* Support: The shelf is supported by something, usually a wall or a bracket. This support provides an upward force that counteracts gravity.
* Equilibrium: The forces of gravity pulling the shelf down and the support pushing the shelf up are balanced. This is called equilibrium, and it's why the shelf stays in place.
Think of it like a tug-of-war: Gravity is pulling one way, the support is pulling the other way, and they're equally strong.
