA and so far all is well. Jim