PHYSICS of Brushing your teeth: I spent a few weeks a few years ago, five minutes here and there, trying to figure out what is the optimal amount of water that you should put in your mouth to get the maximum rinsing effect after you brush your teeth. Do you fill your mouth half full, 1/3 full or what? If you want to rinse out a bottle with water then how full do you fill it before you shake it? With less water the water has more distance to travel. With more water there is more mass involved. Are you looking for energy transfer or momentum, one involves the square of the velocity and one only varies linearly with velocity. This kind of question actually comes up when you analyze tankers full of fuel and motorcycle design where they look at making high speed turns. The forces are not negligeable.