Well, say you have a smartphone. This smartphone can be represented by a rectangle, which is easy to model with an equation (similar to what you suggest with the sine function). However, if you look closely, your phone probably has some rounded edges. Your model equation for the rectangle has to be modified to account for this. If you look even more closely, you'll see scratches. Your model equation has to be modified again. If you look even more closely, you'll start to notice that at the atomic level, things become even more complicated. You'll need to add quantum effects in your equations to account for everything that is happening there. It all depends on the precision you need, but no model will be able to include all effects at all levels of precision. Reality is too complex. In the end, if you really want to know how reality behaves, best you can do is look at reality. This doesn't mean these imperfect models are useless, you just need to know when and how to use them.