the red part is spining forever, but how do i make the yellow ball also go around along with the red ball? they are already welded
It depends how you are spinning the red part. If you are spinning it with physics and the yellow one is welded it should already be following it. Remember that constraints are ignored if both parts are anchored, though.
i am spinning it with changing the CFrame, does that count as spinning it with physics? if not, how do i do it?
No, the physics engine will not notice that the part is spinning. This can still work if the yellow ball is connected with a regular Weld. I don’t think a WeldConstraint will work in this case, and I believe the yellow ball must be unanchored.
oh wait thats why it dosent work, the yellow ball is anchored lol
This is because constraints were reworked to automatically be disabled if more than one part in a constrained assembly is anchored. This allows Roblox to make some optimizations when sending and receiving the physics info.