It is used as a lever either to force apart two objects or to remove nails. Crowbars are commonly used to open nailed wooden crates. Common uses for larger crowbars are: removing nails, prying apart boards, and generally breaking things. Crowbars can be used as any of the three lever classes but the curved end is usually used as a first-class lever, and the flat end as a second class lever. In mining, crowbars are used to break blasted rocks and to remove loose rock on roof sides and the working face, but not much in modern mining.