Poppin’ Physics

The American Association of Physics Teachers hosts an annual photo contest that encourages high school students worldwide to capture and explain, in 250 words or less, the physics around us in the natural and manmade world. This photo, by Charmi Rajiv Patel in Jeffrey Tieman’s class at Maine West High School in Des Plaines, Illinois, earned an honorable mention in the contest’s “contrived” category. Her caption:

This is a photo of a water balloon at the exact moment it is popped. As the balloon is popped the water retains the shape of the balloon until gravity pulls the water down. According to Newton’s first law, because the water was previously at rest, it remains at rest until gravity, the unbalanced force, acts upon it. This photo also includes the physics concept of surface tension. Because the water retains the shape of the balloon, the photo shows that the like molecules are attracting to each other. The wisps of water flying out of the water balloon are created as the needle pops the balloon and the latex retracts over the surface of the water, introducing the concept of skin friction—the drag created by the water as it moves across the skin of the balloon. In the photo, the latex of the balloon is in the process of expanding over the water which is shown toward the bottom left of the balloon where the latex is seeming to pinch the side of the water. Since the force at which the latex expands over the water is proportional to the area of the balloon, the greater the water balloon would have been, the greater the value of its skin friction would have been, and the greater the wisps of water flying out of the water would be. In the end, the photo, while simple on the outside, is a much more complicated action shot when further analyzed.

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