What is an equilibrium in physics?
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It’s the same state of two interacting parts of a system. Think of an inflated balloon in a normal room: the air inside the balloon is at a higher pressure than the air outside. If you open it up, the air rushes out until the pressure difference is eliminated. At that point, the pressures are at equilibrium.
Similarly with a fresh cup of coffee. It’s hotter than its environment. Heat would continue to be transferred from the cup to the air and surrounding surfaces at a decreasing rate as the temperature difference is reduced, until they are the same. That would be an example of temperature equilibrium, when the transfer considerably stops.
For a movement example, a skydiver. Gravity accelerates them downwards. As they gain speed, air resistance (an upward force in this case) increases. Once the air resistance and gravity become equal, the person stops accelerating and falls at a steady rate. The forces are said to be in equilibrium because the change in velocity has gone to zero.
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