Assume I was in a falling box that falls at exactly 50 mph. I drop a penny and this penny is travelling toward the floor of this box at 20 mph. Because the speed of the box is greater than the speed of the penny, what would happen to the penny?
Remember that if you drop something, it doesn't fall at a constant speed, but picks up speed as it falls. This is due to the acceleration of gravity.
If the box is not accelerating, but has a constant velocity, the penny will fall exactly as it would if the box was on the ground. (From a physics standpoint, the box being stationary and in constant motion would look the same.) The penny would accelerate at 32ft/sec squared and would fall as we expect it to on the ground.
If the box was accelerating at the same acceleration as gravity, the penny would just hang there in mid air. If the box was acceleration downward at a greater rate than the acceleration of gravity, the penny would rise and hit the ceiling.
If you aren't in a gravity field at all, the penny would move in whatever direction you sent it with whatever velocity you started it with until it hit a wall.
If the speed of the falling box was only a constant 50 mph then it would obviously not be free falling. Inside the box if you let a penny freefall then after 2 1/2 seconds it would be moving faster than 50 mph and constantly gaining speed. In reality if both the box and the penny were freefalling then they would move at the same speed and the penny would appear to be floating inside the box.