Question

Newton’s third law of motion tells us that forces always occur in pairs of equal and opposite magnitude. Explain how the choice of the "system of interest" affects whether one such pair of forces cancels.

a) The system of interest doesn't affect force cancellation.
b) Forces always cancel regardless of the system.
c) The system of interest determines force direction.
d) Forces within a system cannot cancel.

Answer 1

Newton's third law states that forces come in equal and opposite pairs, known as action-reaction pairs, which do not cancel as they act on different bodies.

The 'system of interest' determines whether these forces are considered internal or external, which affects the system's net force and motion.

Step-by-step explanation:

Newton's third law of motion states that whenever one body exerts a force on a second body, the second body exerts a force that is equal in magnitude and opposite in direction on the first body.

These forces are action-reaction pairs and do not cancel each other because they are exerted on different systems or bodies. The choice of the "system of interest" is crucial because the internal action-reaction forces within a single system do not influence the motion of the system itself but are useful for understanding interactions within the system.

For example, if you take a system of two ice skaters pushing against each other, the force one skater applies on the other is equal and opposite to the force applied back on the first skater.

However, if you choose one skater as the system of interest, then the force from the other skater on them is an external force, which affects their motion according to Newton's laws, and thus these forces do not cancel out within this chosen system.