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Types of Equilibrium

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1. Based on the State of the Object (Mechanical Equilibrium)

A. Static Equilibrium

• The object is at rest and remains at rest.
• Net force = 0 AND net torque = 0.
• Example: A book lying on a table, a building standing still.

B. Dynamic Equilibrium

• The object is in motion but at a constant velocity (no acceleration).
• Net external force = 0 and/or net external torque = 0.
• Example: A car moving at a steady speed on a highway, a box being pulled up an incline at constant velocity.

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2. Based on Stability (Nature of Equilibrium)

A. Stable Equilibrium

• When displaced slightly, the object returns to its original position.
• A restoring force acts opposite to the direction of displacement.
• Center of gravity is low, and lies within the base of support.
• Example: A ball at the bottom of a bowl, an upright cone.

B. Unstable Equilibrium

• When displaced slightly, the object moves further away from its original position.
• The net force or torque acts in the same direction as the displacement.
• Center of gravity is high.
• Example: A ball balanced on top of a hill, a pencil balanced on its tip.

C. Neutral Equilibrium

• When displaced, the object stays in its new position - it neither returns nor moves further away.
• Center of gravity does not shift.
• Example: A ball on a flat surface, a cylinder lying on its side.

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3. Chemical Equilibrium

• Rate of forward reaction = Rate of reverse reaction
• Concentrations of reactants and products remain constant over time (not necessarily equal).
• Governed by the Equilibrium Constant (K).
• Described by Le Chatelier's Principle - if a system is disturbed (by changes in temperature, pressure, or concentration), it shifts to counteract the disturbance.

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4. Thermal Equilibrium

• Two systems are in thermal equilibrium when they are at the same temperature and no net heat flows between them.
• The basis of the Zeroth Law of Thermodynamics: if A is in thermal equilibrium with B, and B with C, then A is in thermal equilibrium with C.
• Example: A cup of hot coffee cooling down until it matches room temperature.

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5. Thermodynamic Equilibrium

• A broader concept combining thermal + mechanical + chemical equilibrium.
• A system is in full thermodynamic equilibrium when there are no unbalanced driving forces (no gradients in temperature, pressure, or chemical potential).

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6. Biological Equilibrium

• Homeostasis - the body's ability to maintain stable internal conditions (temperature, pH, blood glucose) despite external changes.
• Example: Hemoglobin-oxygen binding equilibrium in blood, bicarbonate buffer system maintaining blood pH.

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7. Economic Equilibrium (bonus)

• A market is in equilibrium when supply equals demand, resulting in a stable price.
• Example: Nash Equilibrium in game theory - where no player can gain by changing their strategy unilaterally.

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Summary Table