The 42nd Period | STEM Education Hub

Use this as a quick reference for third-law pairs, internal vs. external forces, and the ideal-string / ideal-pulley shortcuts.

🧭 Plot Summary

Every force you drew on a free-body diagram last lesson has a twin — one you deliberately left off the diagram. Newton's third law says that twin always exists: whenever one object pushes or pulls on a second object, the second object pushes or pulls back on the first, equally hard, in the opposite direction. The catch that trips almost everyone up at first is that a third-law pair always involves two different objects— the normal force and gravity acting on the very same resting book are nota pair, just two forces that happen to balance. This lesson uses that one law to explain why pulling on your own shoelaces can't move you, and then builds the vocabulary — tension, ideal strings, ideal pulleys — you'll lean on constantly for the rest of the unit.

What you'll do in this lesson

Learn Newton's third law as a statement about two objects, not a description of any single object's forces.
See why the normal force and gravity on the same resting object are NOT a third-law pair.
Use Newton's third law to explain why internal forces never move a system's center of mass.
Treat tension as the net macroscopic result of countless internal pulling forces inside a string.
Apply the ideal-string shortcut (massless, inextensible, uniform tension) in multi-object problems.
Recognize when a string or pulley isn't ideal, and what changes about tension when it isn't.

Why it matters

Nearly every multi-object problem for the rest of this course — blocks connected by strings, pulleys, tug-of-war scenarios — depends on correctly pairing forces and knowing exactly when "the tension is the same everywhere" is a valid shortcut and when it isn't.

✅ Self-Check Before You Roll On

Check off each item as you get there. These aren't grades — they're your own signal.

I can state Newton's third law and identify the two objects involved in a genuine action-reaction pair.I can distinguish a true Newton's third-law pair (two objects) from two different forces that happen to act on the same object.I can explain why internal forces between parts of a system cancel out and never change the motion of that system's center of mass.I can describe tension as the macroscopic result of infinitesimal string segments pulling on their neighbors.I know that an ideal string has negligible mass, doesn't stretch, and has the same tension throughout its length.I can explain why a string with significant mass can have tension that varies from point to point along its length.I know that an ideal pulley has negligible mass and friction, and simply redirects a string's tension without changing its magnitude.I can determine when the "same tension throughout" shortcut is and isn't valid, based on whether the string and pulley involved are ideal.

🔬 Deep Dive

Your textbook for this topic: Newton's third law, internal vs. external forces, and tension — including ideal strings and ideal pulleys — with an interactive pair-finder tool.

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🧭 Guided Practice

Step through worked examples for every essential knowledge statement in 2.3.A. Three examples each, revealed one step at a time.

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📝 Assignment

20 questions covering the whole lesson. Every answer gets feedback right away, and a wrong answer tells you the most likely reason why.

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📌 AP Exam Note

Misidentifying the normal force and gravity (on the same object) as a third-law pair is one of the most common points lost on FRQs. Also expect at least one multi-part question built around a heavy rope or chain, where tension changes along its length — a direct test of the ideal-vs-nonideal string distinction.