Measurement of Stopping Force in Low Velocity Impact Cratering

The time dependent stopping force on a ball dropped into a granular medium has been measured using an accelerometer embedded within the ball. The velocity dependence of the force shows two distinct behaviors: (1) for impacts with large (200 $\mu $m) irregularly shaped sand particles, $F(v)\propto v^{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}$; for impacts with 100 $\mu $m spherical glass beads, $F(v)\propto \left( {v-v_0 } \right)$. The accelerator apparatus yields reproducible, low noise data that reveals peculiar features such as a downward acceleration pulse just before the ball comes to rest.