Real time 3D physics simulator applet

The model

The world is made up of 'Atoms' and 'Rods'. An atom is a point mass that has mass, position and velocity. A rod connects two atoms and has natural length, youngs modulus (spring stiffness), damping and an atom at each end.

When you enter this page or press refresh (because you lost the ball), the world contains some test objects: a 'pencil', a 'ball' and and a grid so that you can see the ground.

The 'pencil' consists of two 1kg atoms connected by a 1m rod. It falls and hits the ground, bounces a bit then settles in an upright position. It does not fall over because the top atom is precisely above the bottom. If you move it in pick-up mode you will never get it to balance like this again!

The 'ball' is actually a dodecahedron made of 20 1kg atoms connected by rods of various length. The rods not only connect the edges but also cross-brace the ball so that it springs into shape. As the scene starts it falls and bounces.

The only colisions that can occur are when atoms hits the ground (z <= 0). The atoms themselves have totally inelastic colisions. Objects bounce because, as they land, the rods convert kinetic energy into spring compression causing the top atoms to stop and reverse with sifficient momentum to pull the whole object back off the ground.

Rod colour indicates strain. Compression is red, tension is green. Look at a resting ball; its equator rods is in tension as is bulges but other rods are in compression.

What next

I would like to develop this applet further. If I ever get the time these are some of the things on my wish list:

• Add time vs. energy displays for total kinetic, potential (gravitational) and spring energy.
• Create a list of 'experiments' to demonstrate e.g. conservation of linear or angular momentum, gyroscopic procession - the applet already simulates all this.
• Improve the controls, adding buttons, menus etc. instead of the crude function keys.
• Fix problem that makes the model go erratic when Youngs modulus is too large, objects are too small or forces are too great.
• Improve the efficiency so it can handle larger models (The code is not at all optimised).
• Improve the rendering so it is not restricted to wire-frame. Adding surfaces between the rods.
• Add ability to load scenes from applet parameters and to dump scenes.
• Add object-object collisions (currently it only does object-ground).
• Simulate aerodynamics.
• Make pigs fly.

I originally wrote the Physics simulator as an exercise to lean Java programming. Since it was kind of fun I decided to put it on the web. I hope you like it.