# Can You Make a Hollow Metal Sphere So Big That It Floats?

This works for any shaped object where V is the volume. Notice that this total force from the air only depends on collisions between the air molecules and the surface. It doesn’t matter what the balloon is made of or what it’s filled with. Only the volume matters.

Then why does a party balloon float but a basketball of about the same size falls? It has to do with whether the upward-pushing buoyancy force is enough to overcome the gravitational force that is pulling the object downwards.

Let’s put in some numbers. Let’s assume both a basketball and a balloon have a diameter of 20 cm. Calculating the volume and plugging into the Fair equation, I get an upward-pushing force of 0.049 newtons. That’s tiny.

But the rubber shell of a balloon is thin, so the gravitational force is not very large. And if you fill it with helium, a gas that has a lower density than air, you can compensate for the mass of the balloon’s thin surface and achieve equilibrium. If you can get the mass of the rubber plus the helium gas to be the same as the upward-pushing buoyancy force, the balloon floats.

It doesn’t matter what you put in the basketball; it will still fall. The rubber shell of the basketball is much thicker and heavier than the wall of the balloon. The tiny buoyancy force is essentially insignificant compared to gravity’s pull on an object with this mass, and it can’t overcome it. So the ball drops.