A set of two equal and opposite forces that govern the interaction between two objects, as described by Newton's third law of motion. Whenever object A exerts a force on object B, object B exerts an exact, opposing force back on object A. Identifying these pairs is fundamental to drawing accurate free body diagrams and calculating static equilibrium in structural analysis.
Heard on the job
When you draw the free body diagram for the pin joint, don't forget to include the reaction force pair.
Also heard as
- action-reaction pair