The viscosity of a fluid is directly related to
the pumping power needed to transport a
fluid in a pipe or to move a body through a
fluid.
Viscosity is caused by the cohesive forces
between the molecules in liquids and by
the molecular collisions in gases, and it
varies greatly with temperature.
In a liquid, the molecules possess more
energy at higher temperatures, and they
can oppose the large cohesive
intermolecular forces more strongly. As a
result, the energized liquid molecules can
move more freely.
In a gas, the intermolecular forces are
negligible, and the gas molecules at high
temperatures move randomly at higher
velocities. This results in more molecular
collisions per unit volume per unit time and