# Balloon rocket- What happens inside and why the reaction?

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• Guidog77
In summary, the 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.
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.

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.

Omega0, Guidog77 and vanhees71
Guidog77 said:
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.

Guidog77
Guidog77 said:
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.

Last edited:
Ibix, PeroK and anorlunda
PeroK said:
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//

Guidog77 said:
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.

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.

DaveC426913 said:
few brazillion molecules
It this a reference to some kind of "no-hair" theorem?

Drakkith, russ_watters and DaveC426913
Guidog77 said:
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.

DaveC426913 said:
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 ?

hmmm27 said:
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.

## What is a balloon rocket?

A balloon rocket is a simple experiment that demonstrates the principles of Newton's third law of motion. It involves attaching a string to a balloon and securing the other end of the string to a fixed point. When the balloon is inflated and released, it moves forward due to the force of the escaping air.

## What happens inside a balloon rocket?

Inside a balloon rocket, the air inside the balloon is compressed as it is inflated. When the balloon is released, the air rushes out of the opening, creating a force in the opposite direction. This force propels the balloon forward, causing it to move along the string.

## Why does the balloon rocket move?

The balloon rocket moves because of Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. In this case, the action is the air rushing out of the balloon, and the reaction is the balloon moving in the opposite direction.

## What factors affect the distance the balloon rocket travels?

The distance the balloon rocket travels can be affected by several factors, such as the size of the balloon, the amount of air inside the balloon, the length and tension of the string, and the surface it is launched on. These factors can impact the amount of force and friction acting on the balloon, which can affect its distance traveled.

## What other applications does the balloon rocket experiment have?

The balloon rocket experiment can be used to demonstrate the principles of rocket propulsion and can be applied to real-life situations such as space travel. It can also be used to teach about air pressure, forces, and motion. Additionally, it can be a fun and interactive way to engage students in science and engineering concepts.

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