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Interview Question

  1. Dec 13, 2006 #1

    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?

  2. jcsd
  3. Dec 13, 2006 #2


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    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!

    Last edited: Dec 14, 2006
  4. Dec 14, 2006 #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"?
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