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What is meant by the word EQUILIBRIUM?
 
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The concept of equilibrium is an extremely important one. Here are a number of examples:

1. Pendulum: while it is swinging back and forth it is NOT in equilibrium - there is a net forward, or a net backward direction. But when the pendulum stops and hangs straight down, then it is equilibrium. There is just as much forward as backward direction (both = zero, in this case). Physicists call this static equilibrium, or static steady state.  
2. Toaster: unplug your toaster. The wires are cool - they are neither heating up nor cooling down; they are in a state of inactive equilibrium - just like the pendulum at rest. BUT a toaster has another point of equilibrium - plug it in and turn it on and the filaments get hotter and hotter up to a certain temperature. Then, no matter how much longer you keep the toast "on", the filaments remain at the same hot temperature. Electrical energy is going in, and heat energy is coming out - and the two balance each other. They are in "dynamic" equilibrium, or dynamic steady state.  
3. Biologically, there is a phenomenon known as homeostasis. This is where the cells are taking in food and oxygen, and giving off wastes and CO2 and maybe a little heat. There is active input, and there is active output, and yet the cell remains the same. It is not dead (static equilibrium), but it is in a state of dynamic equilibrium or dynamic steady state.  
4. Chemistry: In the glass of water in front of you there are a few molecules that dissociate into H+ and OH-. The concentration of the H+ remains the same. If it is pure water at 20c, the concentration of H+ = 0.0000001 molar (pH = 7.00), But at any given instant the H+ ions are not the same ones that existed an instant earlier. The ones earlier reassociated with an OH- and another water molecule dissociated somewhere in the glass. Just as many associations are happening as dissociations - another case of dynamic equilibrium or dynamic steady state.  
5. There are many different ways to slice the physics pie. One way is the realm of statics versus the realm of dynamics. Force is in statics; velocity is in dynamics. Dynamics means movement. Dynamic equilibrium says that both the forward movements and backward movements are the same. But the is nonetheless movement in the seething mass of molecules - it's just that they whole mass has no net movement.

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