How do you create objects that "defy" gravity like an hot-air balloon?
Then halfway into animation, how do you make the hot-air balloon drop anti-gravity, as it loses heat?
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What kind of object?
An individual rigid body cannot be made anti-gravity, however you can make the entire scene gravity point upwards by changing Z=-9.8 to a positive number in the Scene Gravity settings:
"Gravity" is a property that can be keyframed, i.e. you can make it change at a given frame.
For cloths and soft bodies you can choose what Gravity each object is feeling. There is a "Field Weight" panel in the Physics settings. There you can change Gravity to a negative number (assuming that you don't change the Gravity scene settings!). The slider won't allow you to do that, but you can just type something like
Again, this can be keyframed.
To reverse gravity at a given location rather than time, you can add a Wind force field (ShiftA > Force Field > Wind), make it point downwards, and changing its falloff type to Tube and falloff Power to something between 1 and 2.
Here is a scene with a Soft Body hot air balloon, with its Gravity Field Weight set to
-0.3 (Scene Gravity is untouched), and with a Wind Force Field at a given altitude (also a Turbolence Force Field for more wiggling):
Here's the blend if you want to look at how it's done