Grade 12 · Week 06 · Friday · Day 05

Interaction Pairs and the Evidence Claim

Learning objective: Explain net force and interaction partners with a quantitative claim–evidence–reasoning report.

45–60 min · Offline-capable · No physical apparatus required

Reading & explanation

Make the claim precise

The ideal block's horizontal acceleration equals the net horizontal external force divided by its mass. Zero net force means zero acceleration but need not mean zero velocity. These statements depend on an inertial frame and the defined system. A good claim does not replace acceleration with speed.

Separate action and reaction

When the book pushes downward on the table, the table pushes upward on the book; those are the two members of an action-reaction pair acting on different bodies. The book's weight is Earth pulling on the book and its partner is the book pulling on Earth. The normal and weight can happen to have equal magnitudes while both act on the book; that does NOT make them a third-law pair.

Support a claim with numbers

A 4 kg block with 14 N right and 6 N left has net +8 N and acceleration +2 m/s². With equal horizontal forces of 6 N right and 6 N left, net force is zero and acceleration is zero. If it initially moves at +3 m/s in that latter case, it keeps moving at +3 m/s under the ideal assumptions.

Qualification is part of science

The simulator assumes a flat support, contact maintained, chosen fixed forces and no automatic drag or friction laws. Explain one factor that would matter in a real motion test, and propose a safe way to gather more evidence. Friday's random ten-question check supports correction and mastery practice; it is not a confidential final examination.

Vocabulary

claim
A precise scientific statement supported by bounded evidence.
evidence
Measurements or calculated results with quantities and units.
reasoning
A valid principle relating data to the stated claim.
interaction pair
Equal and opposite forces acting on different bodies.
correction
A documented revision resolving a mistaken idea.

Worked example

4 kg block: +14 N−6 N=+8 N and ax=+2 m/s². Another case +6 N−6 N=0 N and ax=0. The book's normal and weight are not a third-law pair because both act on the book.

Show direction, units, body boundaries and your calculations.

Independent science journal

Write a CER with two signed numerical force sums, force agents, a counterclaim about velocity or interaction pairs, one limitation and corrections. Complete ten Friday practice questions.

Browser-local journal only. Export a backup for another device.

Friday · 10 Randomized Practice Questions

Public, open-book practice with feedback and an 80% correction target. It is not a supervised final.