NEB Class 12 • Mathematics • Mechanics Option • Concept Guide

Force Triangle & Lami’s Theorem: NEB Class 12 Maths Guide

Master Force Triangle and Lami’s Theorem through clear concepts, representations, a reliable answer method and exam-focused practice.

ConceptsWorked reasoningNEB practice

Core concept

What Force Triangle and Lami’s Theorem means physically

Three concurrent coplanar forces in equilibrium form a closed force triangle. Lami’s theorem relates each force to the sine of the angle between the other two forces.

For equilibrium F₁/sinα=F₂/sinβ=F₃/sinγ, where α is opposite F₁ and is the angle between F₂ and F₃.

The institutional reference gives the wider physical model; the CDC curriculum determines the NEB boundary.

Worked checkpoint with interpretation

If three equal forces balance, the angles between their directions are each 120°, so all sine ratios match.

Do not stop at the number. State what the sign, ratio, spacing, frequency or direction means. A common error is Using angles adjacent to rather than opposite forces, applying Lami to more than three forces without reduction, or ignoring concurrency and equilibrium.

NEB answer method

Model → evidence → equation → check

First identify the system and what the question asks. Label the diagram, apparatus or reaction with SI quantities, then state the principle before selecting an equation. Solve symbolically where possible and check units, limiting cases and chemical or physical meaning.

Stage	Question	Evidence
Model	What is idealised?	Assumption stated
Representation	Which diagram fits?	Labels or balanced equation
Calculation	Which relation follows?	Symbols then SI values
Evaluation	Does it make sense?	Unit, sign and trend check

Make the idea concrete in Nepal

Connect the model to a school laboratory, household technology, health, environment, energy or local material. The example should clarify the science rather than merely mention a place. Compare a prediction with an observation and record uncertainty.

Review the NEB Class 12 Mathematics revision roadmap and the complete Mathematics guide. For online or physical tuition, call 9846662070.

Frequently asked questions

Questions students ask about Force Triangle and Lami’s Theorem

What is the central idea of Force Triangle and Lami’s Theorem?+

Three concurrent coplanar forces in equilibrium form a closed force triangle. Lami’s theorem relates each force to the sine of the angle between the other two forces.

Which equation should I remember?+

For equilibrium F₁/sinα=F₂/sinβ=F₃/sinγ, where α is opposite F₁ and is the angle between F₂ and F₃.

How should I begin a numerical?+

Draw the situation, list known quantities in SI units, choose the governing relation symbolically, substitute once and finish with a unit and reasonableness check.

What is a common exam mistake?+

Using angles adjacent to rather than opposite forces, applying Lami to more than three forces without reduction, or ignoring concurrency and equilibrium.

How can I practise this topic?+

Alternate one concept explanation, one diagram and two numericals. Revisit errors after a day instead of only rereading the solution.

Where can I get tuition support?+

For online or physical KTM Tuition classes, call 9846662070. After Grade 12, the MKS Education panel below gives preparation and pre-counselling contacts.

References and next steps

Scope checked against CDC Nepal; concept support from an institutional reference and OpenStax Science. Continue with the Force Triangle and Lami’s Theorem cluster.

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