During the last Apollo 15 moon walk, Commander David Scott performed a demonstration where he dropped a hammer and a feather from the same height at the same time. Both of these objects fell at exactly the same rate and hit the ground at exactly the same time. This happened because there is no air on the moon, and without air, objects fall at exactly the same rate.

• Here is a youtube link to the Hammer and feather on the moon: www.youtube.com/watch?v=ZVfhztmK9zI .
• Here is a youtube link to Brian Cox doing a version of this experiment on Earth with a bowling ball and a feather in a vacuum chamber: www.youtube.com/watch?v=E43-CfukEgs

In the picture below, you can see a hammer and a piece of paper being held at the same height. If there was no air, they would hit the ground at the same time. With air, you must account for aerodynamics and forces, and the hammer will hit the ground first.

Often in physics, we make an assumption that there is no air to make scenarios and calculations more simple.