Sorry I hadn’t responded to your other post yet.
I’d refer again though to my answer here, particularly the bit about the damping:
The solver in K2 prioritizes convergence to a numerically accurate equilibrium solution in a fast and stable way, since for the form-finding applications which it is primarily intended for this is what actually matters. It minimizes the sum of squared distances from satisfying all the goals, which allows fast and precise modelling of the equilibrium solutions of systems of loads and Hooke’s law springs, even with extremely high stiffness values.
This also means it deliberately removes energy. What it converges to can be physically accurate with the right inputs, but exactly how fast things swing or bounce is only approximate.
The ‘bouncy’ solver option was added later in order to allow something visually like the more dynamic behaviour that the original Kangaroo1 had, but it still uses a modification of the projective method used in the regular K2 solver.
The solver in Kangaroo1 is actually a more classical dynamic simulation, with a force and velocity based symplectic integrator that does preserve energy very well when the damping is set to zero. It also has options for setting the mass of individual particles, and this will correctly affect their momentum (K2 does have mass values internally but they are not exposed in the interface). I believe these falling and pendulum examples would give the expected physical results in Kangaroo1. It also worked well for orbital mechanics and faithfully reproduced classical examples for that.
The big downside with K1 though was that stability was dependent on the timestep, and when using stiff elements you would often have to adjust it down to a very small value to ensure things didn’t explode, which made the simulation run slower, and getting this value right could be fiddly and non-intuitive.
I’ve long had it on the to-do list to have another go at adding an option for an integrator that does a better job of both (stability and energy preservation/realistic dynamics), but (a bit to my surprise) it’s almost never come up over the years, so I’ve got the impression it isn’t important to what most people want to use Kangaroo for.
I promise I will have another look at it though. One option would be to simply add in a solver option using the same integration method as K1, and options for mass, but I think there would be some challenges in then presenting it all in a coherent way that doesn’t compromise or complicate the main form finding use.
There have been some good new papers on the topic of stable energy preserving integrators since I last changed this that might work here, and my preference would be to try and make a single system which can do numerically accurate equilibria of even extremely stiff elastic systems, as well as energy preserving dynamics.