2.1.A.1–2Concept
A system's properties are determined by the interactions between the objects within it. When those internal interactions aren't important to modeling the system's overall (macroscopic) behavior, the system can be treated as a single object.
ExampleDefining the system: block, pulley, and hanging mass1 / 3
A block sits on a table, connected by a string over a pulley to a mass hanging off the edge. You're asked to find the whole setup's acceleration. Compare what counts as "internal" versus "external" if you define the system as (a) the block and the hanging mass together, versus (b) the block alone.
Try it on paper first, then reveal each step to check your work.
Step 1 — System (a): block + hanging mass
The string tension pulls the block forward and pulls the hanging mass upward — but tension acts between two objects that are both inside this system, so it's an internal interaction and cancels out of the system's overall motion equation. Only gravity on the hanging mass (external, since the table/pulley/Earth are outside the system) drives the combined acceleration.