You are in a vacuum chamber, and you drop a feather and a bowling ball (initially at rest) from a great height. Which will hit the ground first?

The key fact here is in free fall everything accelerates at the same rate. Acceleration is equal to 9.81 m/s² (accleration of free fall). This means provided you drop both at the same height, and drop them from rest, they will travel to the ground together with the same acceleration and hit the ground at the same time. But this is not what happens in our usual exerience, so what is really going on? Well, if we were to do this experiement we'd find the feather takes a long time to reach the ground. This is due to air resistance or drag which is present on Earth (not in a vacuum of course, as we've removed all the air!). The molecules in the air collide with the feather slowing it down, which is why on Earth the feather would take a while to reach the ground, but in our vacuum, they reach the ground at the same time.