CoachA thing is in equilibrium — we also say balanced — when the pushes and pulls on it cancel each other out, so it stays exactly as it is. The tug-of-war ribbon standing still between two mighty teams: equilibrium. Not a world without forces — a world where the forces are having a perfect tie.
示例
Example 13.2 (The book on the table)
A book lies on the table, perfectly still. Boring? Look closer with last chapter’s eyes. The Earth pulls the book down — so why does it not fall? Because the table pushes it up, exactly as hard as the Earth pulls down. Two arrows, equal and opposite: a tie. Take the table away and the tie is broken: down goes the book.