A particle with mass is moving with constant speed along a circular orbit (radius ).
The centripetal force is provided by gravitation force from another mass .
A string is connected from mass m to the origin then connected to mass .
Because the force is always in the direction, so the angular momentum is conserved. i.e. is a constant.
For particle with mass m:
The following is a simulation of the above model.
You can change the mass M or the radius r with sliders.
The mass M also changed to keep the mass m in circular motion when you change r.
However, if you change mass M , the equilibrium condition will be broken.