Like it or not, you need some reality in your fantasy. The question is, how much do you need? Every GM has their own style, and every game has its own version of the Reality<->Fantasy slider set at whatever location the world needs. But how much of it do you prefer? How much grounding do you want in the real world, and at what point do you want to knock that crap off and go with the world?->
It seems like a common sense discussion, except for one thing: most fantasy worlds have magic and active gods, which means the type of progress you'd see technologically would be completely different. After all, why waste time inventing guns when you can make magic crossbows that self-load or learn to cast lightning bolts from your hands?
So how much do you like?
I tend to keep things simple. Gravity or something like it exists, and basic Newtonian Physics can generally be counted on for physical objects. Beyond that though? Well, let's just say I'm fond of pointing out to people that our table of elements has 200+ in it, the L5R one has 5, and all 5 can talk to people so we know it's only those 5. If nothing else it makes for more Plato type physics with every element having a rightful place, than the physics we have in the real world.
As for more advanced applications, well, the math for them just doesn't exist. Why would it when there's no need for calculus or advanced physics because studying magic will let you mess with reality as is?