Stage 3 – Page 6
Simple Harmonic Motion (SHM) – Non-Standard Systems & Constraints


1. What is a Non-Standard SHM?

Any system that does NOT look like a simple spring–mass but still follows:

Restoring Force ∝ Displacement

Key skill: Convert the system into SHM mathematically


2. Effective Restoring Force Concept

If net force can be written as:

F = −keff x

Then motion is SHM with:

ω = √(keff / m)


3. SHM on Inclined Plane (Spring Attached)

  • Inclination shifts equilibrium position
  • Does NOT affect time period

T = 2π√(m/k)

Gravity changes mean position, not oscillation nature


4. SHM of Block in a Smooth Circular Track

For small angular displacement (θ):

Restoring force ≈ mgθ

Angular SHM:

ω = √(g/R)

Small angle approximation is the key


5. SHM of Floating Object

For a floating body displaced vertically:

  • Restoring force comes from buoyancy change

keff = ρgA

ω = √(ρgA / m)

Frequently asked in JEE Advanced


6. SHM with Pulley Constraints

When pulleys are involved:

  • Displacement relation reduces degrees of freedom
  • Effective mass changes

Always express motion using a single coordinate


7. SHM in Variable Force Fields

If force varies with position:

F(x) ≈ −(d²U/dx²)eq x

Then:

keff = d²U/dx² at equilibrium


8. Small Oscillation Technique (Advanced)

Steps:

  1. Find equilibrium position
  2. Displace slightly
  3. Linearize force or torque
  4. Extract keff

This method converts complex problems into SHM


9. Common IIT Traps

  • Ignoring constraint relations
  • Using wrong equilibrium reference
  • Missing effective mass change

Stage 3 – Page 6 Key Takeaways

✔ SHM is hidden in many systems
✔ Linearization near equilibrium is essential
✔ Effective k decides everything


— Stage 3 | SHM Deep Systems —

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