Lami's Theorem – Complete Notes, Revision, Important Questions & Downloads
Lami's Theorem provides a direct shortcut for three-force equilibrium problems: when three concurrent coplanar forces are in equilibrium, each force divided by the sine of the angle between the other two equals the same constant — F₁/sinα = F₂/sinβ = F₃/sinγ. NEET tests this as direct numerical problems (find the unknown force or angle given two forces and one angle), as statement-type questions ('State Lami's Theorem'), and as comparison questions contrasting Lami's Theorem with the component-resolution method. Problems with three concurrent forces that include a vertical weight supported by two strings at angles are the canonical NEET scenario for Lami's Theorem.
NEET Weightage — Lami's Theorem
Newton's Laws of Motion (Chapter 4)| NEET Year | Questions from this Topic | Bar | Marks |
|---|---|---|---|
| 2024 | 1 | 4 | |
| 2023 | 0 | 0 | |
| 2022 | 1 | 4 | |
| 2021 | 1 | 4 | |
| 2020 | 0 | 0 | |
| 2019 | 0 | 0 | |
| 6-Year Total (2019–2024) | 1–3 | 4–12 |
Angle sum rule: α + β + γ = 360°. This is the consistency check. If the angles given do not sum to 360°, there is an error in the problem setup or angle identification.
Derivation basis: Lami's Theorem is the sine rule applied to the force triangle formed when three forces in equilibrium are placed head-to-tail. The sine rule: a/sinA = b/sinB = c/sinC for a triangle with sides a,b,c and opposite angles A,B,C.
How to Prepare Lami's Theorem for NEET
Master the angle labelling convention The angle for each force is NOT the angle that force makes with the horizontal — it is the angle between THE OTHER TWO forces. Draw three force arrows at the equilibrium point. The angle α (for F₁) is measured between F₂ and F₃, going around from F₂ to F₃ through the region not containing F₁. Confirm: α + β + γ = 360°. Getting this right is 80% of Lami's Theorem problems.
Memorise the canonical setup The most common NEET scenario: a weight W hangs from two strings making angles with the vertical. At the junction: three forces — W (downward), T₁ (along string 1), T₂ (along string 2). Identify the three angles, apply Lami's Theorem, solve. This specific setup appears in 70% of NEET Lami's problems.
Know the equivalence: component method vs Lami's Both methods give the same answer. Use Lami's Theorem when all three forces and two angles are given (find the third force — one equation). Use the component method when you have more than 3 forces or when the geometry makes components easier. For NEET, Lami's is faster for exactly-three-force problems.
Study Materials — Lami's Theorem
PDF · Cheat Sheet · MCQ Set · PYQSubtopics in Lami's Theorem
2-Column TableRapid Revision — Lami's Theorem
Concept → Trap → Example1) Theorem Statement and Angle Convention
CoreLami's Theorem: When three concurrent coplanar forces F₁, F₂, F₃ are in equilibrium, F₁/sinα = F₂/sinβ = F₃/sinγ, where α = angle between F₂ and F₃, β = angle between F₃ and F₁, γ = angle between F₁ and F₂. These angles are measured going around the point, and α + β + γ = 360°.
- Critical angle rule: α is the angle BETWEEN F₂ and F₃, NOT the angle F₁ makes with any axis. The force F₁ is divided by the sine of the angle of the gap 'opposite' F₁.
- Validity: Lami's Theorem applies ONLY to exactly three concurrent coplanar forces in equilibrium. For 4+ forces or non-coplanar forces, use the component method.
- If one of the angles equals 180° (two of the forces are collinear and opposing), the sine of 180° = 0, making F/sin(180°) undefined — meaning the third force must also be zero. This is the degenerate case.
2) Derivation: Sine Rule on the Force Triangle
DerivationThree forces in equilibrium form a closed triangle (triangle of forces). Let the force triangle have sides F₁, F₂, F₃. The angles of the triangle are (180°−α), (180°−β), (180°−γ). By the sine rule: F₁/sin(180°−α) = F₂/sin(180°−β) = F₃/sin(180°−γ). Since sin(180°−θ) = sinθ, this simplifies directly to F₁/sinα = F₂/sinβ = F₃/sinγ — Lami's Theorem.
