24-10-2025, 10:24 AM
(This post was last modified: 24-10-2025, 10:45 AM by josemendez.)
Hi!
Rotational mass = "how much material is there?" and can be understood as the thickness of the rod, as far as physics are concerned.
Compliance = "how soft is the material?", so it sort of modulates the rotational mass.
In other words: having a very thick but soft rod (high mass, high compliance) is similar to having a very thin but hard one (low mass, low compliance). Even if your material has zero compliance, making the rod very thick (high rotational mass) will make it very hard to bend.
Keep in mind however that compliance affects both linear and angular forces. Linear and rotational masses only affect linear and angular forces respectively.
No. External forces (collisions, wind, gravity, inertial forces, etc) and internal forces (forces that originate inside the object and keep its shape, such as the forces applied by constraints between particles) act on the exact same material (same mass, same compliance) so forces of the same magnitude must result in the same acceleration, regardless of where they originate from.
Think of a foam ball so soft that collapses under its own weight, but that feels hard as concrete when you try to sink your finger on it: this is physically impossible, objects must respond to all forces equally. Any other behavior would mean the object magically changes its mass or its constitutive model depending on the force affecting it, which doesn't make any sense.
You might be able to get comparable behavior by moving the solver component instead of the rod: this kinematically transforms all actors under it which means moving the solver won't inject any forces on the rod (or rather, the % of inertial force injected by the solver is controllable using the solver's linear/angular world inertial scale parameters). However, collisions still will be able to bend the rod as usual.
The combination of controllable inertial forces while moving with regular collision forces might do the trick.
kind regards,
(24-10-2025, 09:35 AM)Qriva0 Wrote: what is difference then, between rotational mass ratio and compliance for bending?
Rotational mass = "how much material is there?" and can be understood as the thickness of the rod, as far as physics are concerned.
Compliance = "how soft is the material?", so it sort of modulates the rotational mass.
In other words: having a very thick but soft rod (high mass, high compliance) is similar to having a very thin but hard one (low mass, low compliance). Even if your material has zero compliance, making the rod very thick (high rotational mass) will make it very hard to bend.
Keep in mind however that compliance affects both linear and angular forces. Linear and rotational masses only affect linear and angular forces respectively.
(24-10-2025, 09:35 AM)Qriva0 Wrote: I understand there is difference I can kind of feel it, but overall both control similar thing. Does it mean that with new system I can set big rotational mass to make rod very rigid, but then set compliance to big value and the result would be rigid rod, but with capability to bend significantly during collision?
No. External forces (collisions, wind, gravity, inertial forces, etc) and internal forces (forces that originate inside the object and keep its shape, such as the forces applied by constraints between particles) act on the exact same material (same mass, same compliance) so forces of the same magnitude must result in the same acceleration, regardless of where they originate from.
(24-10-2025, 09:35 AM)Qriva0 Wrote: Because that is my goal, responsive and "rigid rod" when it comes to movement, but at the same time bendable when colliding with other (static) objects. (so far rigidity meant it behaves like rod made of iron)
Think of a foam ball so soft that collapses under its own weight, but that feels hard as concrete when you try to sink your finger on it: this is physically impossible, objects must respond to all forces equally. Any other behavior would mean the object magically changes its mass or its constitutive model depending on the force affecting it, which doesn't make any sense.
You might be able to get comparable behavior by moving the solver component instead of the rod: this kinematically transforms all actors under it which means moving the solver won't inject any forces on the rod (or rather, the % of inertial force injected by the solver is controllable using the solver's linear/angular world inertial scale parameters). However, collisions still will be able to bend the rod as usual.
The combination of controllable inertial forces while moving with regular collision forces might do the trick.
kind regards,