Q: What does A-Level Physics: 2) Forces & Moments Guide cover? A: From Lorentz force to seesaw balance, this post reverse-engineers Section I Topic 2 of the 2026 H2 Physics syllabus into check-lists.
TL;DR Forces, moments and equilibrium form the control-panel for every mechanics question after WA 1. Nail the four compulsory models - field forces, contact forces, Hooke springs and couple torque - then clamp them together with the twin “net-zero” tests (∑F=0 and ∑τ=0
Reviewed by
Chee Wei Jie·Academic Advisor (Physics)
). The pay-off is a one-line checklist that prevents sign slips and missing forces.
Concrete example: how to use this page
When a mechanics question feels messy, first draw the free-body diagram and label every force. Then decide whether the object is translating, rotating, or in equilibrium. That order keeps force and moment equations from mixing too early.
Force-and-moment route-selection map
Use this map before substituting numbers. It keeps force equations, moment equations, and spring equations from being mixed in the same first line.
Question cue
First move
Equation or check
Common trap
Weight, electric force, magnetic force, or current-carrying conductor
Identify the carrier: mass, charge, moving charge, or current in a wire.
Pick F=mg, F=qE, F=qvBsinθ, or F=ILBsinθ.
Using the right magnitude formula but drawing the field-force direction wrongly.
Normal, friction, upthrust, tension, or drag
Draw a free-body diagram before resolving components.
Every contact force must start at the object and act in its physical direction.
Adding a force because it is familiar rather than because that contact or field exists.
Spring extension or compression
Check that Hooke's law is still within the proportional region.
Use F=kx, with x measured from natural length.
Treating total spring length as extension.
Beam, pivot, seesaw, ladder, or hinge
Choose the pivot that removes the most unknown forces.
Moment uses perpendicular distance to the line of action.
Using the full beam length when the perpendicular moment arm is shorter.
"Equilibrium", "stationary", or "constant velocity"
Apply both translational and rotational tests if the body can rotate.
Require ∑F=0 and ∑τ=0
Revisit earlier/later mechanics chapters in our free H2 Physics notes - it chains Topic 1 foundations to this post and onward to kinematics, circular motion, and power drills for Paper 2/3.
1 Quick syllabus snapshot
Section I Topic 2 sets nine learning outcomes, from describing field forces to drawing vector triangles. They are reproduced verbatim in the SEAB 9478 document so you can tick them off one by one.
2 Field forces every IP student must quote
Situation
Model equation
Vector cue
1
Mass in Earth field
F=mg
Down
2
Charge in electric field
F=qE
Along E
3
Charge moving in B
F=qvBsinθ
4
Conductor length L in B
F=ILBsinθ
The last two come straight from the Lorentz force law and its current-carrying variant.
In equation 4, the current-carrying variant for Lorentz force law, the direction of the length L is given by the direction of the current. I is defined to be the magnitude of the current so it's a scalar. Hence we use the length vector to denote the direction of the current.
2.1 Mini-drill
Identify the carrier (mass, charge or current).
Write the matching formula.
Use the right-hand grip rule on your free-body diagram to determine the direction of the force.
3 Contact forces - four names, one diagram
Normal force: surface reaction perpendicular to contact.
Buoyant force (upthrust): weight of displaced fluid.
Friction: parallel to surface, opposes impending motion.
Viscous drag / air resistance: proportional to speed for low v.
Teach your child to label them exactly as the mark scheme: “normal”, “upthrust”, “friction”, “viscous”. Physics Classroom's taxonomy is a handy poster for the study wall.
4 Hooke's law - the spring you will meet ten times
For an ideal spring
F=kx
where k is the force constant and x is the extension or compression. The law holds until the proportional-limit point. Khan Academy's graph tutorial is perfect for WA data-logger questions.
Parent cue: ask your teen to re-plot their practical data as F vs x and fish out k from the gradient.
5 Moment of a force
The turning effect of a single force about point O is
τ=Fd⊥
where d⊥ is the perpendicular distance from O to the line of action. Engineering Statics gives a brilliant interactive showing how d⊥ shrinks as the force slides.
Moment-arm checkpoint
Before writing a moments equation, decide what each force is trying to rotate the body about and whether its line of action passes through the pivot. This prevents the common mistake of using the visible length instead of the perpendicular distance.
Force position
Moment about the chosen pivot
What to write first
Line of action passes through the pivot
Zero moment
Cross it out from the moments equation, but keep it in the force-balance equation if needed.
Force is perpendicular to the beam or ruler
Moment equals F× the marked distance
Use the marked distance only if it is measured from the pivot to the line of action.
