The helium balloon in a braking car

Question

A helium balloon floats inside a car, tied by a string to the floor, with all the windows closed. The car is moving forward and the driver brakes hard. The passengers lurch forward.

Which way does the balloon swing, forwards or backwards? Why? Then say what happens when the car accelerates, and when it turns a corner.

Accepted Answer

The balloon swings backwards, opposite to the passengers. What the air does. When the car brakes, everything inside it tends to keep moving forward. The air in the car does too: it piles up slightly at the front, so the air pressure at the front of the cabin rises and the pressure at the back falls. This is the same as what gravity does vertically: air is denser and higher-pressure near the floor than near the roof. What the balloon does. A helium balloon rises because it is lighter than the air it displaces; it moves towards lower pressure, away from the denser air. During braking, the "heav…

The helium balloon in a braking car

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