Final answer:
Action and reaction pairs of forces, based on Newton's third law of motion, always act simultaneously and are equal in magnitude but opposite in direction. They act on different systems, and understanding the system of interest is vital in determining if these forces cancel out within a particular analysis.
Step-by-step explanation:
The student's question regarding action and reaction pairs of forces relates to Newton's third law of motion, which states that forces always occur in pairs of equal and opposite magnitudes and act on different systems. To answer the student's choice question, Newton's third law dictates that action and reaction pairs of forces: a. always act simultaneously.
For the multiple-choice questions provided:
- A point acted on by two forces in equilibrium would have forces that b. have equal magnitudes but opposite directions.
- Two equal and opposite forces do not cancel out because c. Because the two forces act on different systems.
Understanding the concept of system of interest is crucial in analyzing physical situations. When analyzing a problem, selecting the system can determine whether the action-reaction force pairs cancel out or contribute to the net force acting on the system. Within a chosen system, action-reaction pairs do not cancel since they act on different parts of the system or on different objects entirely. However, if one were to consider both objects as part of the same system, the internal action-reaction forces would cancel out.