Physics 9702 · AS & A Level · Equilibrium of forces
Equilibrium of forces — practice question
A uniform plank AB, with length $5.0\,\text{m}$ and weight $200\,\text{N}$, is laid across a stream, as illustrated in Fig. 3.1. A man whose weight is $880\,\text{N}$ stands at a distance $x$ from end A. The ground exerts a vertical force $F_A$ on the plank at end A and a vertical force $F_B$ on the plank at end B. While the man walks along the plank, the plank remains in equilibrium.
(a(i))[2]
Explain why the sum of the forces $F_A$ and $F_B$ remains constant whatever position the man takes on the plank.
(a(ii))[4]
The man is positioned $x = 0.50\,\text{m}$ from end A. Use the principle of moments to calculate the magnitude of $F_B$.
(b)[3]
Fig. 3.2 shows how force $F_A$ varies with distance $x$. On the axes in Fig. 3.2, sketch a graph to show how force $F_B$ varies with $x$.
Worked solution & mark scheme
This 9-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Net force is zero” …