An inflated balloon falls slower than an uninflated balloon? How can we explain this?

Answer

An object falling in air (or in water) experiences a frictional force. The whole mechanism is quite complicated, but roughly that frictional force is proportional to the speed squared and dependent on the shape (streamline) of the object. If you drop an object, its speed will increase initially, but as it increases, the frictional force will also increase. So there comes (quite quickly) a moment when the frictional force is equal to the gravitational force and from that moment on the object falls with a constant speed: the so-called terminal speed (parachutists know this all too well). The diameter of the inflated balloon is much larger than that of an empty balloon, so the frictional force at the same speed is greater and that equilibrium is reached faster.