Chemical equilibrium exhibits no change on the macroscopic level. A first class lever in a condition of static equilibrium remains at rest. The sum of all of the moments of the forces acting is zero. A gradient is a description of a pattern or variation. The description includes both the magnitude and the direction of the change.
Light intensity decreases in a predictable manner as the distance from the light source increases.
On a mountain, the direction in which the change of slope is smallest is the most desirable route to build a railroad line. It is an action within one system which causes a similar action within another system. A wine glass can be made to shatter by sound vibrations due to mechanical resonance. It is the belief that certain differences exceed those that would be expected to be caused by chance alone. Validation is a belief that similar relationships obtained by two or more different methods reflect an accurate representation of the situation being investigated.