You measure some gold and some feathers on a good scale. Each is determined to have a mass of 1.00000 kg. Which actually has the greater mass?

A) Gold

B) Feathers

C) Same

Solution: B

Scales are not mass detectors. They measure normal force and then infer mass. So we know that the normal forces are the same for both objects.

Feathers are less dense than gold, so they will have a larger volume. The larger volume will mean a larger buoyant force from the air. The weight force must be a bit larger in order to get the same normal force. So the actual mass of the feathers must be larger.