One jar filled with water, the other filled with snow… which one will roll down faster?
Question: Does a bottle filled with water or snow roll down a hill faster?
Prediction? Take your time. Think about it. Ask your friends, ask your kids, ask your parents.
Make sure to get explanations from yourself and all the people you ask, not just guesses.
Answer: (watch the video)
If you look carefully, the water in the bottle does not rotate, but stays in place. The water wants to stay in place (inertia), and since it is not connected together as tightly as the solid snow, the water will not rotate inside the bottle. The only thing that rotates is the bottle itself. In the snow-filled bottle, the bottle and the snow rotate all together.
Both bottles (jars in my case) have equal amount of potential energy at the top of the hill. As they both move down the hill, the potential energy is converted into kinetic energy. But since the water is not rotating, the potential energy is converted only into translational kinetic energy, and not wasted on rotating the water. In the snow filled bottle, however, the potential energy at the top not only converts into the translational kinetic energy but is also wasted on rotating the snow inside the bottle.
Water-filled bottle: PE (at the top) = KE (translational)
Snow-filled bottle: PE (at the top) = KE (translational) + KE (rotational)
Therefore, the water-filled bottle goes faster down the hill.