I would like to know the force that is acting on a joint's anchor.

I would like to know the force that is acting on a joint's anchor.

To be more precise, I want to simulate a rope with one (or more joints), hang an object from it, move it around and find out the force that is acting on the joint anchor.

I doubt this will suit your needs, but if you instead attach the same rope to something that isn't anchored, you can easily compute the force by seeing how the unanchored object moves.

So two methods spring to mind if you want to show the rope AND compute the force. One is to have an off-camera unanchored copy of everything. The other is to not anchor, and after the force is computed move the "anchor" back to its home position in a script. This may interfere with the physics engine and/or leave visual artifacts, so treat it as just a crazy idea. (Maybe the force computation could be done in FixedUpdate and the re-anchoring in Update?)

However, I have not tried either of these solutions myself, but they might be worth a try if someone else doesn't chime in with a Unity API call that already does what you ask.

If you don't need high accuracy (i.e. like most games) it would probably easier to approximate the force by adding the weight of the rope itself and then deriving some additional force from how hard the end is whipping around.

As far as I'm aware, a fixed joint is essentially just a very rigid spring with zero length. Perhaps you could implement this type of joint yourself in code, thereby giving yourself access to the forces involved. You would essentially just write a script which applies a force (caluclated using a standard dampened spring formula) to the rigidbody which is supposed to be attached, which is proportional to the amount from which the rigidbody has deviated from its fixed position.

i.e. you get the values you're looking for, because you're calculating the force required to keep the object in place yourself.