# Objects with negative mass?

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.

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.

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

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

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

• I'm not using a particle system here, so I guess Bruno's suggestion is not going to work. Commented Apr 6, 2020 at 13:32
• 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. Commented Apr 6, 2020 at 13:57

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.

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.

• 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. Commented Apr 16, 2020 at 20:02