applyExternalForce() has no effect on a body linked to a fixed body.

[WillDudley]

I tried to create a see-saw in pyBullet, then move it via p.applyExternalForce. I made the see-saw by attaching a rectangular body to fixed "pivot" body via a revolute joint. Dropping a ball on one side of the see-saw moves the see-saw as expected. However. using p.applyExternalForce on the see-saw doesn't move the see-saw.

When I remove the fixed base property of the pivot, p.applyExternalForce() now affects the entire urdf file. It seems like p.applyExternalForce() assumes that joints have infinite stiffness?

My toy example can be found here: https://github.com/WillDudley/pybullet-revolute-applyExternalForce-demo

Many thanks. Apologies in advance if I'm missing something trivial.

Pybullet 3.1.7, Ubuntu 18.04.5 LTS