Consider a free-standing film (like a soap bubble) that is suspended across an opening as shown. The gray rectangular frame is fixed, but the green rod can slide without friction. The film’s surface tension will exert forces on the rod to the left as depicted by the small arrows. Suppose that a force, F, is required to hold the rod in place as depicted by the large arrow.

Surface tension is the force per length exerted by the surface. Each surface of the film exerts a force on the rod of g L, where g is the surface tension and L is the length of the rod. Here, g = F/2L.