Modelling Assumptions in Mechanics, and "Hence Show That" Discipline

~40 min · WME01 · 1.1

WME01 · 1.1 · 40 min

A "critique the model" question is not testing whether you remember the vocabulary — it's testing whether you reread the paragraph the model is actually printed in. And a "hence show that" question is not asking you for the right number — the number is already sitting on the page. Both come down to the same discipline: stay tied to what's actually given, never to what you assume this kind of question "usually" wants. One real examiner report names the first failure directly; examiner reports across at least five separate series name some form of the second. Thin as a standalone topic on its own — but two of the quietest ways to lose marks in this whole unit, because a wrong answer to either one can look, right up until the mark scheme explains why, exactly like a right one.

Before you read on

Two or three questions on exactly what this lesson teaches. Being wrong here is fine — it's the fastest way to find out what to pay attention to next.

The vocabulary spec 1.1 names, and what each term actually licenses in your working

Spec item 1.1's own guidance is a single sentence with a long list packed into it: *"Students should be familiar with the terms: particle, lamina, rigid body, rod (light, uniform, non-uniform), inextensible string, smooth and rough surface, light smooth pulley, bead, wire, peg. Students should be familiar with the assumptions made in using these models."* The second sentence is the one that matters for how this actually gets examined: not just naming the term, but knowing what it lets you assume once it's there — because every one of these words is doing real work later in the unit, not sitting in a glossary.

Particle: a body with mass but no size or shape, so every force on it acts at a single point and there is nothing to rotate — this is why the whole of spec sections 2-4 (vectors, kinematics, dynamics) can talk about "the acceleration" of an object as one single number or vector, with no separate question of whether different parts of it are moving differently. Rigid body and lamina: a rigid body keeps every one of its own points the same distance from every other point (it doesn't bend or deform) — a lamina is a rigid body that is flat, with area but no thickness. Rod (light / uniform / non-uniform): a rigid body that is long and thin, where only length matters; light means it contributes no weight of its own; uniform means its weight is spread evenly and can be treated as one force acting at its geometric midpoint; non-uniform means it isn't spread evenly, so the centre of mass has to be given or found rather than assumed at the midpoint (the full worked consequences of this — where the rod-as-model-assumption trap actually costs marks in a real equilibrium question — belong to this unit's own moments lesson, not repeated here).

Inextensible string: cannot stretch — the physical consequence licensed by this one word is that two particles joined by a taut inextensible string, however it's routed, move with exactly the same magnitude of acceleration at every instant the string stays taut. Smooth surface / rough surface: smooth means frictionless — no friction term at all, however hard the surfaces are pressed together; rough means friction is present, governed by $F \le \mu R$ while in equilibrium (spec 5.3) or $F = \mu R$ while moving (spec 4.4). Light smooth pulley: light removes the pulley's own inertia from the problem (nothing needed to spin it up); smooth removes friction at its axle — together, these two words are the entire reason a connected-particles pulley problem can use one shared tension T rather than two separate unknowns. Bead, wire, peg: a bead is a particle with a hole through it, threaded onto a string or a wire so it can slide along a fixed path rather than move freely; a wire is rigid, so — unlike a string, which can only ever pull along its own length — it can push as well, exerting a reaction in any direction perpendicular to itself; a peg is a fixed point (a nail, a smooth pin) that a string can pass over or a body can rest against, playing much the same role in a diagram as a pulley but usually treated as a single point rather than a wheel.

None of this is abstract vocabulary recall in the way it actually gets examined, and it's worth being honest about exactly how thin the real-paper record for spec 1.1 on its own actually is: this topic never anchors a full question by itself anywhere in the archive reviewed for this course. What it does do is show up as a late sub-part riding on a kinematics or dynamics question that's already been solved — asking not "define a particle" but "given the model this specific question just used, what could be refined, and what would that change?" That's the technique the rest of this lesson is actually about.

