Aside from my head exploding with more questions, there is not a consistent amount of force measurable without very specific information. Force applied directionally or on a specific area of the disc or cartridge will be magnified by leverage and opposing tension. Some discs are made of more britte material than others and some are made thicker or thinner. Furthermore, what type of label is used can have an effect on the surface tension. Pressure applied from a point carries more penetrative power than that applied evenly across the surface.

Point is, you're best off just doing something simple to get a rough estimate, as you'll be doing a lot of pointless math and measurement to get specific answers for each item being destroyed. These things are overly complex to collect your data in a practical setting instead of a vacuum.

So, whatever madness you're attempting, consider the practical situation the item will encounter and simulate direction, any opposing forces, the amount of surface to surface contact, and THEN apply weight in that circumstance in a measurable form such as you said, like a book.

But, um... but... yeah.