Five forces plus the static load
Five named forces describe how a load acts on a member. TENSION stretches, pulling ends apart (a tow rope). COMPRESSION squeezes ends together (a table leg). SHEAR is two offset forces sliding one part past another (a rivet). BENDING sags a beam — its top edge compressed, bottom stretched at once. TORSION twists a member about its long axis (a screwdriver). A STATIC load is steady, unlike a dynamic (moving) one.
Lever order — what sits in the middle
Every lever has three points: the FULCRUM (pivot), the LOAD (resistance) and the EFFORT (the force applied). The ORDER is set by which one sits between the other two. FIRST: fulcrum in the middle (see-saw, crowbar). SECOND: load in the middle (wheelbarrow, nutcracker) — the effort arm is always longer, so mechanical advantage is greater than 1. THIRD: effort in the middle (tweezers, forearm) — mechanical advantage less than 1, but you gain speed and range of movement.
Force drives the design
The force in a member decides how to design it. A member in TENSION (a tie) only resists a pull, so it can be thin — a cable or rod. A member in COMPRESSION (a strut) can buckle, so it needs a larger cross-section or a tube. Concrete is strong in compression but weak in tension, so a concrete beam is reinforced with steel near its lower (stretched) edge. TRIANGULATION turns a frame into rigid triangles, so members carry pure tension or compression, not bending.
Drawn from real examiner reports.
Bending misread as compression
Force identification is a repeatedly reported weak spot. Two common slips: reading BENDING as plain compression, and missing TORSION (a twist about the long axis). A beam loaded across its length is in BENDING because its top edge is squeezed while its bottom stretches at once. Name a force from what it DOES — stretch, squeeze, slide, sag or twist.
s23 P42 Q4; s23 P43 Q2a — torsion was confused with other forces, and bending was misread as compression.
Shear at bolts, dowels and joints
A force candidates repeatedly fail to spot is SHEAR: two forces offset so one part tends to slide or be cut across another. It appears wherever overlapping parts are pushed in opposite directions — across a bolt or rivet joining two plates, across a dowel, or at a glued truss joint. Shear acts ACROSS the member, not along it, tending to slice it.
w23 P42/P43 Q9a; s23 P43 Q10c — shear at glued truss joints and on dowels was not identified.
Guessing the lever order
Asked for a lever order and a real example, weaker candidates guess. The reliable method: mark the FULCRUM (pivot), LOAD (what is moved) and EFFORT applied, then see which lies BETWEEN the other two — fulcrum in the middle is first order, load second, effort third. A wheelbarrow pivots at its wheel with the load in the middle, so it is SECOND order, not first.
s23 P43 Q11a(iv); w23 Q4 — candidates were weak at identifying the class of a lever and naming a matching example, tending to guess.
Moments — mixing mm and m
In a moment calculation (force times perpendicular distance) the standard slip is a UNIT mix-up: a distance in millimetres used as if it were metres. Convert every distance to the same unit before multiplying, and state the answer's unit (newton metres). Always SHOW working — method marks are given at each stage, even if the final number slips.
w23 P42/P43 Q9c — in moment calculations the common failure was mixing millimetres and metres; the standing message is to show working and state the unit.
Mechanical advantage vs velocity ratio
Two ratios get muddled. MECHANICAL ADVANTAGE compares FORCES: MA = load divided by effort — how much the lever multiplies your force. VELOCITY RATIO compares DISTANCES moved: VR = effort distance divided by load distance. Examiners note the MA definition (load divided by effort) is often stated weakly. Keep them apart: MA is a force ratio, VR a movement ratio.
w23 P43 Q11a(ii) — the precise definition of mechanical advantage (load divided by effort) was weakly stated.
Static load vs dynamic load
A STATIC load is steady and non-moving — the dead weight of a structure itself, or a stationary object resting on it. A DYNAMIC load moves or changes — a person walking across, wind gusting, a vehicle passing. Candidates blur the two, or call any load static. The test is movement: stays put = static, moves or varies = dynamic.
Every lever makes lifting easier
A testable wrong belief: that every lever reduces the effort, giving a mechanical advantage above 1. True only for FIRST and SECOND order levers, where the effort arm is longer. A THIRD-order lever (tweezers, forearm) puts the effort between fulcrum and load, so its MA is LESS than 1 — the effort exceeds the load but gains speed and range.
Classify a lever by marking F, L, E
Do not answer lever-order questions from memory. Mark the FULCRUM, LOAD and EFFORT on the tool, then state which sits between the other two (fulcrum middle = first, load = second, effort = third) and name a matching example. A quick labelled sketch is often the fastest route.
Show working and state the unit
Section 4 calculations are marked for METHOD, not only the answer. Write the formula, substitute on its own line, then give the answer WITH its unit (newton metres for a moment, a plain number for a ratio). Working earns method marks even if the answer slips.
Name the part and justify the force
On force questions, do not just write 'tension' — say which named part is in tension and WHY (a suspension cable is in tension because the deck pulls it down). Match the command word: explain wants the reason, evaluate wants both sides plus a judgement.
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