What's wrong with Blender physics simulator?

Question

I am trying to make a physics simulation where particles are moving inside the box like an ideal gas. However, there is a problem with the simulator.

I set bounciness for the ball and the box to be 1, all the drag forces to be zero so there is only elastic collisions. The image down below shows the bouncing of a ball, which should bounce infinitely long since there is only gravity force acting on it. However, you can see that Blender gives weird results.

What could be the problem?

Also, the simulator every time behaves differently, sometimes when ball has already stopped, it starts bouncing by itself, or, sometimes ball bounces 3 or 4 times and then stops completely.

Answer 1

Blender's rigid body physics are based on game-engine physics. They are built for good performance, with a minimal number of intuitive parameters, for results that look pretty much right in contexts involving solving crate-based puzzles while under attack by headcrabs.

They are only remotely like real-world physics. One reason, already mentioned elsewhere on this page, is that there is some randomness built into the sims, to prevent problems with pencils balancing on their points. Another reason is that states are sampled rather than solving all the calculus problems we'd need to do things realistically. Most visibly we see this problem manifesting in objects teleporting through colliders, but it can also affect bounces.

Consider this starting state for a rigid body sim:

The spheres start in collision with the ground, but at different depths. Let's advance a few frames and see what happens:

The spheres are pushed out of collision, but with different velocities. The one that was deeper has a higher velocity. What's happening? The rigid body sim sees that the spheres are in collision. It pushes them out of collision, creating velocity. That velocity is different for each sphere, because the distance required to push the sphere out of collision is different for each sphere.

Now, what happens when a rigid body sphere rebounds off a wall? The simulation does not see the exact moment that happens; that would require the simulation to solve some calculus problems, potentially, a huge number of them. It sees one subframe, where the body is not in collision, then, the next sub-frame it looks at, the body is in collision. How deep is it in collision? That depends heavily on the body's velocity, but it also depends, artifactually, on when exactly the sub-frame occurred.

Because of that, rebounds in Blender's rigid body system are not perfect. They are just good enough for typical use. They average out to be about realistic. If you need to solve physics problems precisely, you should be using different software to do so.

Answer 2

So in Blender version 2.93.4 a Plane with the default collision work as expected giving an infinite bounce:

For the ideal gas simulation, I would recommend turning on the Size Deflect option and removing gravity, as well as increasing the randomised velocity, this helps to emulate the different molecule velocities. As an emitter, I just used a subdivided Ico Sphere and as a collider a cube with the default collision.

Hope this helped

Answer 3

Depending on which integrator you use, the particles sometimes bounce higher and higher, like in your example:

https://docs.blender.org/manual/en/latest/physics/particles/emitter/physics/newtonian.html?highlight=bounce#integration