JEE Advanced – Simple Harmonic Motion (SHM)

Examiner Traps & Wrong Option Analysis – Page 4

Purpose: This page is designed to stop silly mistakes and teach how IIT examiners design wrong options.

Trap 1: Assuming Energy Conservation Everywhere

Many students blindly apply energy conservation.

Wrong Thinking:

“Energy is always conserved in SHM.”

Correct Thinking:

Energy conserved only if no damping / no external work
Sudden stops, breaks, sticking → energy NOT conserved

Exam Tip: First check system integrity before using energy.

Trap 2: Confusing Mean Position with Zero Velocity

A very common IIT trap.

Mean position → velocity maximum
Extreme position → velocity zero

Wrong Option Pattern:

“Velocity is zero at mean position”

Why It Works: Students confuse SHM with circular motion intuition.

Trap 3: Mixing Linear & Angular Quantities

Questions combine angular SHM and linear SHM intentionally.

Wrong Mix:

Using v instead of ωθ
Using m instead of I

Correct Rule:

Linear SHM → m, v, a
Angular SHM → I, ω, α

Trap 4: Assuming Phase = Displacement

Phase difference ≠ displacement difference.

Example Trap:

Phase difference = π/2 ⇒ displacement = A/2 ❌

Correct:

Displacement depends on sine/cosine values.

Trap 5: Time Symmetry Assumption

Students assume equal time for equal distances.

Reality:

Motion near mean → fast
Motion near extreme → slow

Result: Time is not symmetric in SHM.

Trap 6: Ignoring Initial Conditions

SHM equations depend heavily on starting point.

Start from mean → sine form
Start from extreme → cosine form

Wrong Option:

Using x = A sin(ωt) for every case ❌

Trap 7: Power at Mean Position

Many students say power is maximum at mean position.

But:

Force = 0 at mean
Power = F · v = 0

Correct: Maximum power occurs between mean and extreme.

Trap 8: Graph Interpretation Errors

Energy vs x, v vs x, a vs t graphs are frequently misread.

PE vs x → parabola
v vs x → semicircle
a vs x → straight line

Exam Tip: Identify equation → then graph.

How Toppers Use Page 4

Eliminate 2 options instantly
Detect trap words (“always”, “zero”, “constant”)
Slow down only at trap-heavy questions
Gain marks without calculation

JEE Advanced SHM – Page 4 (Trap Mastery) Completed ✅

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