a. An area of longitudinal deformation ε moves along the rod with velocity c
to the right. The cross-sectional area of the rod is S, the material density ρ.
What is the rate of change of momentum of the particles of the rod in the area
to the right of the selected section?
b. The momentum carried per unit time through a unit of cross-sectional area
is called the momentum flux density. Why should the momentum flux density
be equal to the normal stress σ in that section? Expressing σ through strain,
define c hence through ε and ρ.