How do i simulate balloon physics

i dont get it how this game was be able to do this, but i tried with serveral properties and objects, even scripts. but it doesnt work either. sometimes unstable or broken.

Update: Used Bodyforce at 20 on the Y axis does work, and some AssemblyLinearVelocity at 1 on the Y axis.

also works the same when replaced with vectorForce, with an attachment

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