Does the air exert buoyant force on all objects in air or only on objects such as balloons that are very light for their size?

Answer 1

Buoyant force is observed whenever an object displaces a fluid. The apparent weight of an object is decreased by the weight of the fluid it displaces. Since air isn't very dense, it normally isn't too noticeable unless the object also has a very low density.

If you weighed an object with a very accurate scale within a vacuum (no air) and then weighed it again in the air, you would see a small decrease in the apparent measured weight due to the buoyant force of the air it is displacing. If the displaced air weighs more than the object displacing it, the object floats in the air. If not, then it settles to the ground.

Most things are a lot more dense than air so we don't notice the buoyant forces because they are trivial compared to the force of gravity on the object - but they are present. When you get into a much denser fluid like water, the buoyant forces are a lot easier to notice. You will find it a lot easier to lift a 50 kg weight from the bottom of a pool to the surface of the water than to lift it the same distance out in the air.