Hello all, I have been arguing about this for a few days. My friend is telling me that a helicopter flies in the air because there is a difference in density on top of the propeller and below the propeller.The air on top of the helicopter has less density and the bottom has more density. This generates a force to push the helicopter up from difference in density. He is calling this a fundamental principle of physics. I am trying to tell him that the helicopter creates a wind aka force push in the air to push it up and there is no change in density. He also has been trying to convince me for days that if i am holding a box in the air, it is a fundamental law of physics that my body density is greater than the air above it, so it stays up. I believe the box stays up because my hand pushes with a force upward greater than gravity, and thus is not pushed down, unrelated to a density difference between my body and the air. Another case he has just tried to convinced me I am certainly incorrect about is that if I blow a piece of paper into the air, he says I have manually changed the density of the air below the paper and the air above the paper is less dense and thus is moves upward. In his explanation, anytime there is a force or pressure the density has to change. I am saying absolutely not, sometimes density will cause things like a helium balloon to float, sometimes it is a force independent of density. Thank you for any explanations.