= Solution
Take a representative tennis-ball <mass> of $m\simeq0.06\,\mathrm{kg}$. The <speed> is nonrelativistic, so the <de Broglie wavelength> is
$$
\boxed{\lambda=\frac hp\simeq\frac h{mv}=\frac{6.626\times10^{-34}}{0.06\times10}\simeq1.1\times10^{-33}\,\mathrm m.}
$$
This is an estimate because the <mass> was not specified. The <wavelength> is vastly smaller than any practicable aperture or obstacle separation: a <diffraction> angle of order $\lambda/d$ is unresolvably small for laboratory-scale $d$. Moreover, interactions with air, thermal radiation and the apparatus destroy observable coherence between macroscopically distinct paths. <Quantum mechanics> still applies; the small <wavelength> and rapid <decoherence> explain why ordinary experiments reveal classical trajectories rather than a measurable <interference> pattern.
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