>>281284441
It's half a semantics and half a physics thing, laymen say weight when they mean mass, so by the layman's definition of weight, yeah, a kilogram of anything weighs the same as a kilogram of anything else.
If you treat kilograms as a unit of mass, as they're meant to be used, a kilogram of anything is also the same mass as a kilogram of anything else.
Now, the issue is that when measuring actual weight (mass*gravity), at first glance, you'd think they also weigh the same, 1KG*9.8= 98N, and, in a literal vacuum, this is correct. But you need to take into account that however small, air does have buoyancy; Normally, this is negligible, but in the case of objects with extremely low density, the difference is a lot larger, and a kilogram of feathers is an example of extremely low density.
The end result is that buoyancy pushes the kilogram of feathers up more than it does the kilogram of iron/steel/whatever metal the person wants to yap about, so the kilogram of feathers is lighter despite being the same mass.
I feel that I explained it like shit but keep in mind I'm not a physicist, just someone who has had to deal with drunken physicists rambling about this whenever someone brings it up
