There is no force pushing the air in the vehicle backwards. The air is stationary until some part of the vehicle that was behind the air at the start, moves forward and collides with the stationary air.

I think the balloon is being pulled forward by the static electrify formed by the driver shifting in his seat.

quote:

Originally posted by kkina:

quote:

Originally posted by senza nome:
There is a very strong fan in that van?

Nothing like that. All the information you need is in the OP.

The van is:

A chrysler pacifica?

According to Einstein and modern physics, all motion is relative. There is no stationary nor moving, unless relative to something else.

In the context of physics, a force is:
An influence tending to change the motion of a body or produce motion or stress in a stationary body.



The van is accelerating forward relative to the outside world.

As the vans forward velocity increases, the air inside the van changes relative velocity as well, accelerating, relative to the outside world, forward with and at the same deltaV as the vehicle. However, the action/reaction of relative velocity change creates a force within and relative to vehicle, that causes the air molecules inside the vehicle to move rearward relative the the vehicle. It's the same force you can feel pulling you back against the seat.

Similar to (the same actually) the effects of gravity, but horizontally rather than vertically, as the van accelerates forward, the air inside is being accelerated forward as well, relative to the outside world, but accelerated rearwards relative to the car. This results in the air at the rear of the van having a higher pressure than the air in the front of the van. The balloon "floats" forward proportional to the difference in pressure between the front of the van and the rear of the van, as the balloon now displaces a volume of helium with less mass than the equivalent volume of surrounding air.

So, it floats forward for the same reason it floats upward. As the van accelerates forward, the air molecules in the van try to move rearward in the van, increasing the air pressure at the rear of the van.

When you hit the brakes, the effect will be reversed.

Edited to change the term "air density" to "air pressure", as I think in this context, pressure is probably more correct.

wait is all this in a vacuum?

Of course the answer is...There is no balloon...

The ballon is lighter than the air surrounding it and would behave as it normally should.

Accelerating forward ---> it will be pushed forward by the relative density and inertia of the surrounding air.
Moving forward at a constant speed ---> would be upright as the air inside the van would have stabilized.
Accelerating backward ---> the ballon would be pushed toward the rear of the van.
Moving backward at a constant speed. ---> would be upright as the air inside the van would have stabilized
Stationary ---> completely upright.

Option A is the correct answer.

The correct answer is: Accelerating forward.

As the vehicle accelerates ahead, the air, which has inertial mass, is compressed in the rear of the cabin (hope I said that correctly for those getting into a relativistic discussion). The balloon responds to the lateral pressure gradient by moving towards the region of lower pressure, namely the front of the van. It is in effect floating sideways. As the van settles into a constant speed, and pressure normalizes, the balloon will once again straighten itself.

Many of you explained the science very well, so well done. This was a toughie. And if you need the video, here it is:

wow, I actually got it right.