- The force triangle angles are the SUPPLEMENTS of the Lami angles. Triangle's internal angles sum = 180°: (180°−α)+(180°−β)+(180°−γ) = 180° → α+β+γ = 360° ✓.
- NEET may ask: 'Lami's Theorem is derived from ___.' Answer: the law of sines / sine rule applied to the triangle of forces.
- Memory aid: Lami → Lamina → the theorem works in any plane (coplanar). The name comes from Bernard Lamy (French mathematician, 1679).
3) NEET Canonical Problem: Weight Hung by Two Strings
NEET-KeyA body of weight W hangs from two strings making angles θ₁ and θ₂ with the vertical (or given as angles with the ceiling). Three forces at the junction: W downward (270° from positive x), T₁ along string 1, T₂ along string 2. Apply Lami's Theorem to find T₁ and T₂.
- Standard setup: T₁ at angle (90°+θ₁) from downward direction, T₂ at angle (90°+θ₂). Lami's: W/sin(angle between T₁ and T₂) = T₁/sin(angle between W and T₂) = T₂/sin(angle between W and T₁).
- For equal angles (θ₁ = θ₂ = θ): T₁ = T₂ = W/(2cosθ). As θ → 90° (strings become horizontal), T → ∞. As θ → 0° (strings vertical), T = W/2 each. This increasing-tension-with-angle fact is a NEET trap.
- Sanity check after solving: T₁sinθ₁ + T₂sinθ₂ = W (vertical equilibrium) should hold.
US Curriculum Gaps — Lami's Theorem
Topics in this section are tested in NEET but organised differently in standard US physics courses.Lami's Theorem (AP Physics 1 Gap)
AP Physics 1 does not include Lami's Theorem by name. Three-force static equilibrium is fully in scope, but AP students use only the component resolution method (ΣFx = 0, ΣFy = 0). NEET tests Lami's Theorem as a named theorem with direct recall ('State Lami's Theorem') and numerical application. The formula F₁/sinα = F₂/sinβ = F₃/sinγ with the correct angle identification is a NEET-specific skill that US-curriculum students must learn separately.
- AP Physics 1: solves three-force equilibrium using component resolution only
- NEET: Lami's Theorem is a named theorem requiring statement, derivation (from sine rule), and numerical application
- Common NEET question type: 'A body is in equilibrium under three forces. Using Lami's Theorem, find the unknown force given two forces and their included angles.'
Named Theorems in Statics (AP Physics C Gap)
AP Physics C: Mechanics covers statics with full vector and calculus methods but does not name theorems like 'Lami's Theorem' or 'Triangle of Forces'. The named-theorem approach (memorise formula + conditions + derivation) is specific to NCERT/NEET preparation. AP Physics C students are comfortable with the underlying mathematics but would not recognise 'Lami's Theorem' as a test item. NEET specifically tests the ability to recall and state named theorems.
- AP Physics C: vector equilibrium solved analytically, no named shortcut theorems
- NEET: 'State and apply Lami's Theorem to three concurrent forces in equilibrium' — named theorem recall required
- Derivation route: sine rule → force triangle → Lami's formula (must be known for NEET)
NEET-Style Practice Questions — Lami's Theorem
4 QuestionsPractice Problems — Lami's Theorem
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FAQ — Lami's Theorem
Notes · Downloads · Revision · Important QuestionsWhat is Lami's Theorem?
What is the condition for applying Lami's Theorem?
How do I identify the correct angle in Lami's Theorem?
How is Lami's Theorem derived?
What is the difference between Lami's Theorem and the component method?
Can Lami's Theorem be used in 3D (non-coplanar forces)?
What happens when one angle in Lami's Theorem is 180°?
Why can't a single horizontal cable support a vertical load without sagging?
Assertion-Reason: Lami's Theorem applies only to three forces. Is the assertion correct and what is the reason?
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