In a classic demonstration of energy conservation, smashing two large steel ball bearings generates sufficient heat to burn a hole through a piece of paper. Although the originator of this demonstration is unknown, Educational Innovations reports that they first learned of the demonstration in 1995 from Physics Teacher Dr. Joseph Wesney.
Video 1: Upgrading a Classic Science Experiment - Burning Paper with Steel Ball Bearings, The Science Classroom YouTube Channel (accessed 2/27/2023)
When ball bearings collide, some of the kinetic energy is transformed into heat and sound. I have used this demonstration in my chemistry class to illustrate the conservation of energy and the difference between an elastic and inelastic collision (Ideal Gas Law), showing that energy can transform into other types of energy but is never destroyed. Students enjoyed smashing a piece of paper between the ball bearings and observing the resulting hole. However, it takes significant persuasion to convince students that the hole was burned and not torn during the collision. You must look closely to see only a slight brown discoloration, evidence that the paper was burned.
Image 1: Slight discoloration is evidence that the holes were burned.
To better visualize the heat the smashing ball bearings produced, I covered a piece of paper in Elmer’s Coloring Changing Glue, which changed from purple to pink when it became warmer. Repeating the experiment created a clear pink circle at the collision site, providing visual evidence of heat production.
The Elmer’s Glue product contains a thermochromic pigment, a molecule that changes color at a specific temperature. The pigment used in this product turns pink at around body temperature (roughly 37℃), so it will remain purple in the average classroom but turn pink when you touch it with your fingers. To prevent discoloration before the experiment, I clamped the glue-covered paper in a retort clamp to be suspended without touching it with my fingers.
Image 2: The Elmer's Color Changing Glue changes from pink to purple at the collision site providing clear evidence for heat production.
You could also use a thermochromic paint and achieve the same effect, but the Elmer’s Glue product is much more cost-effective.