via : http://www.physlink.com/education/askexperts/ae6.cfm
Do falling objects drop at the same rate (for instance a pen and a bowling ball dropped from the same height) or do they drop at different rates? I know a feather floats down very slowly but I would think a heavy object would fall faster than a light object. Thanks for your help. I have a bet on this one.
Asked by: Terri
If no air resistance is present, the rate of descent depends only on how far the object has fallen, no matter how heavy the object is. This means that two objects will reach the ground at the same time if they are dropped simultaneously from the same height. This statement follows from the law of conservation of energy and has been demonstrated experimentally by dropping a feather and a lead ball in an airless tube.
When air resistance plays a role, the shape of the object becomes important. In air, a feather and a ball do not fall at the same rate. In the case of a pen and a bowling ball air resistance is small compared to the force a gravity that pulls them to the ground. Therefore, if you drop a pen and a bowling ball you could probably not tell which of the two reached the ground first unless you dropped them from a very very high tower.
Answered by: Dr. Michael Ewart, Researcher at the University of Southern California