Consider a helium balloon on a string tied to the seat of your stationary car. The windows are closed, so there is no air motion within the car. The car begins to accelerate forward. Which way will the balloon lean, forward or backward

The balloon will move forward.

The density of the air will be greater at the back of the balloon; similar

to the density of air being greater at lower altitudes due to gravitational

attraction because of the weight of the air in an air column.

A block of wood in water rises because of the difference in pressures

on the top and bottom of the block.