Place the beaker full of steel balls and submerge the table tennis ball under the steel balls. The table tennis ball does not float up. Why does it not float up? Do table tennis balls float when the diameter of steel balls is reduced? How to calculate the buoyancy of steel balls? Would it come up without friction?
The ball bearings are behaving as a solid because the forces between the steel balls (i.e. friction) are large enough to hold the balls in position relative to each other.
If you apply enough force to a solid you will cause it to fracture or to cause plastic flow. So for example if you attached a string to the ball and pulled upwards with enough force it would cause the steel balls to flow over each other and the table tennis ball would move up. The force required is related to the yield stress of the solid formed by the steel balls.
You can make the steel balls behave as a fluid by making a gas flow through them. This creates a fluidised bed. The gas pushes the steel balls apart so the friction between them is removed, and in this state the steel balls will behave like a fluid and the table tennis ball would float upwards.
Alternatively just shake the beaker. This is equivalent to adding thermal energy i.e. heating the system until it melts. If you shake the beaker you'll find the table tennis ball floats upwards.
Well, what if the steel balls were extremely small, say molecular size. In that case, the constraining annulus would look like a polished steel collar, and would likely hold down the ball even if the glass was shattered, underwater, in a swimming pool. . (The van der Waals forces, and metallic bonds, would account for that.)
But this example given, shows discreet balls of intermediate size, and unless they are magnetized, their coupling with the container is what allows restraint of the tennis ball. If THIS setup was in the bottom of a deeper pool, and the beaker was shattered, the steel balls would run radially away, and the tennis ball would pop up.
(Note: I answered this as if there was water in the beaker along with the steel balls and tennis ball. But the answer is not changed by my error.)
The term "van der Waals force" is sometimes used loosely for all intermolecular forces.