Equilibrium & stability
Since a stationary point is a point that is momentarily flat, that point is like every point on a flat shelf.
- Place a ball on a shelf and it won’t roll down;
- place a ball at a stationary point and it theoretically won’t fall down.
We might say that the ball is in equilibrium when it in this way has no tendency to move and to release all stored energy. That of course counts for both translation and rotation. No translation means Newton’s 1st law, and no rotation means the rotational version of Newton’s 1st law. So, these two are requirements for what we call ‘equilibrium’:
- ‘An Introduction to Human Movement and Biomechanics’ (book), Andy Kerr and Philip Rowe, Elsevier Evolve, 7th ed., 2019, books.google.dk/books?id=HjaPDwAAQBAJ (accessed Jun. 20th, 2020), ISBN 978-0-7020-6236-0