Thermal energy tends to flow as heat
a. from a system of higher energy to a system of lower energy
b. only between systems that have equal energy
c. only between systems that have equal temperatures
d. from a system of lower energy to a system of higher energy

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.