A U-shaped wire frame holds a soap film; one side is a movable slider of length 5 cm. A force of 4.0 × 10⁻³ N is needed to hold the slider. Find the surface tension of the soap solution. Step 1: A soap film has TWO surfaces (front and back), so total length = 2 × 0.05 = 0.10 m. Step 2: T = F / (2l) = 4.0 × 10⁻³ / 0.10 = 0.040 N/m. Answer: T ≈ 0.04 N/m — about half of pure water's 0.072 N/m, because soap lowers surface tension (that is its job). Forgetting the factor of 2 is the classic mistake here.