Answer: An extended object is in static equilibrium if the net force and net torque are zero.
Explanation:
An object would be at equilibrium if the net force and net torque is equal to zero.
The second Newton's law for this type of object is:
Fnet = M*Acm
Where M is the mass and A is the acceleration of the center of mass of the object.
Now, also there is something called torque, that is the force that makes the object to spin, if you apply torque to an object, this will start to spin, and iff you mantain the torque, the spining velocity will increase.
So for example, if a tree is moving (and maybe rotating at a constant rate) in vacuum with constant velocity, where no forces are afecting the tree and then there are no acceleration of the center of mass, such tree is in equilibrium, and obviusly, if the tree is not moving at all the case is the same.
