Mechanics 1
Topic Brief: Momentum Impulse Sign Convention
Provenance: written 2026-09-29. No new primary-source mining was done here — every citation below is reused, and re-checked against, four existing sources: research/veridian/WME01-verified-facts.md §4 topic 4.3, §6, and §9's lesson-audit entry (the provenance-checked primary quotes); forge-exam-technique-guide.md §1–§2 (the two fully mark-mapped 2025 series' question tables and sign-convention section); signal-frequency-and-boundaries.md §1 (spec-4.3 mark values and the cross-series trap count); and momentum-impulse-sign-convention.ts's own header, confirming its numeric scenarios are VERIDIAN-original, not reproductions of real stems. question-bank/wme01.ts was checked and holds no momentum items, so there's no question-bank overlap. "17 series checked" is the facts bank's own §5 count of series with a genuine primary document reviewed for this paper. This document's job: collapse those sources into one series-indexed, mark-tariff-aware brief for this sub-topic — nothing here is a new fact.
SIGNAL · WME01 Topic Master Brief — Momentum, Impulse, and the Sign-Convention Trap
For the derivations, the worked collision chain, and the method-comparison treatment, use the lesson file (momentum-impulse-sign-convention.ts). This document does one thing only: for this specific sub-topic, which real series tested it, at what mark tariff, in what recurring shape, and what to expect if it shows up on your paper.
1. Series index
Spec 4.3 (momentum and impulse; conservation of momentum for two particles colliding directly, one-dimensional motion only) is confirmed present, by name, in 5 of the 17 series checked for this paper — the other 12 were never searched question-by-question for this specific topic, so they are unconfirmed rather than ruled out (see §4 of this course's own facts bank, which never built a full per-series topic tally).
| Series (question) | What's confirmed | Real context |
|---|---|---|
| Jan 2020 (Q1) | Sign-convention trap | Two-particle direct collision; final answer required as a speed |
| Oct 2022 (Q1) | Sign-convention trap | Two-particle direct collision; final answer required as a speed |
| Jun 2024 (Q1) | Sign-convention trap and particle-choice efficiency note | Two-particle direct collision; impulse asked for on one named particle |
| Jan 2025 (Q3, part of a 12-mark question shared with 3.1/4.4) | Topic presence only — no specific error documented in sources checked | Momentum/impulse combined with suvat and friction in one multi-part question |
| Jun 2025 (Q1, 6 marks, standalone) | Topic presence only — no specific error documented in sources checked | Momentum/impulse, two particles, dedicated question |
Of these five, the identical, quotable sign-convention trap — reporting a signed velocity where the question asked for an unsigned speed — is independently confirmed in 3 of the 17 series (Jan 2020, Oct 2022, Jun 2024), every one of them Q1. That is the single strongest, most independently-confirmed trap anywhere in this paper's record, momentum or otherwise.
Not counted in that tally: January 2025 reproduces the same sign-vs-magnitude failure ("a substantial number gave a negative answer and lost a mark") on a kinematics deceleration question, not a momentum one — real evidence the discipline generalises beyond this topic, but a different spec point's question.
2. Mark-tariff pattern
Only two series in the record carry a documented mark total for a momentum/impulse question, and this brief states only those two rather than guessing at the rest:
- Jun 2025 Q1 — 6 marks, a standalone two-particle momentum/impulse question, nothing else riding in the same question.
- Jan 2025 Q3 — 12 marks total, but shared three ways with suvat (3.1) and friction (4.4) inside one multi-part question; no mark scheme reviewed for this paper sub-divides a question's marks by spec point, so momentum's own share of that 12 is not stated anywhere in the sources checked and is not estimated here.
No mark total was located for any of the three trap-confirmed anchor questions (Jan 2020 Q1, Oct 2022 Q1, Jun 2024 Q1) in the material this brief draws on. What the record does support: in every one of the five series above, the topic anchors an early question — Q1 in four of the five, Q3 (of seven) in the fifth — never a late, high-mark question the way connected-particles/pulley problems consistently are on this paper.
3. Recurring context and angle
The confirmed real angle here is narrow and consistent, not varied the way some other WME01 topics are. Every trap-confirmed citation is the same scenario shape: two particles colliding directly, one-dimensional motion, the question asking either for the impulse on a named particle or for a resulting speed/velocity. The spec's own guidance restricts the topic to exactly this ("Knowledge of Newton's law of restitution is not required. Problems will be confined to those of a one-dimensional nature") — so the narrowness is a real feature of what can be asked, not a gap in what this course has checked.
Two distinct, real failure points sit inside that one scenario shape:
- The final-line sign trap — the working is correct up to and including a genuinely negative velocity, and the mark is lost only by copying that sign into an answer the question specifically called a speed. Confirmed word-for-word in three series: Jan 2020 ("not taking account of the direction of the velocity"), Oct 2022 ("the final answer needed to be positive as speed was required"), Jun 2024 ("the final mark was often lost because it had been left as negative").
- Particle-choice efficiency — confirmed once, Jun 2024 Q1: "the majority of responses used particle A to find the impulse, rather than using particle B which was much easier since it started at rest." Not a correctness error (both routes reach the same right answer), but a real, examiner-documented extra-step cost.
4. If this topic appears — what to expect
- Expect a two-particle, one-dimensional direct collision, asking for an impulse magnitude and/or a resulting velocity or speed — not a multi-body or multi-stage setup.
- Expect it early in the paper: four of the five confirmed series place it at Q1; the fifth at Q3 of seven.
- Before writing a final answer, reread the exact word the question uses — speed and magnitude are never negative; velocity can be. This single check accounts for every confirmed trap in this topic's record.
- If two particles are involved and the question asks for an impulse, check which particle has the simpler known values (often, but not only, the one starting at rest) before choosing which one to base the calculation on.
- If the question bundles momentum with suvat and/or friction in one multi-part item (as in Jan 2025 Q3), don't assume the printed total mark count describes the momentum content alone — it doesn't, and this paper's mark schemes never state the split.
- Nothing above is a probability. It is a description of what five named series, out of the 17 with a genuine primary document reviewed for this paper, have actually contained.
Genuine gaps, flagged
- The remaining 12 of 17 series were not searched question-by-question for this specific topic in the sources this brief draws on; a series absent from §1's table has not been checked for this topic, not confirmed clear of it.
- No documented mark total exists anywhere in this course's WME01 material for the three trap-confirmed anchor questions (Jan 2020 Q1, Oct 2022 Q1, Jun 2024 Q1) — §2 states two figures, not five, rather than filling the rest with an estimate.
- "5 of 17" (topic presence) and "3 of 17" (the specific sign trap) are two different claims answering two different questions — both are stated separately in §1 and should not be collapsed into one number.