State what the principle of moments is.
In a bicycle shop, two wheels are suspended from a horizontal uniform rod AC, as shown in Fig. 3.1. The rod weighs $19\,\text{N}$ and is freely hinged to a wall at end A. The other end C is supported by a vertical elastic cord from the ceiling. The rod's centre of gravity is at point B. Each wheel has weight $W$, and the tension in the cord is $22\,\text{N}$. By taking moments about end A, show that the weight $W$ of each wheel is $14\,\text{N}$.
Determine both the size and the direction of the force acting on the rod at end A.
The cord’s unstretched length in (b) is $0.25\,\text{m}$. Figure 3.2 shows how the tension $F$ in the cord varies with length $L$. State and explain whether Fig. 3.2 indicates that the cord obeys Hooke’s law.
Calculate the spring constant $k$ for this cord.
On Fig. 3.2, shade the area that shows the work done to extend the cord when the tension is increased from $F = 0$ to $F = 40\,\text{N}$.