Physics 1 · Unit 2: Dynamics · 14 min read · Updated 2026-05-11
Newton's Third Law and Free-Body Diagrams — AP Physics 1
AP Physics 1 · Unit 2: Dynamics · 14 min read
1. Newton's Third Law and Action-Reaction Pairs★★☆☆☆⏱ 4 min
Newton's third law (N3L) describes the interaction force between any two objects: whenever object A exerts a force on object B, object B exerts an equal-magnitude, opposite-direction force of the same type back on object A. This is a foundational rule for all dynamics problems on the AP Physics 1 exam.
\vec{F}_{A \text{ on } B} = - \vec{F}_{B \text{ on } A}
Always act on *two different objects* (never the same object)
Always the same *type of force* (e.g., gravitational, normal, frictional)
Equal in magnitude at all times, even if objects accelerate or have different masses
Exam tip: On any question asking you to identify an action-reaction pair, first eliminate any pair that acts on the same object — that is never a Newton's third law pair.
2. Drawing Correct Free-Body Diagrams★★☆☆☆⏱ 4 min
A free-body diagram (FBD) is a standardized graphical tool to isolate all forces acting on a single target object or system, so you can correctly apply Newton's second law. The AP Physics 1 exam regularly awards points for correct force direction, correct labeling, and excluding invalid forces.
Represent the target object as a single point (all forces act on the center of mass for rigid bodies)
Draw each force vector starting at the point, pointing outward in the direction the force acts
Label every force with a standard label indicating type and source
Never include net force, or forces the target object exerts on other objects
Exam tip: If you are ever tempted to draw a 'force of motion' or 'force of inertia' on an FBD, stop: all forces must come from an interaction with another object, so these fictitious forces are never allowed, and including them will lose points.
3. Internal vs. External Forces and System Selection★★★☆☆⏱ 4 min
When solving problems involving multiple interacting objects, you can choose to treat each object as a separate system or treat multiple objects together as a single combined system. This choice is valid because of Newton's third law, and it drastically simplifies calculations for connected objects.
This method lets you find the acceleration of the entire system first, without solving for internal forces like tension first. You can then go back and solve for internal forces after you know the acceleration.
Exam tip: If an FRQ asks for the acceleration of a system of multiple connected objects, always use the combined system method first to get acceleration quickly, then solve for internal forces separately.
4. AP-Style Concept Check★★★☆☆⏱ 2 min
Common Pitfalls
Why: Students confuse balanced forces (two forces on the same object that sum to zero) with Newton's third law pairs, which always act on two different objects.
Why: Students confuse normal force direction with the direction of weight, or assume normal force always opposes gravity.
Why: Students intuitively think moving objects need a force to keep them moving, from everyday experience with friction, leading to this misconception.
Why: Students forget internal forces cancel per Newton's third law, leading to under or over-counting net force and wrong acceleration.
Why: Students remember 'equal and opposite' so they assume the forces sum to zero, leading to a wrong conclusion of zero acceleration.