Why Does a Helium Balloon Tilt Forward in an Accelerating Car?

[HHveluj]

Homework Statement

Why is helium balloon in accelerating car tilting forward?

Homework Equations

The example that I think that I understand - is simple pendulum in accelerating car - and everything there is clear - g(effective)=g-A (all of them are vectors).

The Attempt at a Solution

So, the difference from the pendulum is only presence of buoyant force that's making balloon to float. So why is it tilting forward?

P.S. This question is in section 'Mechanics in Noninertial Frames', I'm not sure if it should be placed in this forum, but not advanced... Maybe the problem is harder than I imagine? Or easier?... :)

 

[OlderDan]

Think first about what makes a helium balloon float in air. Think in terms of the forces acting on the balloon. Then think about what must happen to the air molecules in an accelerating car. Are they distributed in the same way as they are in a car that is not accelerating?

 

[HHveluj]

So, the only difference is that in case of balloon - it's lighter than air => the air will move to the back of the car and thus push balloon forward?...

 

[OlderDan]

So, the only difference is that in case of balloon - it's lighter than air => the air will move to the back of the car and thus push balloon forward?...

That is basically it. The balloon floats in still air because the pressure is higher on the bottom of the balloon than it is on the top. The pressure difference comes from the weight of the air between the top and bottom. In the accelrating car there will be a pressure difference because the air at the back of the car provides the force needed to accelerate all the air in front of it, and there is more air to push as you go farther back in the car. The pressure difference will be more than enough to accelerate the lighter than air ballon, so it will move forward relative to the car.

The same thing happens when the car goes around a turn. The balloon will lean into the turn.