From what I see when we change density of the actor with "Set Density" block the weight attribute of this actor is also changed to the same value (when script is running). Are density and weight synonyms in Tynker Physics? I would expect that density should be linked to weight through area of the actor (for 2D). Also density should be useful for working with liquids.
The density is used to set the mass of a body, measured in kilograms by square meter. Higher density means heavier bodies, and this is used to differentiate between two same-sized objects on the screen like a balloon and a bowling ball.