The Stone the Air Did Not Have to Push

After the hand lets go

Throw a stone and ask the awkward question at exactly the right moment: once the hand has released it, what keeps it moving?

An influential Aristotelian answer gave the surrounding medium a role in carrying the projectile onward. By the fourteenth century, that explanation felt increasingly strained. In lectures at Paris, Jean Buridan argued that the thrower impressed an impetus into the stone. The motion continued because the moving body carried this acquired tendency, while air resistance and gravity gradually altered it.

This is a modern projectile reconstruction, not Buridan’s calculation. Its changing velocity arrows make visible the separation that later mechanics would achieve between forward motion and downward acceleration.

A wrong theory can move the question

Impetus was not Newtonian inertia. Buridan thought it could be spent or corrupted, and he did not build a universal mathematical mechanics from it. That is precisely why the episode matters.

The history of physics is not a relay race in which each thinker hands over a finished law. Sometimes progress is a change in where the explanation is allowed to live. The motion no longer needed to be pushed from outside at every instant. Something about the state of the moving body itself had become part of the account.

Two centuries later, Galileo would make motion slower and measurable. Buridan had already made it possible to ask what a body keeps when the hand is gone.