Thanks for these modifications.

It seems to be a problem while if r > 0.6, g don't change anymore.

As the gravitation field is concerned, would it be possible to have it as an option because I work with 15 years old students and it will be difficult for them to understand the meaning of such a field ?

Can we have instead a cursor where m of the object can be changed and the value of force would appears in the simulation ?

thanks in advance,
best regards,