Balloon rocket- What happens inside and why the reaction?

[Guidog77]

TL;DR Summary

I understand that the balloon will go in the opposite direction than the air that comes out. But other than Newton's law, I can't find an explanation of why the balloon goes in the opposite direction.

The law of action and reaction is the explanation that I see everywhere. But I can't find anywhere what exactly pushes the balloon in the opposite direction as the air coming out.

Air molecules come out to where pressure is less. What exactly moves the rubber balloon in the opposite direction?
Can anyone explain this without referring to a principle?

Thank you.

 

[PeroK]

The air pressure inside the balloon is greater than the air pressure outside. While the balloon is sealed it pushes approximately equally in all directions. If one end of the balloon is opened, then there is no longer any pressure on that side. That leaves an unbalanced force on the balloon.

A simpler scenario would be two people pushing outwards on opposite sides of an object. If the object opens at one end, that person tumbles out and the object starts to move under the force from the other person.

 

[Baluncore]

Summary:: I understand that the balloon will go in the opposite direction than the air that comes out. But other than Newton's law, I can't find an explanation of why the balloon goes in the opposite direction.

Can anyone explain this without referring to a principle?

Welcome to PF.

The circular hole at the back where the air comes out, is directly opposite an equal sized circle at the front, where the air does not escape. It is the unbalanced pressure between those two small areas that drives the balloon. The pressure on the inside at the front pushes the balloon forwards.

 

[DaveC426913]

Air molecules come out to where pressure is less. What exactly moves the rubber balloon in the opposite direction?
Can anyone explain this without referring to a principle?

For an ultra-simplistic visual explanation, let's pretend there are only ten "air molecules" in the balloon, as suggested by the diagram below.

They all push against each other to create a pressure in the balloon, pushing outward.

In the diagram, you can see eight "air molecules" pushing outward (red arrows), but they are in pairs - one opposite another. Their forces cancel out. The balloon goes nowhere.

But one molecule, blue, shoots straight out of the balloon to the right. It applies no force to the balloon.

But it also means that one molecule - out of all ten of them - the green one - has no opposing force. It pushes on the front of the balloon, giving it a shove forward.

Multiply that by a few brazillion molecules, and you have a net force toward the left.

 

[256bits]

A simpler scenario would be two people pushing outwards on opposite sides of an object. If the object opens at one end, that person tumbles out and the object starts to move under the force from the other person

... force from the person tumbling out//

 

[russ_watters]

The law of action and reaction is the explanation that I see everywhere. But I can't find anywhere what exactly pushes the balloon in the opposite direction as the air coming out.

Air molecules come out to where pressure is less. What exactly moves the rubber balloon in the opposite direction?
Can anyone explain this without referring to a principle?

Action-reaction means that when something pushes the air in one direction the air pushes the something in the other direction. The balloon is full of air, so if a parcel is pushed out, the air that is pushed out pushes back on the air still inside the balloon...and that pushes on the balloon itself.

 

[Joseph M. Zias]

Very good analysis of balloon motion. Interestingly, a century ago some thought the air leaving had to push on the surrounding air and thus a rocket could not work in outer space.

 

[jbriggs444]

few brazillion molecules

It this a reference to some kind of "no-hair" theorem?

 

[Omega0]

Can anyone explain this without referring to a principle?

Sorry but physics is made from principles, called laws of nature. In this case it is mainly the conservation of momentum. Why shouldn't we referr to the laws, to the principles? I personally like them.

 

[hmmm27]

In the diagram, you can see eight "air molecules" pushing outward (red arrows), but they are in pairs - one opposite another. Their forces cancel out. The balloon goes nowhere.

But one molecule, blue, shoots straight out of the balloon to the right. It applies no force to the balloon.

But it also means that one molecule - out of all ten of them - the green one - has no opposing force. It pushes on the front of the balloon, giving it a shove forward.

View attachment 288332

Multiply that by a few brazillion molecules, and you have a net force toward the left.

Pretty diagram. So... does the balloon distend from the green arrow poking it to the left ? Translational movement and attendant aerodynamics aside, if you were (lightly) holding the balloon, would a teepee form at the front ?

 

[DaveC426913]

Pretty diagram. So... does the balloon distend from the green arrow poking it to the left ? Translational movement and attendant aerodynamics aside, if you were (lightly) holding the balloon, would a teepee form at the front ?

No.
Just because there's no net force in the red "air molecules" doesn't mean there's no force at all.

Every air molecule except the blue one is pushing the balloon outward with equal force. So you get a sphere, not a teepee.