Expert: Daniel Mazur - 1/19/2010

Question
can you explain why an inflated balloon shoots off when is air is released? age 13

Answer
Yes Ayesha,

But let's start at the beginning. Most balloons are pressurized in order to inflate. This means that when inflated, the walls of the balloon are stretched. The stretching tension is equal to the overpressure between the air inside and outside. When you pull the air plug, the balloon will use the opportunity to even out the pressure inside with the pressure outside - it releases the air.

Why is the air release accompanied by the balloon shooting off in the opposite direction? This is explained by one important law of Nature, which we call the "conservation of momentum". It works like this:
Every material thing has some mass, right? Everything also has a velocity. Things that stand still have velocity as well, only the magnitude of the velocity is zero. The "momentum" I referred to above is simply a product of mass and velocity.
Now, say we have something that is made of two parts. It can be two people together, but it can be a balloon with air inside. First, both parts are standing still. The balloon has zero velocity and the air inside has zero velocity, and so the full pressurized balloon also is standing (lying, hanging,...) still. The total MOMENTUM is zero. Once you pull the plug, the air is rapidly flowing from the inside outward with an average velocity that is NOT zero. Suddenly, the air does not have a zero momentum.
The "conservation of momentum" says that if something starts at rest and nobody moves it from the outside, it must stay at rest for eternity. In our problem the air part of our balloon is suddenly moving out in some direction and the only way for the whole balloon to obey the Law is that the balloon shell (or skin, whichever way you call it) starts moving in the opposite direction to the air flow. Only that way the momentum (velocity times mass) of the balloon skin can subtract (as they go in opposite directions, we subtract) from the momentum of the released air and the result of subtraction will be zero.
You can say shortly that the center of mass of the "system balloon+pressurized air" will have the same momentum (velocity) before and after the release of the air. The principle is used in all our rockets as propulsion, called the "jet propulsion".

Take care,
Daniel