Reaction is defined as

• A. A force provided by a support that is equal but opposite to the force transmitted from a member to its support
• B. The upward reaction at supports
• C. The internal force within a member
• D. The moment at a hinge

Answer: (A)

Reactions are the forces that supports exert on a member to keep it in static equilibrium. When a member pushes on a support, the support pushes back with an equal and opposite force. That force from the support on the member is the reaction. Its direction and the way it manifests depend on the support type and loading: it’s typically a vertical force at simple vertical supports, but can also include horizontal components or a bending moment at fixed supports. This aligns with the idea that a reaction is the force provided by the support that balances the force transmitted from the member to the support. Internal forces inside the member are different—they’re internal, not the boundary reaction at the support.