Mechanism

Why a "critique the model" answer has to be tied to the stem's own words, not to modelling in general

Every model in a Mechanics question is a stated list of simplifications, not an open invitation to list simplifications in general. A question that models a falling object under gravity alone, ignoring air resistance, has made exactly one named trade: real air resistance for zero air resistance, in exchange for equations simple enough to solve by hand. "Critique the model" is asking you to identify that specific trade and reason about undoing it — not to produce any true statement about the real world that happens to sound like physics. This is why a technically-correct observation (real objects experience air resistance; a rod has some width; a string has some mass) can still earn nothing: if the question's own stated model never claimed the opposite of that observation, there's no named simplification for the observation to be relaxing, and the answer is disconnected from what was actually asked. A real examiner report on exactly this question type is direct about the consequence: candidates need to appreciate that a late modelling part refers to "the model," and the model is the one "clearly described" in the question's own paragraph — so only a modification to THAT model, the one actually printed in front of you, receives credit. The discipline is small and mechanical: find the sentence that states the simplification, name what it gave up, and say what changes if that trade is undone — in that order, every time.

"Hence show that" — a command word with its own marking discipline, used across the whole unit

A "show that" (or "hence show that") part prints its own target: the number, or the algebraic form, you're meant to reach is already sitting in the question. That single fact changes what's actually being marked. On an ordinary "find" part, the final number IS the thing worth marking — a candidate who reaches it by any valid route earns the marks. On a "show that" part, simply writing down the printed number proves nothing at all, because a candidate could get it from the page without doing any Mechanics whatsoever. The marks on an ag ("answer given") part are for the ARGUMENT that reaches the printed target, not for matching it.

The facts bank's own record of this is a hedge worth repeating exactly as it stands, not upgrading into something stronger than it is: examiner reports across at least five separate series (Jan 2020, Jan 2023, Oct 2022, Jun 2024, Oct 2023) each repeat SOME FORM of the same standing instruction — *"when a question asks candidates to 'hence show that', ... a full explanation, including all steps, is required to earn full marks, where the explanation must use one or more of the previous parts of the question."* Two separate requirements are packed into that one sentence, and both are load-bearing. First, "a full explanation, including all steps" — an ag part specifically punishes skipped working in a way an ordinary part doesn't, because skipped working is indistinguishable from having copied the printed answer. Second, "must use one or more of the previous parts" — the word "Hence" in "hence show that" is not decoration; it's an instruction to genuinely reuse a value you already found, not to re-derive the whole thing from scratch by some other, unconnected route.

Two real, verified abbreviations from the general marking guidance matter specifically for this kind of part, in addition to the M/A/B system this unit's kinematics lesson already introduced: "DM", a dependent method mark — one that can only be awarded if a specified earlier method mark was itself earned, exactly the shape "hence" is describing in words. And "cso" — *"correct solution only... There must be no errors in this part of the question to obtain this mark"* — the mark code that captures why an ag part's accuracy mark is stricter than an ordinary one: on a normal part, a slipped sign that still lands somewhere reasonable might survive on a follow-through; on a cso mark, the entire chain leading to it has to be clean, because the whole point of the part is that the chain is what's being checked, not just its destination.

Worked, in full

Reading a "critique the model" sub-part correctly — reread before you answer (VERIDIAN-original scenario, built around the real Oct 2019 Q6 technique)

01
Locate and reread the exact paragraph describing the model — not a general sense of what this kind of question usually wants. Stem: "A skydiver's fall is modelled in two phases. During the first phase (free fall), air resistance is ignored, so her acceleration is g throughout. During the second phase, after the parachute opens, she descends at a constant speed, modelling the total resistive force as exactly balancing her weight." A later part asks: "State one way this model could be refined to be more realistic for the first phase, and describe the effect this would have on the calculated duration of that phase."
Earns: This is the step a real examiner report on exactly this question shape names as the one that decides everything downstream — not the physics that follows, but whether the model's own printed wording was actually reread before answering.
02
Identify precisely which named simplification the question is pointing at. The question asks specifically about the FIRST phase, and the paragraph names exactly one simplification for that phase: air resistance is ignored. The second phase's own separate assumption — a constant resistive force exactly balancing weight — belongs to a different part of the model, for a different phase, and is not what this sub-part is asking about.
Earns: Two named simplifications exist in this stem, for two different phases; picking the one this specific sub-part is actually about is a separate skill from having read the paragraph at all.
03
Propose a modification that genuinely loosens THAT named simplification — not an unrelated new detail, however scientifically true it might be on its own. "Include a resistive force during free fall (e.g. proportional to speed)" is a direct relaxation of "air resistance is ignored." "Her mass might change slightly during the fall" would not be: nothing in the paragraph named mass as fixed or ignorable, so relaxing an assumption that was never actually made earns nothing under this same discipline, however plausible it sounds in isolation.
Earns: This is the exact fork a real examiner report's finding describes: a modification tied to the stated model, versus a true-sounding statement about the real world that isn't tied to anything the model actually claimed.
04
State the connected, directional effect — not just that something changes, but which way, and why. Including a resistive force during free fall means her acceleration would be LESS than g throughout that phase, since resistance opposes motion; reaching any given speed or height would therefore take LONGER than the model's own prediction — the calculated duration of the free-fall phase increases.
Earns: Naming the direction of the change, tied to the mechanism causing it, is what turns "the model isn't perfectly realistic" — true of literally every model, and worth nothing on its own — into a specific, markable answer.

