You work for a balloon delivery service and you are delivering a single, helium-filled balloon in your car. To prevent the balloon from bouncing around on the ceiling while you are driving you have tied a string with a weight on it to the balloon. The weight is resting on the floor and the balloon is floating just below the ceiling.
When you accelerate, does the balloon stay where it is, move backward, or move forward? What does it do when you make a turn?
Assume all the windows are closed and the vents are turned off so there is no air flow inside the car to affect the balloon.