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

{\bf Question}

Use Bernouilli's law to explain why a ping-pong ball can be
supported by an upward jet of air.

\vspace{.25in}

{\bf Answer}

\begin{center}
\epsfig{file=215-5-1.eps, width=45mm}
\end{center}

Bernoulli's law along the particle path shown gives $ p +
\frac{1}{2} \rho u^2 = p_A,$ (RHS because evaluating at A) where
$u$ is the speed of flow.

Therefore $p = p_A - \frac{1}{2} \rho u^2 $ (*).

As the fluid moves away from $A$ its speed $(u)$ increases and
hence from (*) $p$ decreases.  Thus the pressure is greater on the
lower half of the ping-pong ball than the upper half.  Pressure
acts normally on the ping-pong ball and so there is a net upward
pressure force that can oppose downward gravity.  As a consequence
of pressure acting normally there are net opposing lateral
condition on each side that ensures the ball is \lq trapped'.



\end{document}