If you have a object inside a car which has equal amount of inertia as car, will it stay stationary at any level of acceleration without going back?

Question

A 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

Answer 1

Assuming the car is closed and the ballon is in the car, when car starts moving forward the air has some inertia and will start building up at the end of the car.

The air density will bi larger at the back of the car due to the air build up, and the ballon density does not change. This will push the ballon forward.

Here is a nice experiment:

https://youtu.be/y8mzDvpKzfY