6. A pebble is dropped from a high vertical cliff. The collision of the pebble with the ground below is seen 1.5 s after the pebble is dropped. With what speed did the pebble hit the
ground?
(A) 10 m/s
(B) 15 m/s
(C) 48.6 m/s
(D) 100.4 m/s
(E) 343 m/s

The space between two 15-in.-long concentric cylinders is filled with glycerin (viscosity = 8.5 × 10-3 lb·s/ft2). the inner cylinder has a radius of 1 in. and the gap width between cylinders is 0.1 in. determine (a) the torque and (b) the power required to rotate the inner cylinder at 180 rev/min. the outer cylinder is fixed. assume the velocity distribution in the gap to be linear.