From Mechanics

4 tonnes = 4000 kg \displaystyle \ \ cm = 4000 \displaystyle \ \ u = 25 \displaystyle \ \ v = 8

\displaystyle \begin{align} & I=mv-mu \\ & 4000\times 8-4000\times 25 \\ & =32000-100000 \\ & =-68000\text{ Ns} \end{align}

Magnitude of the Impulse = 68000 Ns