Two identically sized balls that have different weights are dropped from the same height in a non-vacuum environment.
Will they arrive simultaneously or will one ball land before the other?
The heavier ball will reach the ground first. This is calculated using the buoyancy force as determined by Archemedes' Principle. The buoyancy force acts in opposition to the gravitational force and is related to the weight of the displaced air.
Try thinking of it this way: Imagine two identical balloons. One of them is filled with air and the other one with hydrogen. Although they are the same size, the gases inside the balloons have different weights. The one filled with air is sinking toward the ground while the other one is staying in the air.