Our culture owes a lot to the influence of Newtonian mechanics in shaping the thinking in other areas, such as in Psychology with the notion of "energy". Even though this usage is obsolete within professional circles, the expression is still an active part of the popular culture.
The fabric of our thinking also has another Newtonian concept as a background: the mechanical "static equilibrium". We usually see stability in a way that is contradictory to what happens in life, where equilibrium is a delicate balancing of multiple forces woven in a dynamic exchange.
This cultural influence extends to ideas on project management. We tend to see projects as a collection of static endeavors where everything is predictable, punctuated in between with checkpoints at which some change might be added, in a predictable way, into the system.
I would rather refer to Heraclitus "everything changes" idea when thinking about project management. And that implies continuous adaptation, not just at staggered checkpoints or milestones.
Juggling has always been a great metaphor for project management, for although we don't think about it while watching their mesmerizing performance, jugglers are constantly adapting to where the balls go. Even though they kind of have an idea of a ball's trajectory through their experience, their greatest skill is going after it in the nick of time.
The following video by Fatboy Slim is a homage to adaptive project managers of all times, from Heraclitus to Agilistas today, with eyes on the ball and ears on the rhythm. Enjoy...