I had a helium balloon in my car tied so it hangs down by string toughing nothing. Windows are rolled up. Heat and AC are off. Air inside the car is not moving and car is parked with motor running. When I step on the gas pedal and car takes off forward balloon goes forward towards the windshield. Repeat the same experiment with air inside the balloon hanging from ceiling by string. When car takes off forward balloon goes forward too. Next experiment hand a rock from ceiling with string just like the balloon when car takes off forward rocks goes to the rear and balloon goes forward. Inertia makes the rock stay in place as the car takes off. I assume inertia makes air in the car stay in place too this produces a high pressure area in the rear and low pressure area in front so air pressure pushed balloon forward?