Source — Examiner report, Oct 2019

"Candidates need to appreciate that the question refers to 'the model' and the model is clearly described in the second paragraph of the question. Hence only modifications to that model received credit."

In your own words

In one sentence: what do a "critique the model" part and a "hence show that" part have in common, in terms of what they're actually checking that a plain right-looking answer doesn't prove on its own?

Marked, line by line

A block, modelled as a particle P of mass 2 kg, lies on a rough plane inclined at angle α to the horizontal, where sin α = 3/5 and cos α = 4/5. P is connected by a light inextensible string, passing over a light smooth pulley fixed at the top of the plane, to a particle Q of mass 8 kg, hanging freely. The coefficient of friction between P and the plane is 1/2. The system is released from rest with the string taut, and Q descends vertically, pulling P up the line of greatest slope. Take g = 9.8 m s⁻². (a) Find the normal reaction on P, and hence show that the friction force acting on P has magnitude 7.84 N. (b) Find the acceleration of the system. (c) Hence show that the tension in the string is 31.4 N, correct to 3 significant figures. (d) State one further modelling assumption made in this question (other than that P is modelled as a particle), and explain how relaxing it would affect the working used in parts (b) and (c). (VERIDIAN-original question — not a reproduction of any real paper. The incline-and-pulley setup is a vehicle for the modelling-critique and show-that technique this lesson teaches, not new dynamics content: every equation of motion is spelled out inline, and the masses/μ are deliberately different from connected-particles-pulleys-and-lifts.ts's own incline scenario. Every value below was computed with exact fractions and cross-checked using two independent equations of motion — see part (c)'s own inline check.)

10 marks available

(a) — 2 marks

01
Resolve perpendicular to the plane for P (no acceleration in this direction): $R = m_P g \cos\alpha = 2 \times 9.8 \times 0.8 = 15.68$ N
Method mark for resolving perpendicular to the plane with the correct number of terms — the general marking guidance's own principle applies directly: omission of mass from a resolution is a method error, so both the mass and the cos α factor have to be present.
M1
02
$F = \mu R = \tfrac{1}{2} \times 15.68 = 7.84$ N, as required.
cso — 'correct solution only... There must be no errors in this part of the question to obtain this mark.' This line's own target (7.84 N) is printed in the question, so the mark is for a clean derivation that reaches it, not for the number alone.
A1 cso

(b) — 4 marks

101
For P, up the incline positive: $T - m_P g \sin\alpha - F = m_P a \Rightarrow T - 11.76 - 7.84 = 2a \Rightarrow T - 19.6 = 2a$
Method mark for an equation of motion for P with the correct number of terms, each one dimensionally a force: the applied tension, the weight component along the plane (found in part (a)'s own working, m_P g sin α), and the friction force found in part (a) — omitting any one of these would be a method error, not an accuracy slip.
M1
102
For Q, downward positive: $m_Q g - T = m_Q a \Rightarrow 78.4 - T = 8a$
Method mark for a second equation of motion, for Q. This is only a valid second equation because the same a is used for both particles — the direct consequence of the string being inextensible (both particles move with the same magnitude of acceleration) named in this lesson's own teach block above.
M1
103
Adding the two equations to eliminate T: $(T - 19.6) + (78.4 - T) = 2a + 8a \Rightarrow 58.8 = 10a$
Dependent method mark — 'DM indicates a dependent method mark i.e. one that can only be awarded if a previous specified method mark has been awarded' (the general marking guidance's own definition, verbatim). This step is only available once BOTH equations of motion above exist and share the same unknowns T and a.
dM1
104
$a = 5.88$ m s⁻²
Accuracy mark, dependent on the method marks above.
A1

