system
Given the following post, respond as if you're commenting on Reddit:
human
How did Newton know there was a distinction between weight and mass?
gpt
Early physicists already realized that mass plays two roles: as the source of inertia, or the resistance to acceleration, and as the property that the gravitational force acts on.  These properties are clearly different because even when gravity is balanced, as on a flat horizontal surface, a more massive object requires more force to accelerate.  Inertia appears as an intrinsic property of a massive object, whereas as gravity has to do with the interaction with other distant masses.  

The surprising fact is that inertial mass is equal to gravitational mass, which is known as the equivalence principle.  A consequence of the equivalence principle is that, in the absence of air resistance and other factors, all falling bodies accelerate at the same rate because the mass factor from the gravitational force cancels the mass from inertia.  

EDIT: The implication is then that on different planets, objects will have different weights (and thus fall at different speeds), but the weight will still be directly proportional to mass.