Answer to Interview Question: What Happens to a Helium Balloon in a Car?

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In summary, the conversation discusses the behavior of a helium balloon in a car when the brakes are applied. The interviewer explains that due to the balloon's lower density, it will move backwards as the surrounding air is pushed forward. They also mention a similar effect when the car accelerates. The interviewer then poses a related question about substituting the air and helium with void and a region of space with lower energy density.
  • #1
terimakasi74
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Hi,

Ive been to a assessment yesterday and they tossed this question at me.

A Helium balloon is floating free in a car. The brakes are applied hard. What does the balloon do? Why?

I told them that I think it will move slightly to the front of the car as the balloon still has the relative velocity over the ground as the car stops (because its not connected to the car) so the balloon moves towards the front of the car. Is that wrong?

The assessor told me that the balloon will move to the back, because the Balloon with the helium has less mass than the air and therefore the air will overtake the balloon ( on it's way to the front) so that the balloon will move backwards.

I also concluded that the mass of the "system" balloon is greater then the mass of the air, which he denied. Is the mass of the baloon helium system greater or smaller?

I then threw in that one should also consider that the balloon will fly to the ceiling and stays there and puts some force to the ceiling. So there will be also some friction and the balloon would only roll and not move freely.

What do you think?

Thanks
 
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  • #2
I asked this question to my students and let them test this themselves. Your interviewer is correct.

The fact that the balloon floats means that it has less density than the surrounding air. When the car brakes, the surrounding air will be pushed forward, displacing the balloon, so it will then move backwards.

The same non-intuitive observation will occur when the car accelerates. The air will be pushed back, while the balloon will tend to tilt forward.

BTW, this is what is going on when you let go of the balloon in air. If you consider that we are actually accelerating upwards at the rate g, then the more dense air is being pushed down towards the ground, displacing the balloon upwards. So essentially, you have already seen this effect many times!

Zz.
 
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  • #3
Exactly ZapperZ, good answer.
I have a related question: Since not only mass but energy as well is affected by gravity or an accelerating field, what could happen if, in the previous effect, we substitute the word "air" with "void" and "helium" with "a region of space which energy density is lower than that of the void"?
 

FAQ: Answer to Interview Question: What Happens to a Helium Balloon in a Car?

What happens to a helium balloon in a car?

When a helium balloon is placed in a car, it will move towards the back of the car due to the air resistance created by the moving vehicle. This is because the helium balloon is less dense than the air inside the car and will be pushed towards the area of lower pressure.

Why does a helium balloon move towards the back of a car?

The air inside a car is constantly moving towards the back of the car as the vehicle moves forward. This creates an area of lower pressure towards the back of the car, causing the helium balloon to be pushed in that direction.

Will a helium balloon behave differently in a moving car compared to a stationary car?

Yes, a helium balloon will behave differently in a moving car compared to a stationary car. In a stationary car, the air inside the car is not moving, so the balloon will not be pushed in any particular direction. However, in a moving car, the air is in motion and will push the helium balloon towards the back of the car.

What happens to a helium balloon in a car when the car is accelerating or decelerating?

When a car accelerates, the air inside the car moves towards the back of the car at a faster rate. This will cause the helium balloon to be pushed towards the back of the car with more force. On the other hand, when a car decelerates, the air inside the car moves towards the front of the car, causing the helium balloon to move towards the front of the car as well.

Does the size or shape of a helium balloon affect its movement in a car?

Yes, the size and shape of a helium balloon can affect its movement in a car. A larger balloon will have more surface area and will experience more air resistance, causing it to be pushed towards the back of the car with more force. On the other hand, a smaller balloon may not be affected as much by the air resistance and may not move as dramatically in a moving car.

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