Balloons in Accelerating System

We are familiar with the sensation of being pressed backward into our seat when a car accelerates. However, a helium balloon behaves differently, moving forward. This is because helium is less dense than air. In accelerating or decelerating systems, not only do objects inside the system respond to the motion, but the air inside the system does as well. The denser air rushes forward, pushing the helium balloon backward, making it appear to move in the opposite direction to everything else in the system. Isaac Newton's law of inertia explains this phenomenon: a body at rest or in motion tends to stay that way until acted upon by an external force. When the car suddenly accelerates, the air inside briefly lags behind due to inertia and moves toward the rear of the car, creating a force that pushes the helium balloon forward. Inertia is dependent on an object's mass. The surrounding air has more inertia than the helium-filled balloon but less inertia than, for example, a balloon filled with a denser gas like carbon dioxide.