  In this paper we consider the semi-linear wave equation: $u_{tt}-\Delta
u=u_t|u_t|^{p-1}$ in $\mathbb{R}^N$ where $1<p\leq 1+\frac2{N-1}$ and $p<5$ if
N=1, $p\neq 3$ if N=2. We give an energetic criteria and optimal lower bound
for blowing up solutions of this equation.
