When a system of forces acting on an object produces no motion, the system is said to be in static equilibrium.
A rigid body is in mechanical equilibrium when the sum of all forces on all particles of the system is zero, and also the sum of all torques on all particles of the system is zero.
A rigid body in mechanical equilibrium is undergoing neither linear nor rotational acceleration; however it could be translating or rotating at a constant velocity.
A system is in mechanical equilibrium if its position in configuration space is a point at which the gradient of the potential energy is zero.
Because of the fundamental relationship between force and energy, this definition is equivalent to the first definition. However, the definition involving energy can be readily extended to yield information about the stability of the equilibrium state.
Reference: Mechanical equilibrium wikipedia.org
What is the rationale for relaxing the abdomen and withdrawing the coccyx (or tailbone)?
Qi is stored in the Dan Tien as a result of using consciousness to sink the Qi to this point. From here Qi should circulate to the whole body. If Qi just remains in the Dan Tien, then the abdomen will have the sensation of being stuffed. Only when Qi circulates throughout the body will the abdomen be relaxed and pliable. After a time, the abdomen will acquire some “bouncy” or “springy” effect and Qi would have been circulating to the whole body. Qi can be occluded or absorbed into the backbone. The Song of the Thirteen Postures says, “If the abdomen is thoroughly relaxed, then the Qi will rise.” So do not just store the Qi in the abdomen otherwise it will simply bloat. Having the coccyx withdrawn means there is no protrusion of the buttocks while making sure at the same time that the hip joints are not “sliding” forward. This must be combined with relaxing the abdomen and both requirements must be met at the same time. Otherwise, there is no rootedness while the waist is stiff, resulting in vertical imbalance or disequilibrium. It is important to maintain the uprightness of the central axis of the body in order to achieve central equilibrium. A test can be made as follows to see whether all this has been done correctly all along: use one thump to press the abdomen and release the thumb suddenly. There should be a bouncing or springy effect of the abdomen. At the same time, the seat of the buttocks behind should be very soft to the touch.
Relax the chest.
Raise the back.
Enclose the solar plexus.
Protect the cheekbones.
Lift the head.
Suspend solar plexus.
Loosen the shoulders.
Sink the elbows.
Taiji is born of Wu Chi. It is the origin of dynamic and static states and the mother of yin and yang. If they move, they separate. If the remain static, they combine.
We are centered, stable and still
Our Chi sinks to the tan-t’ien and
we are as suspended from above.
Our Spirit is concentrated within and
our outward manner perfectly composed.
Receiving and issuing energy are
both the work of an instant.
Just stay centered in the Now.
Taiji quan Kongfu of Master Zhu Datong 2
Taiji quan Kongfu of Master Zhu Datong 3
Taiji quan Push-hand of Master Zhu Datong 2
Song of Central Equilibrium dyhr.com