when car accelerates, why does the balloon in the car move forward?

Answer 1

The law of inertia says that all objects tend to keep their state of movment, so the baloon should "move" back in comparison to the car, but in fact it doesn't move. There could be 4 reasons why it moves forward: the windows are open, wind comes in the bach reflects to the front moving the baloon / the car accelerates by breaking (negative acceleration) / you push the baloon forward.

Or a bit wearder: imagine the air in the car as a liquid, the car accelerates, it moves back, to much presure in the back now, the car stops accelerating, the air moves forward faster moving the baloon to the front

Answer 2

A balloon in an closed car is really a "marked" area of the internal air in the car. Just as the air in your car doesn't get pushed noticeably to the rear when you accelerate, the balloon stays more or less put, too.

Gentle acceleration is probably not enough force to overcome the random motion of the air within the car. If you accelerate rapidly, the balloon and everything else not tied down will move accordingly.

Answer 3

Could you do the experiment again and ask again? Give us the details, were your windows and vents closed, was the balloon filled with lighter-than-air gas or with regular air?

Try to use two balloons, one with regular air and one with lighter-than-air gas. Be sure to have the car's fans turned off--heater, air-conditioner.

I don't know, but I would nearly bet that a balloon filled with regular air moves backwards and a balloon filled with lighter-than-air gas moves forwards. The lighter-than-air gas balloon after all floats upwards, and while accelerating you change enough parameters in the environment to make upwards = forwards while accelerating. That's my guess!

This experiment sounds kind of fun. If you did it, I would like to hear the results--however wrong I might be in my assumptions.