habahu inertia and mass are equivalent in the real world
In the real world they are related and inversely proportional, but aren't equivalent. Our physics constraint has to fudge a whole lot of things so real world accuracy isn't the goal, but the names should make some sense. We are open to improving the names.
For physics constraint, inertia is not used when applying forces, instead it is used when a bone moves and the physics controlled properties (x/y/rotation/scaleX/shearX) need to be adjusted to simulate lagging behind the action. The constrained pose from physics constraint is achieved by "offsetting" the properties from the unconstrained pose based on bone movement. Inertia controls how much bone movement is transferred to the properties' physics offset. 100% inertia means the property's offset is affected maximally by the bone movement, 0% means it is not affected at all.
Spring forces are applied over time to reduce the physics offset to zero, effectively causing the constrained pose to (try to) return to the unconstrained pose. Mass (1 / mass actually) is use to scale those forces (Newton's second law: force = mass * acceleration). Given the same amount of force, a larger mass is affected less and a smaller mass is affected more.
habahu Mass and Strength have the same effect in controlling vibration frequency
All of the properties are interconnected, so changing one affects the others. You can achieve a reduction in oscillation multiple ways. Damping is most straightforward, but you can also use less strength, making the forces returning the offset to zero weaker and therefore less likely to overshoot zero. Mass is a multiplier for the forces, so it can also make them weaker to reduce oscillation.
Tuning physics is a bit of an art. Many different behaviors can be achieved with various slider configurations. Some of them give similar results but there can be subtle and not so subtle differences.
habahu Wind and Gravity are the same thing but in different directions, why not use the same name and separate X and Y?
By the same name do you mean windX and windY? I prefer the names wind and gravity. People don't often think about wind in the up/down directions. The 2 sliders could be placed in the UI on the same row to reduce vertical space, but would be the only sliders placed that way and would require the tree to be wider to make them usable.
habahu they seem to be just external forces that cause the spring oscillator to deviate from its initial position, so I don't know how to set the parameters if I want to simulate free falling motion.
Enable translate Y and set gravity so you have a downward force. Strength is what returns the property to the unconstrained pose, so set strength to zero and your bone will fall downward forever. Set damping to zero (or near zero) so gravity's constant acceleration results in a continuous linear increase in velocity (ie it falls faster and faster over time).
