\documentclass[a4paper,12pt]{article}
\newcommand{\ds}{\displaystyle}
\begin{document}
\parindent=0pt

{\bf Question}

A gun fires a bullet of mass $m$ with speed $U$ into a block of
wood of mass $M$ which rests on a frictionless plane.  If the
bullet becomes embedded in the block determine the subsequent
velocity of the block.

\vspace{.25in}

{\bf Answer}

Conservation of linear momentum.

$mU = (M+m)V$ where V is the speed of the block and the bullet
after impact.

$\ds \Rightarrow V = \frac{m}{M+m} U$


\end{document}