201
Substitute $a = 5.88$ into Q's equation: $78.4 - T = 8 \times 5.88 = 47.04$ , so $T = 78.4 - 47.04 = 31.36$ N
Method mark for substituting the value found in part (b) into an equation of motion — the 'hence' in 'hence show that' pointing directly at this reuse. This is why 'ft' isn't in play the way it sometimes is on an ordinary part: with a printed target ahead, a wrong value of a carried in from part (b) would not, in general, land on 31.36 at all — the mismatch itself is the signal something upstream needs rechecking, which is exactly the extra scrutiny an ag part is designed to apply.
M1
202
$T = 31.4$ N (3 s.f.), as required.
cso — the printed target is 31.4 N to 3 s.f., and this line has to be the last step of a shown derivation reaching it, not a value copied in from the question. Independent check: substituting a = 5.88 into P's OWN equation instead gives $T = m_P a + m_P g \sin\alpha + F = 2(5.88) + 11.76 + 7.84 = 11.76 + 11.76 + 7.84 = 31.36$ N — the same value, reached from the other particle's equation entirely, confirming the answer without relying on Q's equation a second time.
A1 cso

(d) — 2 marks

301
"The pulley is modelled as light and smooth."
Independent mark for naming a genuine assumption actually used in THIS model — light and smooth are both named explicitly in the question's own opening sentence, so this names something the stem itself stated, not a generic Mechanics fact.
B1
302
"If the pulley had mass or friction at its axle, the tension would not be equal on both sides of the string, so a single symbol T could not be used in both P's equation and Q's equation in part (b) — the addition step used to eliminate T would no longer be valid, and two separate tensions would need to be found instead."
Independent mark for a connected explanation — tied to the specific equations used in parts (b) and (c), not a restatement of 'the pulley might not really be light and smooth' on its own. Naming the assumption earns the first mark; explaining its SPECIFIC consequence for the working already shown earns the second.
B1

Named traps

critique-the-model-answer-not-connected-to-the-stated-model: Confirmed verbatim, and the only real past-paper anchor spec 1.1 has as a standalone question type: "Candidates need to appreciate that the question refers to 'the model' and the model is clearly described in the second paragraph of the question. Hence only modifications to that model received credit" (Oct 2019, Q6). A true-sounding statement about the real world — however scientifically accurate — earns nothing if it isn't a relaxation of a simplification the question's own stated model actually named. Find the sentence that states the trade-off before answering, not after.
show-that-final-value-stated-without-the-full-explanation: Examiner reports across at least five series (Jan 2020, Jan 2023, Oct 2022, Jun 2024, Oct 2023) each repeat some form of the same standing instruction: "when a question asks candidates to 'hence show that', ... a full explanation, including all steps, is required to earn full marks, where the explanation must use one or more of the previous parts of the question." On an ag part, the printed target being reached is not, by itself, evidence of anything — a candidate could read it straight off the page. The marks are for the shown argument that arrives there, using a previous part's own value, not for the number matching.
modelling-term-treated-as-decorative-rather-than-load-bearing: A related trap confirmed on a rod-modelling question: "There was general appreciation that modelling the beam as a rod meant that it did not bend, [but] some candidates failed to achieve the mark for including wrong or irrelevant extra statements. The most common incorrect answers related to the mass (or centre of mass) of the rod and comments such as 'clockwise moments equal anticlockwise moments'" (Jan 2020, Q2(b)). "Clockwise moments equal anticlockwise moments" is the EQUILIBRIUM CONDITION, not a consequence of the rod-modelling term — it would be true whatever object the beam had been modelled as. Every term in spec 1.1's vocabulary list licenses a specific, nameable consequence (see the teach block above); a question asking what a modelling term assumes wants THAT consequence, not the technique used to solve the rest of the question.

Retrieval — with feedback on every choice

Question 1

2 marks

A "show that" part asks candidates to show that a tension is 31.4 N (3 s.f.). A candidate correctly applies Newton's second law throughout and arrives at the unrounded value T = 31.36 N, which they then round and state as T = 31.4 N. Have they satisfied what the marking discipline for this kind of part actually requires?

Question 2

2 marks

A bead, threaded on a fixed circular wire, is held in equilibrium at a point on the wire by its own weight and the wire's reaction alone (no friction). Why does the object need to be threaded on a WIRE rather than a STRING for this equilibrium to be possible in general?

