Have you ever seen a child with a balloon? It’s fun to watch. Kids pay attention to the world around them: They know that when you let go of something, it falls. Balloons don’t follow these rules, and it’s that exception that makes balloons so fascinating.
But what about adults? We still love seeing things that don’t seem to follow our normal rules. Parade balloons seem to cheat physics in order to move through the sky. Of course, they aren’t cheating physics. It is because of physics that they are able to float.
Why Doesn’t the Balloon Fall?
There is indeed a force pulling down on these massive balloons. This gravitational force is proportional to the mass of the object. Both the outer material and the gas within have mass that results in a weight of perhaps 2,000 Newtons (450 pounds). Yet even with so great a downward force, the balloons stay aloft. There must be an upward force at work on the object. This is buoyancy force, and it is caused by a differential air pressure on the top and bottom of balloon.
You can think of the air as a bunch of balls bouncing around. When these air-balls hit a surface (like the side of a balloon), they bounce off. Since the ball changes momentum, it must be pushing against the balloon with some force. This force then depends on the number of air-balls that hit the surface as well as the speed and mass of the air-balls. But here’s the cool part. In order for all these air-balls to not just fall down on the ground, they must have more collisions in the upward direction than the downward direction. This means that as you go lower in the atmosphere, the density of air increases, resulting in greater pressure.
But how much does this air push on an object like a balloon? The easiest thing is to consider a block of air floating in air. Yes, that might seem silly but there is a reason for this. If there is no wind, that block of air in air should remain stationary. That means that the net force pushing on this air must be zero Newtons. Here is a diagram showing all the forces on this floating block of air.