A uniform wooden plank that is 5.0 m long and has a weight of 350 N rests…
PHYSICS · Multi-part · A Level · Physics · Mechanics and materials · Mechanics and materials
Problem
A uniform wooden plank that is 5.0 m long and has a weight of 350 N rests horizontally on two identical sawhorses, one positioned at each end of the plank. A painter who weighs 800 N begins to walk slowly from the center of the plank toward the left end while the plank remains in static equilibrium.
Throughout this question, assume the plank is perfectly rigid, the supports exert only vertical forces, and air resistance is negligible.
(a) Identify all forces acting on the plank–painter system while the plank is still perfectly horizontal, and state where each force is applied along the plank.
(b) Explain qualitatively—without using numerical calculations—why the upward forces exerted by the two sawhorses are equal when the painter stands exactly at the midpoint of the plank.
(c) As the painter walks toward the left end, describe how the magnitudes of the upward forces at the left and right sawhorses change. Use reasoning based on rotational equilibrium to justify your description.
(d) The plank begins to tip once the painter crosses a certain point. Explain, in words, how the condition for static rotational equilibrium is violated at the instant the tipping starts and why that violation causes the plank to rotate about the left sawhorse rather than the right one.