In previous years, I’ve used old AP Physics B free-response questions for practice quizzes. With the shift to AP Physics 2, I’m still using these questions, but I’m adding an additional conceptual question to the end that requires students to justify their answer. Today’s practice quiz featured the classic example of a large tank of saltwater with a small drain with a plug and had students solve for the force on the plug, the velocity of the water once the plug breaks free, and the volume flow rate. What I added was:
The saltwater is replaced by an equal weight of freshwater. Indicate whether the speed of the water as it leaves the hole increases, decreases, or remains the same? Justify your answer.
Few students answered this correctly with the correct justification, and it lead to a great discussion. Some students realized and shared that the velocity of the fluid leaving the drain was only dependent on the height of the fluid in the tank and not the density of the fluid. (They basically derived Torricelli’s theorem.) Few students, however, appreciated that the saltwater was being replaced by an equal weight of freshwater. In order to have an equal weight of freshwater, a greater volume, and therefore greater height, of freshwater must be in the tank. Since the height increases, the velocity of the fluid leaving the drain would also increase.
It’s tough to anticipate the style of the questions on the new AP Physics 2 exam. I hope I’m on the right track.