Lesson–4 : Laws of Motion

STAGE–4 (Part–V) : Friction – Static, Kinetic & IIT-Level Thinking

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🔹 1. IIT VIEW OF FRICTION (VERY DIFFERENT)

In school level, friction is treated as a simple opposing force. In IIT, friction is a self-adjusting force.

IIT Truth:
Static friction takes only the value needed, up to its maximum.

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🔹 2. STATIC FRICTION (MOST IMPORTANT)

Static friction acts when there is no relative motion between surfaces.

Magnitude:

0 ≤ f_s ≤ μ_sN

This means:

• Static friction is NOT always μ_sN
• It adjusts according to applied force

This single concept removes 50% IIT mistakes.

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🔹 3. LIMITING FRICTION

When applied force just tends to cause motion:

f_max = μ_sN

This condition defines the threshold of motion.

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🔹 4. KINETIC FRICTION

When surfaces are in relative motion:

f_k = μ_kN

• μ_k < μ_s
• Direction always opposes motion

Once motion starts, static friction disappears.

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🔹 5. DIRECTION OF FRICTION (IIT TRAP)

Friction does NOT always oppose applied force.

It opposes relative motion or tendency of motion.

Golden Rule:
First assume direction of motion, then assign friction opposite to it.

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🔹 6. FRICTION ON INCLINED PLANE (CORE IIT)

For block on incline:

• Downward force = mg sinθ
• Normal = mg cosθ
• Friction = μN

Acceleration down the plane:

a = g(sinθ − μcosθ)

Condition for rest:

tanθ ≤ μ_s

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🔹 7. FRICTION + MULTI-BLOCK SYSTEM

In IIT problems:

• Each block may have different μ
• Friction direction may differ for blocks

Never assume same friction direction for all bodies.

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🔹 8. COMMON IIT TRAPS (MUST AVOID)

• Using μN blindly for static friction
• Wrong friction direction
• Forgetting normal changes in inclined plane
• Ignoring system approach

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🔹 9. CONCEPTUAL IIT QUESTIONS

Q: If applied force is small, does friction act?

Ans: Yes. Static friction balances applied force.

Q: Can friction accelerate a body?

Ans: Yes. Example: walking.

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🔹 10. FINAL MENTAL CHECK (FRICTION)

• Is friction static or kinetic?
• Is value fixed or adjustable?
• Direction assumed logically?
• Did I check threshold condition?

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✅ STAGE–4 (Part–V) Completed
FRICTION MASTERED – IIT LEVEL 🚀