Why Does Releasing Air from a Balloon Propel It?

[nolachrymose]

Hi all,

I'm relatively new to physics, and was curious: when one does let the air release out of a ballon, why does it, and in which way is its force acting? I thought maybe the direction of the force was in the opposite direction it was traveling, but that doesn't seem to make much sense, since that would mean it wouldn't move because of Newton's third law. And does the amount of air surrounding the balloon have any effect on its force (i.e. would it move differently in a vacuum?)? Any information would be greatly appreciated! Thanks! :)

 

[bino]

basically what's happening is the air pressure in the balloon is what pushes the air out of the balloon. and since every action has a reaction the air then pushes the balloon. it would move the same in a vacuum. this is kind of how spaceships move in space.

 

[geometer]

basically what's happening is the air pressure in the balloon is what pushes the air out of the balloon. and since every action has a reaction the air then pushes the balloon. it would move the same in a vacuum. this is kind of how spaceships move in space.

This is exactly how spaceships move in space. They accelerate a gas of somekind, rocket exhaust, steam, compressed air, etc. out a nozzle and the reaction force acclerates the spaceship. Newton's Third Law at work!

 

[neutrino]

Rocket motion not only involves the third law, but also the law of conservation of momentum. As the rocket loses mass in the form of exhausts it gains velocity.

 

[Gonzolo]

If it were in a vacuum, I believe the balloon would fly in a parabola (or a straight line if there is no gravity). I think the funky flightpath we see is due to aerodynamical effects caused by fluctuations in balloon shape as it shrinks.

Balloon rubber elasticity causes pressure. This pressure pushes [air] in one direction and [air + ballon] in the other direction. Because of symetry (cylindrical symetry), all other forces in all other directions cancel out. Only one pair of forces remain, and Newton's 3rd law takes care of it.

 

[nolachrymose]

Thank you for the information.
However, I have one more question: if the only two forces (as you said all others cancel out) are the equal and opposite forces, then how is it that the balloon moves, rather than remaining stationary?

 

[Tide]

Thank you for the information.
However, I have one more question: if the only two forces (as you said all others cancel out) are the equal and opposite forces, then how is it that the balloon moves, rather than remaining stationary?

The forces are equal and opposite - they don't "cancel." The expelled air moves one way and the balloon moves the other way.

 

[Gonzolo]

Thank you for the information.
However, I have one more question: if the only two forces (as you said all others cancel out) are the equal and opposite forces, then how is it that the balloon moves, rather than remaining stationary?

I said that the pressure pushes [(more) air] in one direction and [(less) air + (all the rubber from the) balloon] in the other. This is because there is a hole only on one end. For it to remain stationnary, you would need another hole, opposite to the first.

 

[Gonzolo]

The forces are equal and opposite - they don't "cancel." The expelled air moves one way and the balloon moves the other way.

It is the forces acting laterally to balloon trajectory that cancel out. Not the pair of opposites that causes the trajectory.

 

[antiflag403]

FYI- The idea that a spacecraft could use expelled gases as thrust was first proven by an american rocket scientist by the name of Robert Goddard. He is known as the american father of rocketry and is ranked up there with Oberth and Tsiolkovsky. One of his first, and simplest experiements that proved this theory was conducted by placed a pistol inside a vacuum and then shooting blank rounds. The gun, which was connected to a pipe which was free to rotate, spun in circles. This was the first time the theory had been proven correct.
Most of you probably already knew this... but i thought it was an interesting part to the story.

 

[enigma]

Well, actually Tsoliovsky figured it out first, but Goddard didn't have access to the research, so it was credited to him as well.

Welcome to PF anti-flag!

 

[antiflag403]

First of all thank you enigma for the welcome.
Secondly... i guess that's what i get for believing things i read out of biographies. Was goddard at least the first to experimentally prove it??
Thanks again

 

[enigma]

Yes, Goddard was.

From http://www.spaceline.org/history/21.html on Tsiolkovsky (you had the spelling right... I can never remember it):

Tsiolkovsky completed a draft of his first design of a reaction thrust motor on August 25, 1898.

Although practical strides in rocketry were being made at this time in other parts of the world, Tsiolkovsky never saw his designs materialize. His rocket motors were neither built nor tested, primarily due to Russian political instability, lack of resources and inadequate technical personnel.

Goddard was 16 at that time according to http://www.time.com/time/time100/scientist/profile/goddard.html and his first rocket test was on March 16th, 1926.