Why is torque zero when the force is applied along the line passing through the pivot point?
Torque τ = r×F depends on the perpendicular distance (rsinθ). If the force acts along the line through the pivot, θ = 0∘ or 180∘, so sinθ = 0, making torque zero.
Ref: NCERT Class 11 Physics, Thermal Properties and Gravitation.