Force acts at an angle
Moment equals F×d⊥, or one resolved component times its distance
Draw the perpendicular from the pivot to the line of action before substituting numbers.
Several unknown reactions meet at a hinge
Choose the hinge as pivot if possible
This removes those unknown reactions from the moments equation because their moment arms are zero.
Misconception check: the pivot does not have to be at the physical support named in the question. You may choose any convenient point, as long as every moment is taken about that same point.
6 Torque of a couple - pure rotation, zero translation
A couple is two equal, opposite, parallel forces whose lines of action are separated by distance s. They cancel translationally but create a pure moment
τcouple=Fs
7 Centre of gravity
For weighted assessment (WA) problems, the weight may be treated as acting at a single point - the centre of gravity (CG). Britannica phrases it as “the imaginary point where the total weight is thought to be concentrated”.
Hack: on uniform beams, centre of gravity (CG) is at the midpoint; on composite beams split into rectangles and take moments about a pivot to find the weighted average.
8 Principle of moments - the seesaw rule
Sum of clockwise moments = sum of anticlockwise moments (about the same point)
Save My Exams calls it the “balanced torque” condition and has an annotated beam diagram you can photocopy.
9 Translational vs rotational equilibrium
Condition
Mathematical test
Typical cue
Translational
∑F=0
Object moves at constant velocity v (constant speed and same direction); No linear acceleration
Rotational
∑τ=0
Object turns at constant angular velocity ω (constant angular speed and angular direction); No angular acceleration
For the system to be in full equilibrium, we need translational and rotational equilibrium - ∑F=0 and ∑τ=0.
10 Free-body diagrams & vector triangles
Isolate the body.
Arrow for every force, labelled.
Check heads and tails form a closed triangle if the body is in equilibrium.
Free-body diagram audit checkpoint
Before resolving components, audit the diagram by force source. This prevents adding a familiar force that is not actually acting on the isolated body.
Audit question
What to draw
Equation impact
Common trap
What object is isolated?
Draw only forces acting on that object, not forces it exerts on other objects.
Every term in ∑F must belong to the same body.
Mixing an action-reaction pair on one diagram.
Is the force from a field?
Weight acts at the centre of gravity; electric or magnetic forces follow the relevant field rule.
Field forces can act without contact.
Drawing weight from a support point instead of through the centre of gravity.
Is the force from contact?
Normal, friction, tension, upthrust, or drag starts at the contact or interaction region.
Contact forces disappear if that contact is removed.
Adding friction when no rough contact or tendency of motion is stated.
Do the chosen axes match the motion?
Resolve along and perpendicular to the slope, string, or acceleration direction when useful.
Components enter separate scalar equations.
Resolving weight horizontally and vertically on a slope, then forgetting the normal is tilted.
Can the body rotate?
Keep each force's line of action visible.
Moment equations need perpendicular distance to the line of action.
Moving a force arrow to a neater position and changing its moment arm.
Worked check: for a block held at rest on a rough slope by a string, isolate the block only. Draw weight vertically downward, normal perpendicular to the slope, tension along the string, and friction along the slope opposing the possible slipping direction. Then resolve parallel and perpendicular to the slope before writing ∑F∥=0 and ∑F⊥=0.
Misconception check: a free-body diagram is not a picture of everything touching the object. It is a force inventory for one isolated body.
Wired's tutorial plus Physics Classroom's “Equilibrium and Statics” page give clear worked examples.
11 Three WA timing rules (copy to your phone)
Use syllabus pacing as a guide: Paper 2/3 average ~1.6 min/mark; Paper 4 ~3 min/mark.
Write units before numbers in moment questions to avoid cm-m mix-ups.
Keep working lines for method marks, especially in vector components.
12 Bridge to Paper 4 practical
Clamp a metre rule to a pivot, add masses, and verify ∑τ=0.
Use Google Sheets =LINEST() to grab gradient ± standard error when testing Hooke's law.
Need structured practice on Forces and Moments? Our H2 Physics tuition programme covers this topic with weekly problem sets and Paper 4 practical drills.
Comprehensive revision pack
9478 Section I, Topic 2 Syllabus outcomes
Candidates should be able to:
(a) describe the forces on a mass, charge and current-carrying conductor in gravitational, electric and magnetic fields, as appropriate.
(b) show a qualitative understanding of forces including normal force, buoyant force (upthrust), frictional force and viscous force, e.g. air resistance. (Knowledge of the concepts of coefficients of friction and viscosity is not required.)
(c) recall and apply Hooke's law (F=kx, where k is the force constant) to new situations or to solve related problems.
(d) define and apply the moment of a force and the torque of a couple.
