Since quantum-mechanical theory is universal, it applies to all objects, regardless of size. therefore, according to the de broglie relation, a thrown baseball should also exhibit wave properties. why don't we observe such properties at the ballpark?

We don't observe such properties at the ballpark because the mass of the ball is so large, its de Broglie's wavelength is minuscule.

Since the quantum-mechanical theory is universal, it applies to all objects, regardless of size. therefore, according to the de Broglie relation, a thrown baseball should also exhibit wave properties.  we don't observe such properties at the ballpark because the mass of the ball is so large, its de Broglie's wavelength is minuscule.

Explanation

The de-Broglie relationship is:λ=h/p = h/(mv)here, λ is the wavelength, m is the mass, v is the velocity, h is the planks constant.
Now, from this relation, we see that the wavelength is inversely proportional to the mass. So, in the ballpark, the mass of the ball, is very large. Due to which the wavelength is very small.