A pendulum tied to the roof of a car moves backwards when the car accelerates and vice versa when the car brakes. What happens if we substitute the pendulum with a balloon? Smarter Everyday explains.
Unlike the pendulum, the balloon moves backwards when the car brakes and forwards when the car accelerates. A bubble at the top of a jar of water demonstrates similar physics. When we move the jar in one direction, the water moves backwards and the bubble moves in the direction of the accelerating jar. This is because the bubble is pushed to the front by all the water rushing to the back of the jar. The helium balloon in the car experiences the same thing. When the car accelerates, air in the front sloshes backwards and the balloon moves forward, because the front of the car has less dense air.