Question 3

2 marks

A question states: "A car decelerates uniformly from 24 m s⁻¹ to rest, modelled as a particle moving in a straight line." A later part asks for one way to make the model more realistic and its effect on the calculated stopping distance. Which answer best satisfies the marking discipline named in this lesson?

Reference — not a study method, a lookup

Particle = point mass, no rotation to consider. Rigid body/lamina/rod keep their shape. Light = no weight of its own.
Inextensible string ⇒ same |a| for both connected particles. Light smooth pulley ⇒ same T on both sides, no extra inertia term.
Smooth ⇒ no friction at all. Rough ⇒ F ≤ μR (equilibrium) or F = μR (moving). Wire can push AND pull; string can only pull.
"Critique the model" parts: reread the exact paragraph the model is stated in. Only a modification tied to what's actually named earns credit.
"Hence show that" parts: full explanation, reusing a previous part's value, arriving at EXACTLY the printed value/expression — the argument is marked, not just the final number.
cso = correct solution only, no errors anywhere in that part. DM = a method mark that depends on an earlier method mark already being earned.

Not affiliated with or endorsed by Pearson Edexcel. The Oct 2019 Q6 quotation (used in the teach block, the worked-chain's embeddedEvidence, and the lead trap-taxonomy item) and the Jan 2020 Q2(b) quotation (reused, on-topic, from moments-rods-on-two-pivots.ts's own trap-taxonomy — see this file's header note for why that reuse is legitimate) are both single, verified citations, matched against WME01-verified-facts.md. The "hence show that" instruction quoted in the second teach block and the second trap-taxonomy item is NOT a verbatim single-series quote — the facts bank itself presents it as "verified across multiple reports... all repeat SOME FORM of this" (Jan 2020, Jan 2023, Oct 2022, Jun 2024, Oct 2023), and this lesson repeats that same hedge rather than upgrading it into five separate citations. No WarrantCheck gate is used anywhere in this lesson: that gate requires one real, single-series examiner-report citation of a candidate fudging a show-that derivation backward from the printed answer, and the facts bank does not supply one for WME01 — the ag discipline is taught here through an ordinary marked-solution part and its commonWrongPath instead. The entire incline-and-pulley scenario in the marked-solution block (particle P, 2 kg; particle Q, 8 kg; sin α = 3/5; μ = 1/2) is VERIDIAN-original — no real spec-1.1 mark scheme with attached figures survives in the research bank, since this topic never anchors a full question on its own — and every value in it was computed with exact fractions and cross-checked using two independent equations of motion (shown inline in part (c)), not carried over from any real paper.

Question 12 marks

Yes — the working shows a genuine method throughout, and the final rounded value matches exactly what was printed to be shown
Correct. Both requirements the marking discipline for 'show that' parts documents are here: a full explanation reaching the target (not the target asserted on its own), and a final value that matches the printed one exactly, to the stated number of significant figures. Rounding 31.36 to 3 s.f. giving 31.4 is the expected last step, not a mismatch.
BNo — since the unrounded value, 31.36, does not exactly equal 31.4, the working never actually reaches the printed target
This treats the printed target as if it meant 'the exact value, with no rounding permitted,' but the target itself is stated as '31.4 N (3 s.f.)' — a rounded form. Rounding a correctly-derived unrounded value to the stated precision is exactly what reaching this target looks like, not a failure to reach it.
CNo — a 'show that' answer must be given as an exact fraction, since decimal answers are never acceptable for this type of question
The target itself is explicitly stated to 3 significant figures — a rounded decimal is exactly the form being asked for here, not a fraction. This invents a form requirement the question's own wording doesn't ask for.
DNo — because the candidate used g = 9.8 rather than g = 9.81, and 'show that' questions require the more precise value
This gets the real convention backwards: the general marking guidance states that use of g = 9.81 (not 9.8) 'should be penalised once per complete question' — 9.8 is the standard value this unit's own convention expects, not a shortcut to be corrected.

