A familiar experience in a vehicle is of a forward jerk when the vehicle comes to a stop and a backward jerk when the vehicle starts suddenly. But a helium balloon kept in the vehicle does the opposite. What could be the reason?
The factor that leads to this difference is the density of helium which is less than the density of the air in the vehicle.
When the vehicle is set into motion, the air inside it rushes at the back which builds up more pressure at the back than in the front. Helium being less dense than the air moves in the direction of decreasing pressure due to bouyant force. A similar effect is there when the vehicle stops or takes a turn.
This leads to the strange behavior of a Helium Balloon.