I am a dental student. I have noticed that dentists tend to say that Class III levers (eg. the human mandible) are "the least efficient type of lever." I told my friends that this was incorrect, because there is no more inherent energy loss in a Class III lever than I or II. The next day my colleague told me that Class III levers are the least efficient because, for a given input of Work, you get the least amount of Work back out. But I don't think this is right either. From what I remember from college physics, all three types of levers conserve Work. So you get out the same amount of Work that you put into the lever. The only things that are changed are the Force and Distance -- right? So am I right in thinking that Class I, II, and III levers are all equally efficient, whether you define efficiency in terms of Energy or Work? Or, if my colleagues are right, how are Class III levers the least efficient? Thanks!