Physical weathering crush a rock's physical structure changing its appearance by friction or impact, therefore, to provide a demonstration of this principle I would take the hammer and smash the rock changing its appearance with the impacts
In contrast, chemical weathering occurs when a chemical reaction of any kind alters a rock's chemical composition changing its appearance through erosion and degradation, therefore, to contrast this phenomenon with the physical weathering I would put acid on the rock to demonstrate the chemical degradation
Mountains wouldn't not be mountains. If the world was flat, then that would necessarily mean that the tectonic plates won't shift or rub up against each other.