Traps tested: Rounding convention mistaken for a mismatch · Overclaimed form requirement not actually asked for · G convention reversed

Question 22 marks

A wire is rigid, so its reaction can act in any direction perpendicular to itself — including pushing the bead, not only pulling it — whereas a string can only ever pull along its own length
Correct. A string, however it's arranged, can only exert a tension along its own length, pulling the bead toward wherever the string runs taut to. A rigid wire is a genuinely different kind of constraint: it can supply a reaction in any direction perpendicular to itself, including holding a bead up from underneath — which a string threaded the same way could not do.
BA bead cannot physically be threaded on a string, only on a wire
This isn't a real physical restriction — beads threaded on strings are a genuine, common M1 setup (e.g. spec 4.2's pulley-and-peg scenarios use exactly this idea in different dress). The actual distinction is about what DIRECTION of force each constraint can supply, not whether threading is possible at all.
CA wire is heavier than a string, so it can supply a larger reaction force
The weight of the wire itself isn't the relevant property here (and in most M1 modelling, wires and strings are both treated without their own weight mattering to this question) — what matters is the KIND of force each can exert (any perpendicular direction, versus pull-only), not how much force either one weighs enough to supply.
DThere's no real difference — "bead on a wire" and "bead on a string" describe the same setup with two different names
Collapsing two distinct modelling terms into one is exactly the vocabulary-as-decoration failure this lesson exists to prevent. A wire and a string genuinely license different physical consequences (push-or-pull versus pull-only), the same way "light rod" and "non-uniform rod" license genuinely different consequences rather than being interchangeable labels.

Traps tested: Physical threading impossibility invented in place of the real distinction · Mass of the constraint confused with the kind of force it can exert · Distinct modelling terms collapsed into one

Question 32 marks

"Braking is unlikely to be truly uniform — it's often harder initially and lighter near the end. If the true deceleration is smaller near the end of braking, the true stopping distance would be GREATER than the constant-deceleration model predicts."
Correct. This is a direct relaxation of the one simplification the stem actually names — 'decelerates uniformly' — and it states a specific, connected direction of effect on the exact quantity asked about (stopping distance), not just that something changes.
B"The car should be modelled as a lamina rather than a particle, since it has an area"
The stem's own named simplification is 'decelerates uniformly,' not the choice of particle versus a flat 2D shape — and a car isn't flat in the first place, so a lamina is also the wrong term to reach for even setting that aside. This doesn't engage with what the question actually flagged.
C"Air resistance could be added, since it always affects real cars"
True of real cars in general, but nothing in this stem's own paragraph names air resistance as a simplification being made — the one thing it explicitly flags is the uniform deceleration. A generically true statement not anchored to what THIS stem actually stated is the same disconnected-answer trap named earlier in this lesson.
D"Nothing needs to change — a uniform-deceleration model is always accurate enough for stopping-distance questions"
The question explicitly asks for a refinement and its effect; asserting none is needed doesn't answer what was asked, and ignores that the stem's own word 'uniformly' is itself a stated simplification, not a proven fact about how cars actually brake.

Traps tested: Unconnected modelling term substituted for the one actually named · Plausible but disconnected answer not anchored to the stated model · Dismisses the question instead of engaging with the stated model

Practice this for real

This site teaches the mechanism; the exam is sat on Pearson's own real questions. Go find and attempt these yourself — nothing here substitutes for actually sitting a timed paper.

Examiner report: Oct 2019 · Q6 — cited directly in this lesson

Pearson's official past-papers portal

Select International Advanced Level → Mathematics → any series, then look for WME01.

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Up next

Resultant Forces (Resolving vs. Cosine Rule/Lami's Theorem) and Bearings

Two forces meeting at a point have exactly one resultant, and this whole topic is really two different ways of finding it — resolve into i/j components and use Pythagoras, or draw the triangle the two forces make and hit it with the cosine rule (or Lami's theorem) directly — plus one conversion, bearing to component, that the exam tests far more literally than it looks: get sin and cos the wrong way round and the wrong answer isn't vague, it's a specific number an examiner report has watched real candidates hand in.

50 min

Modelling Assumptions in Mechanics, and "Hence Show That" Discipline

The vocabulary spec 1.1 names, and what each term actually licenses in your working

Why a "critique the model" answer has to be tied to the stem's own words, not to modelling in general

"Hence show that" — a command word with its own marking discipline, used across the whole unit

Reading a "critique the model" sub-part correctly — reread before you answer (VERIDIAN-original scenario, built around the real Oct 2019 Q6 technique)

Deep dive

Practice this for real