If I was to make a bet on what would hit the ground first, a feather or a baseball, both dropped from equal height, of course I would pick the baseball. But my decision depends a lot on air resistance, not just mass. Air resistance is what allows a plastic bag to lazily float to the ground, and a falling human body to reach its terminal velocity (around 122 mph). The effect of air resistance is dependent on the shape and speed at which the object is moving. In a perfect vacuum, there is no air to move through, so the the feather and baseball will hit the ground at the same time. That might be hard to believe, but it's true. So in your case, the air friction on the balls is not extremely important. Assuming that the balls are made of the same dense material, the 50 lb. ball will just barely beat the 10 lb. ball. The reason is that the air friction is approximately proportional to the area
of the ball, but the mass (and thus the gravitational force) is proportional to the volume
. So the air friction plays a bigger role, compared to gravity, for the smaller ball.
There is a cool video that shows a feather and a ball dropped in a vacuum athttp://www.youtube.com/watch?v=_XJcZ-KoL9o
(published on 07/16/11)