Drag the yellow probe up and down — shear stress peaks at the neutral axis and drops to zero at the edges.
Shear stress in a cross-section follows τ = VQ/Ib — V is the applied shear, Q is the first moment of area above the point of interest, I is the moment of inertia, and b is the section width at that depth. The result: stress peaks at the neutral axis and drops to zero at the top and bottom fibers. For a rectangle, τ_max = 3V/2A — 50% higher than the average.
When a transverse load pushes down on a beam, internal shear forces try to slide one horizontal layer of the cross-section past the next — like a deck of cards. This sliding action produces shear stress (τ) on every horizontal plane through the section. Unlike bending stress (which is zero at the center and maximum at the edges), shear stress is maximum at the center and zero at the top and bottom.
The formula τ = VQ / Ib explains why. Q is the first moment of area above the point you're checking — it's largest at the neutral axis (lots of area above, with a large lever arm) and zero at the extreme fibers (no area above the top). For a rectangle, the peak shear stress at the neutral axis is exactly 1.5× the average: τ_max = 3V / 2A. The probe in the tool below lets you see this variation directly.