(e) show an understanding that a couple is a pair of forces which tends to produce rotation only.
(f) show an understanding that the weight of a body may be taken as acting at a single point known as its centre of gravity.
(g) apply the principle of moments to new situations or to solve related problems.
(h) show an understanding that, when there is no resultant force and no resultant torque, a system is in equilibrium.
(i) use free-body diagrams and vector triangles to represent forces on bodies that are in rotational and translational equilibrium.
Concept map (in words)
Begin with a labelled diagram that separates field forces (weight, electric, magnetic) from contact forces (normal, friction, tension, upthrust). Resolve each vector into perpendicular axes before applying the two equilibrium tests. If a system rotates, check for couples and compute their moments. Link to Hooke's law when springs provide the restoring force, and to friction models when impending motion is mentioned.
Key definitions & relations
Quantity / idea
Expression / reminder
Resultant force
∑F - zero for translational equilibrium
Moment of a force
τ=Fd⊥ about chosen pivot
Principle of moments
∑τcw=∑τacw at equilibrium
Couple
Two equal, opposite, parallel forces → net torque τ=Fs
Hooke's law
F=kx (within elastic limit); energy stored E=21kx2
Lami's theorem (triad)
sinαF1=sinβF2=sinγF3
Centre of gravity
Point where resultant weight acts; for uniform rod = midpoint
Derivations & reasoning to master
Resultant of non-perpendicular forces: resolve forces into components, then reconstruct magnitude via Pythagoras - R=Fx2+Fy2.
Ladder against a wall: write the floor and wall reaction forces (with friction shown qualitatively as a labelled contact force) on a free-body diagram, then apply ∑F=0 and ∑τ=0
Springs in series/parallel: show that keq1=k11+k21
Couple work: prove that work done by a couple is W=τΔθ - independent of pivot choice.
Worked example 1 - non-uniform beam
A 3.0 m beam (mass 18 kg) is supported at the left end and by a cable at the right end making 30° with the horizontal. A 160 N weight hangs 1.2 m from the left. Find the tension in the cable and the reaction at the hinge.
Sketch solution: take moments about the left hinge to solve for tension - Tsin(30∘)×3.0m=160N×1.2m+18kgg×1.5m. Then resolve vertical/horizontal forces to obtain hinge reactions.
Worked example 2 - crate held on rough slope
A 25kg crate rests on a 20∘ rough slope. It is held in place by a light rope parallel to the slope. The slope provides a friction force of magnitude 35N on the crate, acting parallel to the slope. Find the rope tension in the two cases where (i) the friction acts up the slope, and (ii) the friction acts down the slope.
Strategy: draw a free-body diagram showing weight, normal reaction, tension and the given friction force. Resolve along the slope and apply ∑F∥=0 for each direction of friction. Note: 9478 treats friction qualitatively, so its magnitude is given in the question rather than computed from a coefficient.
Practical & data tasks
Use a metre rule with hanging masses to verify the principle of moments (∑τ=0); include quantitative uncertainty analysis.
Build a spring system (series and parallel) and record load vs extension to obtain keq from the gradient.
Film a door closing and plot torque vs angle to appreciate how moment arm changes with geometry.
Common misconceptions & exam traps
Forgetting to convert cm to m before computing moments (units inconsistency).
Missing reaction components at hinges or pins (both horizontal and vertical).
Assuming friction direction incorrectly; always consider impending motion cue.
Using resultant force instead of couple torque when two equal/opposite forces act.
Quick self-check quiz
What two equations must hold for a rigid body in static equilibrium? - ∑F=0 and ∑τ=0.
Define a couple. - Two equal, opposite, parallel forces with different lines of action producing pure rotation.
A 40N force acts 0.25m from a pivot. What is the moment? - 10N⋅m (sense depends on direction).
In Hooke's law, what happens to k when two identical springs are placed in parallel? - It doubles.
What is the common mistake when drawing free-body diagrams of ladders? - Omitting the friction at the contact points or misplacing the weight's line of action.
Revision workflow
Solve two ladder/beam questions from past-year papers; verify each line with both equilibrium equations.
Create a “force library” flashcard set (normal, tension, upthrust, drag, weight) with sketches.
Practise translating word problems into vector component equations within 90 seconds.
Summarise spring combinations and couple properties on a one-page cheat sheet for last-minute review.
Practice Quiz
Test yourself on the key concepts from this guide.
Parents: book a 60-min “Forces & Moments” clinic one week before the first mechanics WA.
Students: stick the moment equation (τ=Fd⊥) on your desk and test it on tonight's textbook questions.
Last updated 14 Jul 2025. Next review when SEAB releases the 2027 draft.
