Analyzing linear and angular momentum conservation in digital videos of puck collisions

We describe a new undergraduate mechanics laboratory experiment that illustrates principles of linear and angular momentum conservation. Students observe that angular momentum is conserved in systems where the initial conditions involve only translational motion, and yet the final conditions include rotational motion. To do this, students make a digital video of a two-object collision on an air table. One object has an asymmetric mass distribution and gains rotational kinetic energy as a result of the collision. Using their digital video, students create a stroboscopic image of the two objects' trajectories and show that both linear and angular momentum are conserved to within a few percent, the limit of experimental uncertainty. Students also show that angular momentum conservation is independent of the choice of origin, that the center of mass of the asymmetric object follows a linear trajectory, and that the center of mass of the whole system follows a linear trajectory. c 2000 American Association of Physics Teachers.