Final answer:
A roller or rocker in physics refers to a simple machine known as a lever. The number of reactions a roller or rocker has depends on the class of lever it belongs to.
Step-by-step explanation:
A roller or rocker in physics refers to a simple machine known as a lever. A lever has a fulcrum, which acts as the pivot point, a load or resistance on one end, and an effort or force on the other end. There are three classes of levers, and the number of reactions a roller or rocker has depends on the class of lever it belongs to.
For a first-class lever, where the fulcrum is positioned between the load and the effort, there are two reactions - one at the fulcrum and another at the point where the lever touches the load or resistance.
For a second-class lever, where the load is positioned between the fulcrum and the effort, there is only one reaction - at the fulcrum.
For a third-class lever, where the effort is positioned between the fulcrum and the load, there are two reactions - one at the fulcrum and another at the point where the lever touches the load or resistance.