As you saw, a steel ball floated in mercury, so mercury, being very dense, had displaced enough mercury to equal its weight before it sank. How does this relate to helium filled balloons floating in air?

Explanation

Using equation

U-W=ma

U is uptrust or the buoyancy force

W is weight

There is a net upward, or buoyant force on any object in any fluid.

Let analyse the three situation

1. The buoyant force is greater than weight of object, then, the object will float.

2. If the weight is higher than the buoyant force, the object will sink.

3. Now, If the buoyant force equals the object's weight, the object will remain suspended at that depth.

Helium balloons work by the same law of buoyancy.

The helium balloon displaces an amount of air (just like the empty bottle displaces an amount of water). As long as the helium plus the balloon is lighter than the air it displaces, the balloon will float in the air. It turns out that helium is a lot lighter than air.