This report approaches the dynamical behavior of brittle materials by developing and analyzing Lagrange’s equations for cracks, representing crack opening and radius as two generalized coordinates. It is assumed that cracks are penny-shaped, so the radius is a well-defined quantity. Strain and surface energy are computed, and Lagrange’s equations for crack opening and growth are derived based on these results. These equations provide analytic results that are consistent with those for crack stability and crack opening. It is shown that Lagrange’s equations allow for an energy integral, and that this energy integral gives rise to an expression for crack speed that is consistent with measured data, involving only elastic properties. The author examines steady-state behavior, and obtains a quasi-steady approximate solution that allows for an estimate of how crack speed depends on internal pressure or on far-field stress, a relation that involves the surface energy as well. These analytic results are compared with a numerical solution of the corresponding ODEs, and it is shown that the agreement is very good, demonstrating also the usefulness of quasi-steady approximation. Finally, it is shown that the approach can be extended to formulate a rule for the dependence of surface energy on stress intensity factor from the known dependence on crack speed. This allows for calculation of the effect of creep-like behavior in unsteady problems.