  We investigate the force of flowing granular material on an obstacle. A
sphere suspended in a discharging silo experiences both the weight of the
overlaying layers and drag of the surrounding moving grains. In experiments
with frictional hard glass beads, the force on the obstacle was practically
flow-rate independent. In contrast, flow of nearly frictionless soft hydrogel
spheres added drag to the gravitational force. The dependence of the total
force on the obstacle diameter is qualitatively different for the two types of
material: It grows quadratically with the obstacle diameter in the soft, low
friction material, while it grows much weaker, nearly linearly with the
obstacle diameter, in the bed of glass spheres. In addition to the drag, the
obstacle embedded in flowing low-friction soft particles experiences a total
force from the top as if immersed in a hydrostatic pressure profile, but a much
lower counterforce acting from below. In contrast, when embedded in frictional,
hard particles, a strong pressure gradient forms near the upper obstacle
surface.
