Concept of Equilibrium

A wooden plank rests in equilibrium on two boulders on opposite sides of a narrow stream. Three forces of size \(P, Q\) and \(R\) act on the plank.

How are the sizes of the forces related?

Each diagram shows a metal plate with four parallel forces acting on it. These are the only forces acting on the plates. In which diagram is the plate in equilibrium?

A spacecraft is travelling in space with no resultant force and no resultant moment acting on it. Which statement about the spacecraft is correct?

Which statement gives a complete description of any object that is in equilibrium?

The diagrams show four objects \(\mathbf{A}, \mathbf{B}, \mathbf{C}\) and \(\mathbf{D}\). The centre of mass \(\mathbf{M}\) of each object is marked on the diagrams.
Which object is not in equilibrium?

The diagram shows four objects on a flat surface.
The centre of mass of each object is marked M.
Which object is about to fall over?

A plane lamina with centre of mass \(X\) touches the ground at point \(P\).
Which diagram shows the lamina in equilibrium?

Two objects \(X\) and \(Y\) are placed on a balance.
The balance tilts as shown.

                                             
What does this show about the masses and weights of objects \(X\) and \(Y\) ?

\[
\begin{array}{|c|c|c|}
\hline & \text { masses } & \text { weights } \\
\hline \text { A } & X \text { has less mass than } Y & X \text { has less weight than } Y \\
\text { B } & X \text { has less mass than } Y & X \text { has more weight than } Y \\
\text { C } & X \text { has the same mass as } Y & X \text { has less weight than } Y \\
\text { D } & X \text { has the same mass as } Y & X \text { has more weight than } Y \\
\hline
\end{array}
\]