Easy way to show abstract concepts in a way that nearly everyone can understand. And even though you may only use this a few times in class during the year, it's a highly effective demonstration of gravity, mass, red-shift curvature, orbits, time dilation, and other fun things that you can think of to add below in the comments. I have seen several of these devices, but was never inspired enough to make one until I attended the Penn State workshop - Black Holes. Because of the class, and the resources it provided, I am able to add a substantial amount of depth to my current content. I will post more about the workshop in the near future, mixed in with the usual "I do other stuff too" postings
This is a pretty easy build, and you probably have enough in your pocket now to buy what you need.
Things you'll need
2- 2x2x8s around $1.25 each
1- yd of swim suit liner. It's usually 60 inches wide, and it's actually called swimsuit liner. Will cost
4- screws longer than 2". I'm sure you know someone that keeps jars of screws in their garage
4- shots of wood glue. Get that from the person with the jars of screws
Many- staples to hold the fabric
Color Sharpie, or fabric marker
Mass- Resonable mass relationship. a small red bouncing ball, a 3-5lb medicine ball ($4 at 5below)
and I use a 20 lb kettlebell that I keep for use during my planning period
Easy Button- Have a student do this as a project
You can be as precise as you like. You can cut 36" with 45 degree angles etc, or just eyeball it. I just eyeballed it and cut 4 ~36" lengths of 2x2. This gives you a rectangle when it's assembled without 45s but that's okay. Screw and glue the frame together. Stretch the liner over the frame just so it's not wrinkled or sagging, and attach it with a staple gun. You're pretty much done at that point, but you may want to make a grid, and add a light wave to show red-shift. Be aware, however, that the marker will bleed through to whatever support is underneath, a couch cushion in my case, but it's all for the good of science!
High Mass Stars