(a)[2]
Explain what is meant by stretched elastically.
(b)[2]
Demonstrate that the SI units of energy per unit volume are $\text{kg m}^{-1}\text{ s}^{-2}$.
(c)[3]
For a wire stretched elastically, the elastic energy per unit volume $X$ is expressed by $X = C\epsilon^{2}E$, where $C$ is a constant, $\epsilon$ is the strain of the wire, and $E$ is the Young modulus of the wire. Show that $C$ has no units.