.Tuesday, March 10, 2009 ' 8:59 PM Y
Dynamic equilibrium :
A system in dynamic equilibrium is a particular example of a system in a steady state. In a steady state the rate of inputs is equal to the rate of outputs so that the composition of the system is unchanging in time. For example, a lake is in a steady state when water flows in at the same rate as water flows out. The term dynamic equilibrium is used in thermodynamics for systems involving reversible reactions. It is said that the rate of the forward reaction is equal to the rate of the backward reaction. Both reactions do in fact occur, but to such a minuscule extent that changes in composition cannot be observed.
Static equilibrium :
We have seen that objects do not move when the sum of the forces acting on them is zero. We can use this fact to find the conditions for "static equilibrium": the condition an object is in when there are forces acting on it, but it is not moving. The conditions for static equilibrium are easy to state: the sum of the forces must equal zero, and the sum of the torques must be equal to zero .
We will illustrate the use of these conditions with several examples from the human body. In each case, notice that the idea is to keep the relevant body part from moving in either the horizontal or vertical directions, and, once a convenient pivot is chosen, to keep the part from rotating around that pivot.
