Question

Which object is in static equilibrium?

a rolling ball that has a net force acting on it
a cruising car that has no net force acting on it
a book that has no net force acting on it and is sitting on a table
a vase that has a net force acting on it and is sitting on a shelf

Answer 1

Static equilibrium means the object is at rest and the net force (and net torque) on it is zero.

Only the book (no net force and sitting at rest on the table) is in static equilibrium.

(For reference: the cruising car is in dynamic equilibrium (moving with constant velocity), the rolling ball and the vase with a net force are not in equilibrium.)