What defines a first-class lever?

A first-class lever is defined by the position of the fulcrum in relation to the load and the effort. In this type of lever, the fulcrum is positioned between the load and the effort. This arrangement allows for the direction of the effort to be changed and the ability to lift heavier loads with less input force.

By placing the fulcrum between the load and the effort, the lever system takes advantage of mechanical advantage, which means that a smaller force can be used to lift a larger load by extending the distance from the fulcrum to where the effort is applied. This configuration is fundamental in many common tools and applications, such as seesaws and scissors, where balancing and lifting are achieved through this strategic placement.

The other options do not accurately describe the characteristics of a first-class lever: the load and effort being at the ends (first option) would describe a different configuration, applying effort in the middle (third option) reflects a second-class lever, and limiting the lift to a single angle (fourth option) does not pertain to the fundamental definition of levers in general.