1. Air resistance versus gravity
Falling with style
Air resistance versus gravity

2. Galileo’s cannon balls
In 1589, so the story goes, the Italian scientist Galileo Galilei took two cannon balls of different weight to the top of the Tower of Pisa and dropped them off the side. Both balls landed at the same time. This showed that the weight of an object does not change how fast it falls - heavy objects do not fall faster than light ones.
Image credit:
The image was drawn by Theresa Knott and shared under the Creative Commons Attribution-Share Alike 3.0 Unported license.
:26 Theresa Knott 487×405× (66176 bytes)
Pisa experiment by Galileo Galilei.
Drawn by Theresa Knott

3. Feathers on the Moon
When an object falls through the air, it needs to push the air aside. This slows the fall and is called air resistance. The larger the area that the air resistance can act on then the slower the object will fall. This why feathers fall slowly, not because they are light weight.
Image credit:
The image was created using clip art from Microsoft Powerpoint.

4. Feathers on the Moon
To prove the point, in 1971 during the Apollo 15 moon mission, David Scott dropped a hawk feather and hammer at the same time in front of the cameras. As there is no air on the moon to get in the way, both feather and hammer hit the ground at the same time 1.2 seconds later.
Image credit:
Image of Apollo 15 Lunar Module and Rover courtesy NASA/JPL-Caltech. The Lunar Module 'Falcon' appears on the left in this image. The name of the space craft is the reason that a hawk, or falcon, feather was used in the feather and hammer experiment.
Video of David Scott performing the hammer and feather drop is available at
Image Courtesy NASA/JPL-Caltech Video of drop available at

5. Forces acting on the paper as it descends
Drag force
Centre of gravity
Teacher notes:
As the paper falls, air resistance acts against it generating drag (a force) at right-angles to the surfaces. As the sum of all the drag is less than the gravity force, the paper falls down.
It is worth noting that the force slowing the falling paper is drag and not lift. Lift acts at right angles to the direction of air flow, whereas drag is a result of air resistance and acts in the same direction as the air flow.

6. The drag force (green) can be split into two separate parts: vertical (blue) and horizontal (red)
Side view
Teacher notes:
If we look at the drag forces from the side, we can actually split the direction of the forces that are acting on each face of the paper in two parts or components. These component forces are calculated by doing a projection of the drag forces (in green) onto the vertical and horizontal axis, shown in blue and red respectively. In the given situation, the vertical forces add up, but the horizontal forces will cancel out, as they are pointed in opposite directions.

7. Adding a weight to the paper causes it to tip to one side and changes the angles of the forces
Drag force
Teacher notes:
When you add paperclip weight to one edge of the paper, you cause this edge to tip down as the paper falls: the centre of gravity has been moved. As we have now changed the angles of the faces of the paper relative to the fall and so the forces acting on the paper are slightly different.
Centre of gravity

8. Now the horizontal forces are not balanced, so the paper glides rather than falling straight down
Side view
Teacher notes:
The whole sheet is now angled. The horizontal forces on the face opposite to the paperclips increase due to the steeper angle of the paper. However, the face next the paper clips is almost flat, decreasing the horizontal force generated by this face. The horizontal forces now don’t cancel each other out; there is an imbalance with an overall horizontal force pointing in the direction of the paperclips. The paper does not fall vertically, but slides towards the tipped down edge and the paper becomes a glider.

9. Acknowledgements
This presentation was developed by Paul Lancelot and Juliet Jopson as part the AMEDEO Marie Curie Initial Training Network project, funded from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement no
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, please attribute to AMEDEO ITN, EU FP7 Grant no