Coin Stack

Every year the American Association of Physics Teachers (AAPT) holds a photo contest for high school students around the globe. The competition offers students and their teachers an opportunity to learn and explore the physics in natural, everyday situations and in staged or contrived situations set up to illustrate specific concepts. More than 1000 photos and accompanying captions were submitted this year. A committee of high school teachers winnowed the field down to the top 100 entries—50 natural and 50 contrived. They were displayed at the 2012 AAPT summer meeting and the winners chosen by popular vote.

Zixuan Zhang, in Matt Zeitz’s class at Helias Catholic High School in Jefferson City, Missouri, won the contrived category with this photo and caption:

In this picture, there is a coin stack that is almost off of the table. This stack was built by stacking coins on top of each other so that the force of each coin was evenly distributed between the bottom two coins. We can also explain this using the property of a lever, in which the center of the coin is the fulcrum. The coin on the top of the other coin applies a positive torque; at the same time gravity applies a negative torque, which prevents it from falling apart. Therefore, we can see the coin stack as a whole. As long as the center of mass is on the table, it will remain in static equilibrium.

To submit candidate images for Back Scatter, visit http://contact.physicstoday.org .