which would be $v_{eject}/L_m \sim 5$ rad s$^{-1}$. $v_{eject}$ is the velocity of the marble when ejected from launcher, and $L_m$ is the radial distance from the pivot to the ejection point. The backspin must be caused by uneven friction or suction when the marble exits its holder. Measurements for the marble spin are discussed in more detail in section \[sec:data\].

Craters in our sandbox were increasingly reproducible after mechanical vibrations were reduced. The pendulum rod is a hollow aluminum pipe, replacing a narrower and heavier steel rod that flexed upon impact. The aluminum rod still flexes some on impact and this might be the cause of the ejected marble’s backspin. The pendulum pivot is clamped to a lab table to prevent bounces and vibration during the impact between pendulum and horizontal stop-bar. We adjusted the angle of the marble holder so that the marble is ejected in the same plane of the pendulum. The connection between marble holder and pendulum rod was shimmed and tightened so that it did not rotate. We added super glue to the the thread at the top of the pendulum rod to prevent it from turning. Prior to each impact experiment, we checked that 