The red balloon

Classical Mechanics Level 3

A balloon filled with Helium is floating, attached to the floor of a car by string. Suddenly, the car begins to move forward with constant acceleration $\vec{a}_\text{car}$ .

What happens to the balloon?

Assumptions

The windows of the car are closed.

It remains in place relative to the car It moves toward the back of the car It moves toward the front of the car It moves straight down, toward the floor of the car

5 solutions

Michael Mendrin
May 22, 2014

Pressure of air is greater in the back of the car because of acceleration.

I chose moves closer to the floor since it is tied with a string and the only sure thing is that it will be getting closer to the ground if it moves forward or backwards.

Hatim Zaghloul - 7 years ago

Yes its right that it will move towards floor.....I tried this in cut the rope game and this was the result....

Rahul Shete - 6 years, 10 months ago

THe ballon will move back word

Syed Azam - 7 years ago

Yes, I remember seeing this in a lecture by Walter Lewin.

Anish Puthuraya - 7 years ago

When car moves forward with constant acceration, then 2 forces will act on balloon , i.e , one pseudo force in opposite direction to car's movement and other is buoyant force vertically upwards. So, how will balloon move towards the front of the car?

Akhil Bansal - 5 years, 8 months ago

it wont feel sudden jurk

Rachit Goel - 7 years ago

Looked like a fairly simple problem on pseudo forces.Alas.

Aritra Chattopadhyay - 7 years ago

I'm sorry I don't get it?? How could it move to the front with a sudden movement of the car.. What about the inertia of the balloon ??

Mohammad Salman - 7 years ago

I agree that the balloons will move to the front. But that is with respect to the ground. Relative to car, they will remain at rest

krishna havish - 7 years ago

relative to the car, the observer will feel that the entire system is at rest. So relative to car, the balloons will remain at rest

krishna havish - 7 years ago

Petru Lupsac
May 24, 2014

The pressure (and thus density) of the air at the back of the car will be greater than in the front, and the baloon will try to "float" towards the front part of the car. There is a great video showing this experiment: https://www.youtube.com/watch?v=y8mzDvpKzfY

Indeed, a great video.

Henrique Coelho - 7 years ago

This video was the only reason I knew the answer, I believe it was also mimicked with an F=ma where a car would drive in circles. Which direction should the balloon tilt?

Spock Weakhypercharge - 7 years ago

Henrique Coelho
May 25, 2014

In this case, the importance of Fluids Mechanics is greater than the importance of Newton's Second Law, due to the fact that the mass of the baloon is very small. When the car accelerates forward, the fluid (in this case, the air inside the car) moves to the back of the car, which creates a small pressure area in the front of the car. Thus, the baloon moves forward too.

let us assume that car moves towards....east...... bouyant force acts at helium baloons.....in northern direction..........so we got 2 vectors in north and east direction........so its resultant will be along north east....which means towards the front of car

Harish kp - 7 years ago

Wuu Yyiizzhhoouu
Nov 25, 2014

the heavier air sloshes to the back of the air, thus the pressure in the front of the car is lower and the light balloon will move to the front of the car

Jaivir Singh
Aug 30, 2014

BECAUSE DENSITY OF He IS LESS THAN THE DENSITY OF AIR

The red balloon

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