If you tie a balloon of hydrogen to a seat in a car that is accelerating forwards, will the balloon go forwards or backwards? That sir said the ballon will go fowards  as it is light and will have a greater acceleration than the car also stated that a ballon of same volume  filled with air will go fowards but slower , and tennis ball of same volume backwards as it is very heavy . My idea is that all will move backwards as all masses experience inertia
 A: The air is more dense than the balloon and will be forced to the back of the car when it accelerates.
The balloon will then move forward relative to other fixed objects in the car, just as the balloon would lift up in air outside the car.
The air pressure increases with depth outside the car and in the accelerating car the air pressure would increase with depth but also with distance from the front of the car.
A: The balloon would move in the direction of motion of the car.
When Einstein began his explanation of his General Theory of Relativity, he considered someone standing in an elevator car that was being accelerated in a straight line at a constant rate of g through space, and that that person would be unable to tell that he was not in a stationary lift car on Earth. https://www.thegreatcoursesdaily.com/einsteins-experimental-elevator/
In the same way, if the person with the balloon couldn't see out of the car, if the car was accelerating at g (which a Tesla Model S apparently could) but for several seconds and gravity could be removed, he or she would be in exactly the same situation. Just as a balloon would float to the top of an elevator car because it was constantly accelerating through space in the direction of its roof, so the balloon would be forced to float towards the front of the car. (and they would be able to stand horizontally on the back seat). Now reintroduce gravity and the resultant acceleration for everything  in the car, by vector addition would be $√2$ backwards at $45°$, so the balloon would float in a position $45°$ forward of vertical from the point where the string was held.

A: Think buoyancy: on object will move in direction of the gravitational force if it has a higher density than the medium, in the opposite direction if the density is lower. Just apply this to the car: if the car is accelerating, the force is backwards, if it is braking the force is forwards. A balloon filled with a gas lighter than air will go in the opposite direction, one filled with a heavier gas in the same direction. If the balloon is filled with air, it will stay in the same place (all relative to the car). Only if there is a vacuum in the car would they all move backwards when accelerating (forwards when braking).
