July 24, 2021

# Where do balloons go? | Science

The soap bubbles are so light (weightless and gentle, as Machado would say) and fall so slowly that the minimum resistance of the surrounding air does not deform them. Very small raindrops, less than a millimeter in diameter, are also spherical; but the larger ones, due to the resistance of the air, which "crushes" the base, take the form of a round muffin. Little to see, therefore, with the usual tear shape that has been imposed as an icon of rain. Why do we represent raindrops in this way? It is tempting to respond that we associate them with tears (when it rains, the sky cries); but we do not usually see tears falling from the eyes in the shape of a tear, worth the redundancy. There has to be another explanation …

We were wondering last week What happens if we direct the air jet of a fan towards a balloon? Intuition tells us that the current of air will separate the balloon, and yet it will catch it (the word game fans will not have escaped that "catch" is an anagram of "will separate"). So much so that, if we move the fan, the balloon will move keeping within the air jet. This curious behavior is a consequence of the Venturi effect: the pressure of the moving air is slightly lower than that of the surrounding static air, and that is what causes the balloon to remain trapped in the fan jet.

A remarkable consequence of the Venturi effect is the Magnus effect, or "effect" to dry if we are talking about a ping pong ball or other balls that spin quickly while moving in the air: the speed difference of microcurrents Surrounding the rotating spheres results in a pressure difference on both sides that makes the path slightly curved. In the case of the very light ping pong ball the effect can be very marked; but even an object as heavy as a soccer ball can manifest the Magnus effect ostensibly, giving rise to the spectacular technique known as "thread" or "chanfle", which allows the ball to enter the goal from the corner point.

When a car turns to the right, the occupants, by inertia, tend to move to the left side of the vehicle; but our balloon last week will do the opposite: it will move to the right. Why?

### How high?

When a helium balloon escapes from the hands of a child, a rather frequent event, how high does it rise? The density of the air decreases with the height, but also the pressure, with which the helium of the interior of the balloon will expand, its density will decrease and it will remain less dense than the surrounding air … until the balloon explodes. And if it did not explode? If the latex could expand indefinitely without breaking, how far would the fugitive balloon ascend?

Carlo Frabetti He is a writer and mathematician, a member of the New York Academy of Sciences. He has published more than 50 scientific dissemination works for adults, children and young people, among them Damn physics, Damn mathematics or The big game. He was a screenwriter The Cristal ball