I am working on a new game that I need a script to make it so any part with any material can float, and I need a way to change the velocity via a script. The material is different each time, meaning the gravity will be different each time.
You need to get the mass of the object using the GetMass method and then multiply the mass by gravity, which is 196.2 on Roblox. Also note that you should use BodyForce and not BodyVelocity for this.
Part.BodyForce.force = Vector3.new(0, Part:GetMass() * 196.2, 0)