INVESTIGATIONS OF PHYSICS LECTURE DEMONSTRATIONS: TWO-BULLET PROBLEM AND PARAMETRIC EXCITATION OF U-TUBE OSCILLATIONS

Lecture demonstrations strongly convey physics concepts and theories. In addition, realistic issues arise that are often not in a model theory. Investigations of lecture demonstrations are published in scholarly journals and are Navy-relevant due to use in classrooms. We investigate two lecture demonstrations. One is the “two-bullet problem,” where a body is released from rest at a height, and another is simultaneously projected at the same height. In a vacuum, it is well known that both strike the floor simultaneously. For the common case of quadratic drag, however, the dropped body is predicted to strike the floor first. We develop a classroom demonstration that clearly exhibits the effect, but the result is suspect because numerical simulations show that the time difference is too small to be clearly discernible. Arguments to resolve the inconsistency are made. In the other demonstration, we describe the first successful parametric excitation of oscillations of the liquid in a U-tube. This is difficult to achieve due to a large drive amplitude threshold. Only a nonlinearity can limit parametric growth and thus lead to steady-state motion, but the observed motion is in the Hooke’s law regime, and the dissipation is very nearly linear. Possible reasons for the steady-state motion are discussed. Also surprising is that the free decay transitions from one damping parameter to a greater one at small amplitudes. We propose a possible reason for this behavior.Ensign, United States NavyApproved for public release. distribution is unlimite