air power using balloons

[perritos2]

I am doing a project of air power using balloons. I filled to same size balloon with different amounts of air. To my surprise the one with less air traveled faster. I repeated the experiment and the same thing happened. Why is that? I appreciate your help.

[ghariman]

Hi,

I think the velocity of the travelling balloon is not determined by the amount of air that is in the balloon. Instead the velocity is determined by the size of the opening (mouth) of the balloon that lets the air out. And of course by the weight of the balloon + total air. So perhaps the balloon with less air is less in weight. And also perhaps another contributing factor is the shape (aerodynamics) of the balloon. A fatter balloon will have worse aerodynamics than a thinner balloon. And so maybe this also contributes to the thinner balloon with less air travelling at a greater velocity.

The amount of air in the balloon I suspect will determine the distance travelled.

So the balloon with more air will travel slowly but then end up travelling further. While the balloon with less air will travel faster but will only end up at a shorter distance.

Is this what you saw in your experiment ?

[deleted-71576]

I think what you are seeing is a demonstration of Poiseuille's law, which basically states that the flow through a tube is proportionate to the radius of the tube to the 4th power. It is also directly proportional to the driving pressure.

Why is this important to your balloon? Well, a doubling of pressure in your balloon will double the air flow out of the balloon. But if you double the size of the opening at the end of the balloon where the air comes out, it will cause an 8 times increase in the flow through the balloon.

I think what is happening is that it is harder to keep the more inflated balloon tied up, and your way of attaching it to the car is constricting the outlet more than with the smaller inflated balloon. This would have a dramatic negative effect on the flow out of the balloon.

There are also possible issues with inertia with differing sized balloon openings. If you took similar balloons with similar pressures, but different openings, the balloon with the larger opening would allow out a larger flow of air for a shorter amount of time. The balloon with a smaller opening would allow a lesser flow of air for a greater amount of time. It will accelerate for a longer period of time, and I think reach a greater speed. Rockets don't produce massive amounts of thrust for short periods of time. The allow time for the rocket to accelerate by having moderate amounts of thrust for long periods of time. It is also why 2 stage rockets go higher than larger single staged rocket.

You can test this phenomenon with bottles of Diet Coke and Mentos candies, with different sized openings in the top. Small opening - long slow stream of Diet Coke. Large opening - short, fast stream of Diet Coke.

[deleted-71576]

To clarify - the two phenomena I mentioned will have opposite effects on the distance travelled. Some combination of the two will be causing what you are seeing.