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I'm trying to simulate balloons gathering under a tent. I've finally figured out the stiffness and gravity of the balloon using a combo of soft body and rigid body modifiers, but the string is now the only issue.

(View from bottom looking up) View from bottom looking up

The string just continues to gather at the top instead of hang down. How do I make the balloon have negative gravity, and the string have positive gravity? Or.. is there another solution for it to remain hanging down?

The eventual goal is to add maybe 50 balloons to the scene to have them fill the tent with their strings hanging down.

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You can create two separate soft body simulations - one for the balloon and the other for the string - and link them together via the Copy Location constraint.

Start by creating the balloon and then, in Edit mode, select just the vertex where you want to attach the string. On the Mesh properties add a new Vertex Group and name it 'Knot'. Assign the selected vertex to the group.

Create the string as a separate mesh as a string of vertices (you can add thickness via a Skin modifier). In Edit mode, select just the 'top' vertex and, again, add a vertex group and assign the selected vertex to the group. In Object mode, add Soft Body and use the new group in the Goal properties so only that vertex is pinned. Increase the Default goal strength to 1.0. Also, set the origin of the 'string' mesh to the location of that top vertex.

Now, add a Copy Location constraint to the string, specifying the balloon as the Target. Set the Vertex Group on the constraint to the 'Knot' vertex group. This should result in the string being tied to and following the balloon while still being a separate soft body (so they can have different Field Weights (ie, negative gravity for the balloon, positive gravity for the string)).

This should produce a result similar to the following :

animated balloon

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