Grade 12 · Week 07 · Wednesday · Day 03

Newton’s Third Law: Two Bodies, One Interaction

Learning objective: Identify the force pair acting on different objects and distinguish it from balanced forces on one object.

45–60 minutes · Offline practice · Local student notebook

Read and reason

Every interaction has two directions

When object A pushes object B to the right with a force of 12 N, object B pushes object A to the left with an equal magnitude of 12 N. These force vectors are equal and opposite, but they act on different objects. Consequently, they do not cancel in either individual object's free-body diagram.

Different masses, different accelerations

Suppose a synthetic model isolates a 12 N horizontal interaction as the only net horizontal force on each object. If A has mass 3 kg and B has mass 6 kg, A accelerates at −4 m/s², while B accelerates at +2 m/s², with right positive. Equal interaction-force magnitudes do NOT imply equal acceleration magnitudes: a=F/m.

Interaction pairs are not the same as equilibrium

A book’s weight is Earth pulling downward on the book. The third-law partner is the book pulling upward gravitationally on Earth. The table’s normal force on the book belongs to a different interaction, paired with the book’s contact force on the table. Both the weight and normal act on the book; even if they are equal, they are not a third-law pair.

Where simplification enters

Two objects may experience other external forces, so you cannot automatically conclude aₐ=−F/mₐ and aᵦ=+F/mᵦ from one known contact force alone. Those acceleration formulas in the virtual pair lab require a stated assumption: the chosen interaction is the entire horizontal net force on each body. If friction or other forces act, their vector sums must also be included.

Five key terms

interaction pair
Equal-and-opposite forces on two distinct interacting bodies.
contact force
A force arising from physical contact between bodies.
action–reaction
Informal name for forces in one Newton’s-third-law pair.
system boundary
The decision about which body or collection of bodies is modeled.
free-body diagram
A drawing of forces acting on one selected body only.

Write an original example showing how the term applies.

Worked physics example

A pushes B right with +12 N; B pushes A left with −12 N. If those are each object's only net horizontal forces, mA=3 kg gives aA=−4 m/s²; mB=6 kg gives aB=+2 m/s².

Write each variable, direction, algebraic substitution and SI unit in your own solution.

Student science notebook

Draw separate free-body diagrams for A and B. Label each force with its agent, the acceleration sign and each mass. Correct the claim that third-law partners cancel on one object.

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Daily · 4 Retrieval Questions

Open-book formative practice with explanatory feedback and an 80% correction target. Scores are browser-local and not authenticated midterms or final exams.