What is the unit of force in get_physx_interface().apply_force_at_pos(pathString, force,position)

It seems it is not N.I applied 980 unit of force on a object whose mass is set 100 .The object can’t move in a stable velocity or stay still

The units are based on your stage units: mass * distance / seconds^2

Where distance can be either m or cm or something completely different based on what units your USD stage is. You can see what units are used when you click on the root layer in the layer window. Then in the properties you will see meters per unit value. (default is 0.01 - meaning cm units).

If your units are in m then you pass in SI units, however if your units are in cm, you need to pass in modified values based on the meters per unit. It is expected that all physics values are in the units the current USD stage is in.