How to understand invisible forces

Anyone that has ridden in a car knows that when the driver speeds up quickly, you are pressed back into your seat, and that when the driver brakes quickly, you are pushed forwards. But what would you think if the force that pushes you backwards was pushing something else forward at the same time?
For this experiment, you will need:
- a helium balloon
- an automobile
Sit in the back seat, so that you will not disturb the driver.