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I'd like to simulate balloons floating up, while other objects are falling down, which I cannot fix by reversing gravity. Is there any way to simulate this, simulating Archimedes' Law?

This is more of a general question about Blenders physics, but for the scene at hand I'm working on about a hundred balloons rising, and confetti and serpentines falling. The balloons have numbers on 'm, so these were generate through a script. It is not a particle system.

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3 Answers 3

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Another solution using RigidBodies, Zero gravity and a Texture Field:

Note that all the balloons here are single user object instances all with RigidBodies.

  1. turn RigidBodies World Gravity to 0.0
  2. Add a texture field which uses RGB values of the texture to determine the force (using a Color Ramp). R or G or B values of 0.5 correspond respectively to 0.0 force in the X, Y or Z axis. 0.0 R, G or B values correspond to +1.0 force and 1.0 R, G, B values correspond to -1.0 X,Y or Z force. Its means that a downward Z axis force of -1.0 corresponds to R=0.5,R=0.5,B=1.0, and an upward Z axis force or 1.0 is R=0.5,G=0.5,B=0.0. Thus the blueish/yellowish texture below. See https://docs.blender.org/manual/en/latest/physics/forces/force_fields/types/texture.html for details

Also note that I am using a Clouds texture here which is 3-dimensional which explains why the balloons are sometimes switching direction, when they pass through a 3D space where the 3D texture is changing color. May be using an image texture or tweaking how the texture is build would prevent that.

enter image description here

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Option using a combination of Gravity, Forcefield and flow option: 1. adjust your gravity down enter image description here

  1. add a Forcefield of type plane just below the emitter (very close to the origin). Check out its settings here:

    enter image description here

This is making the balloons go up and down but I am sure that you can tweak it to your desire.

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  • $\begingroup$ I'm not using a particle system here, so I guess Bruno's suggestion is not going to work. $\endgroup$
    – eezacque
    Apr 6, 2020 at 13:32
  • $\begingroup$ OK no problem. Please add some more detail on the context and your constraints in your original post. You can use the Edit button to do so. This way, that will avoid confusion. $\endgroup$
    – Bruno
    Apr 6, 2020 at 13:57
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One possible solution is to use contraints to tie the balloon's position to that of a hidden "anti-balloon" (represented bellow by the smaller cube), inverting the Z component, however.

enter image description here

Apply regular Rigid Body physics to the anti-balloon and, by tweaking the constraint's influence setting, you can get yourself a more natural-looking animation, seeing as balloons accelerate upwards at a much slower rate than (anti-)gravity would have them do.

Make sure to hide the anti-stuff and disable them in renders, possibly also in viewports.

N.b.: it might suit you better to copy only the inverted Z component of the anti-ballon, or to use two constraints: one for the Y and X components, with full influence, and another one for the Z component, with reduced influence.

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  • $\begingroup$ I played a bit with this, but it is too limited for my purpose. For instance, this does not work for a cloth simulation. I can animate the balloons, but I just want to benefit from Blenders physics simulation. Looks like Blender is not very useful for anything but the most trivial simulation. $\endgroup$
    – eezacque
    Apr 16, 2020 at 20:02

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