Balloon in a Car

You are sitting in a motionless car, which is tightly sealed, i.e. no open windows, holes in the car, etc. A helium balloon on a string is tied to the floor. If you start accelerating the car, is the balloon going to move back, forward, or stay in place?

The reason the balloon floats up in the air is that the helium has a lower density, so when gravity pulls the denser air around down, the balloon gets pushed up. Similarly, when the car accelerates, the air around gets drawn to the back of the car, which makes the helium balloon go forward.

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  1. Because the air in the car is so dense, the air will be pulled to the back of the car when it accelerates. so the ballon will go foward.(And yes i did this before)

  2. The helium balloon will move backwards because of something similar that happens with us when we’re in a car and it suddenly accelerates or brakes move backwards during acceleration and forwards while breaking so I believe something similar will happen to the balloon

    1. Hello Joanthan. This is a good argument, but the reason you feel pushed back to your seat when the car starts accelerating is because of inertia. After a few moments inside, you will again feel comfortable in your seat. However, there is a very interesting (and surprising) effect that occurs with the balloon because of the light air inside it:)

  3. i think that the baloon will go back because the car is going fowards

    1. Hello Armando. This indeed seems very intuitive, but the answer is different. That’s what makes the question so puzzling!

  4. Forward, because there’s no open windows, holes, or doors, so the balloon will move the same way as the rest of the car.

    1. Hi Ariela. This is a good point, but the question is how the balloon will move relative to someone who is inside the car, not someone who is outside:)