ece105: add statics

docs/ce1/ece105.md

@@ -194,3 +194,20 @@ $$
 v_{cm} = \sqrt{\frac{2}{1+c}gh} \\
 a_{cm} = \frac{g\sin\theta}{1+c}
 $$
+
+## Statics
+
+An object at **static equilibrium** has no rotational or translational velocity with zero net force and torque.
+
+An object at **dynamic equilibrium** has a constant rotational and translational velocity with zero net force and torque.
+
+$$
+\sum\vec F = 0 \\
+\sum\vec\tau = 0
+$$
+
+Whether an object *stays* at static equilibrium depends on the 
+
+- It is at **unstable equilibrium** if the object moves away and does not return to equilibrium
+- It is at **stable equilibrium** if the object returns to its original position and equilibrium
+- It is **neutral** if the object does not move