this, let's consider an old experiment. Take a horseshoe-shaped wire
with a handle and with a rod across the horseshoe which is free to
slide along the horseshoe. Dip the whole apparatus in a soap solution
and remove it to reveal a soap film which pulls the slider towards the
handle of the horseshoe.
Figure 3: The Force of a Soap Film
Thus, the tension in the film produces a force acting on the slider.
We see that the tension acts along the length of the sliding rod, so we
give the tension units of force per unit length and define the surface
tension to be
where F is the force and £ is the length along which the force acts.
There is another thing to notice too. We can hang a weight from
the slider to exactly balance the force toward the handle, and no
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