  We explore the process of initiating motion through a granular medium by
measuring the force required to push a flat circular plate upward from
underneath the medium. In contrast to previous measurements of the drag and
penetration forces, which were conducted during steady state motion, the
initiation force has a robust dependence on the diameter of the grains in the
medium. We attribute this dependence to the requirement for local dilation of
the grains around the circumference of the plate, as evidenced by an observed
linear dependence of the initiation force on the plate diameter.
