The petrographic nature of the rock and its resistance to crushing forces (characterized by strength, work index, etc.) are used first to determine what type of crusher is needed and then in part to determine the power requirement of the crusher. When a jaw crusher is selected outright, the physical properties of the rock can be used to determine the mechanical features of the jaw crusher, but for the most part they are used along with the feed size and desired throughput to determine the size of the jaw crusher and its power requirement.