Q.Explain, using de Broglie's own reasoning, why the wave nature of matter is never observed for everyday moving objects such as a thrown ball or a moving car, even though the de Broglie relation applies, in principle, to every moving object.

The formula and what it depends on. De Broglie's relation $\lambda=h/mv$ makes the wavelength INVERSELY proportional to the particle's momentum $mv$. It applies to every moving object, in principle, but $h$ itself ($6.63\times10^{-34}\ \text{J s}$) is an extraordinarily small constant.

Everyday objects. For a macroscopic object -- even something as light as a table-tennis ball, moving at an ordinary speed -- the momentum $mv$, expressed in ordinary SI units, is already many, many orders of magnitude LARGER than $h$ itself, so dividing $h$ by it gives a de Broglie wavelength many orders of magnitude smaller than the diameter of a single atomic nucleus (Numerical 6 works through an explicit case). No experiment, and no physical process, can detect or is affected by a wavelength that small, so the object's motion appears purely particle-like, with no observable wave character at all. …

Note the inverse dependence of $\lambda$ on momentum, then contrast the huge momentum (and hence negligible $\lambda$) of a macroscopic object agai …

• Claiming macroscopic objects have NO de Broglie wavelength at all -- they do, in principle, by the same formula; it is simply far too small to ever be observed, not literally zero or non-existent. …