
The van der Waals Equation 249
This means that the net effect of the fluid surface on the blade is to exert an up-
ward force cancelling the force due to gravity. It will therefore require work to be
done to lift the blade out. The force required to pull the blade out is proportional to
its perimeter, and for a circular blade of radius a, the required force is 4
π
a
σ
(not
2
π
a
σ
!), where
σ
is called the surface tension of the liquid and has the units of
Newton/meter. What could be confusing is that the force on the blade acts normal to
it.
An experiment that can also be used to measure
σ
can clarify this. If we prepare a
rectangular soap film with an arrangement ...