The Roots of the Scientific Revolution 43
The acceleration of a falling body is explained as due to impetus
continually being added to a body due to its weight. The ability of a body
to take on or retain impetus was thought to be proportional to its density.
Thus a feather falls more slowly than a stone but a large stone falls at the
same rate as a small stone. The heavenly bodies that moved above the
atmosphere did not lose impetus because of an absence of air resistance.
The formulators of impetus were just on the verge of the concept of
inertia, the idea that a body will remain in a state of constant rectilinear
motion until some force changes its motion. The concept of impetus did
not arise in response to new observations but rather from a different
interpretation of the known facts.