A forum member of Astrotreff-Deep-Sky sent me the link to the space time simulator (also see Prettygoodphysics).
It is a simple yet very effective way to teach how gravity/space-time works.
Some may also know the coin funnels (eg. here) where a coin slowly spins towards the center, some malls and zoos had those. Same thing, less flexible.
Long story short:
I have to build one of these. As you can see there are a lot of experiments you can do with it, and as I volunteer at a local school, this would be the perfect way to demonstrate all of it instead of reading boring worksheets.
In my opinion a experiment you can touch, change and watch will be even more successful in teaching such complicated things to fifth-graders then animations on TV or the computer.
One of my favorite things about this thing is, that you can even let a small „moon“ circle around the „planet“ while both orbit around the sun-center. Unfortunately I need to get more marbles as I only have one type, BBs where too light to really work.
Of course friction and gravity will prevent the marbles from running around the center for more then a few rounds, but it should be good enough to experiment and observe.
Surely this model is rather simplifying the physics, but so was the atomic model we learned at school. Both have their limits, but they teach the basics quite well and are easier to understand.
Let’s build! 🙂
5-6 x M16x2000mm installation tube/conduit , 2 x M25x2000mm installation tube; aprox. 5€
12 x M16 Clips, 12 x M25 Clips €3
-M16 connecting tubes or if you just build one set, another M16 pipe will be the cheaper option
-10-20% elastan/Spandex stretch fabric 150cmx150cm
-Clothpins with soft tips or Document clips
-Big sphere or weight
-Marbles of different sizes
The stretch fabric is a bit expensive, but you can get it at sales or fabric remnants at ebay sometimes. I got some for 2€ for the running meter with 150cm width, surprisingly the fabric with 8% worked better then the one with 10%, probably because it was thinner.
-Wider rolls are harder too find and more expensive.
You may be able to stitch two rows together with flexible thread, but the smaller size with this set-up has a few advantages. It will be easier to build, cheaper, lighter… But the marbles will not circle the center as many times as with the bigger set-up.
My construction was ment to be cheap and portable, the rigidity is not perfect but OK. You may get away with longer legs, but I would advise against it. The height is ideal to use with younger kids or while seated, or you can put the thing on several tables instead.
Thanks to the adjustable height you can change how hard the weight dents the fabric and control the behavior of the marbles quite well.
The first step would be to glue the clip holders together, with a 90 degree offset. Instant-glue-gel worked great as it is not as runny and makes contact well. With kids a glue-gun or nuts&bolts may be a more secure solution.
While the glue does what it should, take one of the thinner 16mm pipe and cut it in half with a saw. Stick it together tightly with two whole ones, creating a pipe 4.5 meters long. Secure the connections with duct-tape. Bend it into a loop and connect the ends, also tape that connection. To avoid it loosening over time or forming a oval you can take a short pencil to splint/reinforce the connection with duct-tape.
Take the two thick 25mm pipes and cut them into 66cm pieces. These are the legs of the construction.
With the clip-connectors you can now assemble the whole thing. When you are done building, you will see it is pretty easy, much simpler then my stopmotion-film-box or the dobsonian mount for flea-market-telescopes.
I apologize for the mess, I had to make room to assemble the big thing and we had half a dozens of boxes from stuff I ordered that all arrived at the same day 😉
Now you will need to put the fabric across the pipe-ring. Start on one side, and stretch it across („north/south“). Then do the same thing from side to side („east/west“). Then secure the whole ring carefully. Do not use more then about ten clips, else it will be difficult to make adjustments.
It should stretch, but still be flexible.
Also make sure the ring is leveled, sometimes the clip slide down after using the construction.
Transport and storage are easy-peasy. If you remove the duct-tape, it will be even easier to move around.
Now put a weight in the center. It does not have to be a sphere, just small and heavy enough.
Unfortunately I only made a quick video test, but you should get the idea. For ideas and experiments see the original (A, B).
Youtube Video Test
-round and elliptical orbits
-Planet moving in eights around a binary star system
-Why planets orbit in one direction
-Smaller moon orbiting larger marble while orbiting sun
-… see original video 🙂
More coming soon…
 Space-Time-Simulator & prettygoodphysics.wikispaces.com/PGP-Modern+Physics
 Funnel/Trichter, Coin-Funnels / Münztrichter
This 10% Lycra/Elastan fabric worked
http://www.ebay.de/itm/0-5-Laufmeter-Jersey-beige-mit-Lycra-N18-nur-2-94-m-/121118505141?pt=DE_M%C3%B6bel_Wohnen_Stoffe&hash=item1c3339b8b5 with 8%.
Not tested, but 20% Elastan:
If someone knows sources for broader sheets or how to sew stretchy fabric, let me know 🙂
Wenn jemand eine Bezugsquelle für breiteren Stoff kennt, oder Tipps zum „elastischen Nähen“ hat – nur her damit 😉
After setting up the simulator once more I think it would be best to wrap the connections with even more tape or glue them together. Perhaps pre-bending the pipes would decrease the risk of breaking or popping out. But for a first prototype the whole thing works well.
elastan http://www.funfabric.de/halli.php?01_11025 and others