PART–4 (ii) : IIT / JEE Previous Year Questions – Set 2

This set continues topic-wise PYQs from Laws of Motion. Focus is on method selection, Free Body Diagrams, and examiner traps.

---

PYQ–6 (IIT–JEE)

Question:

A block of mass m is placed on a rough inclined plane of angle θ. Coefficient of friction is μ. Find the acceleration of the block when it slides down.

Method Selection:

Use Free Body Diagram and apply Newton’s Second Law along the incline.

Solution:

Forces along incline:
Component of weight = mg sinθ (down the plane)
Friction = μmg cosθ (up the plane)

Net force along plane = mg sinθ − μmg cosθ

Using F = ma,

a = g (sinθ − μ cosθ)

Examiner’s Intention:

To check correct resolution of forces and friction direction.

Common Mistake:

Taking friction in the wrong direction.

---

PYQ–7 (IIT–JEE)

Question:

Two blocks of masses m and m are placed one over the other. The lower block is pulled with a force F on a smooth surface. Find the maximum value of F so that the upper block does not slip.

Method Selection:

Treat blocks separately. Use friction as the only accelerating force on the upper block.

Solution:

Let acceleration of system = a

For upper block:
Friction = ma

Maximum friction = μmg

So, ma ≤ μmg ⇒ a ≤ μg

For both blocks together:

F = 2ma

F_max = 2mμg

Examiner’s Intention:

To test understanding of relative motion and friction limits.

Common Mistake:

Assuming both blocks always move together without checking friction condition.

---

PYQ–8 (IIT–JEE)

Question:

A body of mass m is suspended by a string in a lift. Find the tension in the string when the lift is moving downward with acceleration a.

Method Selection:

Use Free Body Diagram and take downward direction as positive.

Solution:

Forces: Weight mg downward, tension T upward.

Applying Newton’s second law downward:

mg − T = ma

T = m(g − a)

Examiner’s Intention:

To test non-inertial frame concepts and sign convention.

Common Mistake:

Writing T = mg without considering acceleration.

---

PYQ–9 (IIT–JEE)

Question:

A block of mass m is pushed against a vertical wall with force F. Coefficient of friction between block and wall is μ. Find the minimum force required to prevent the block from sliding down.

Method Selection:

Use equilibrium condition with friction acting upward.

Solution:

Normal reaction N = F

Maximum friction = μF

For equilibrium:

μF ≥ mg

F_min = mg / μ

Examiner’s Intention:

To test understanding of friction without horizontal motion.

Common Mistake:

Taking normal reaction as mg.

---

PYQ–10 (IIT–JEE)

Question:

A body is moving in a straight line with constant velocity. What can be said about the net force acting on it?

Method Selection:

Pure conceptual question based on Newton’s First and Second Laws.

Solution:

Constant velocity means acceleration is zero.

Therefore, net force acting on the body is zero.

Examiner’s Intention:

To test the equivalence of rest and uniform motion.

Common Mistake:

Assuming force is present because the body is moving.

---

Directional Solving Tips (Must Read)

• Always decide direction of acceleration first.
• Friction always opposes relative motion.
• Check limiting conditions before final answers.
• Velocity zero does NOT mean force zero.

---

PART–4 (ii) Completed.
10 PYQs covered so far. We will continue until 30+ years coverage is achieved.

📚 Physics Thinking Library – How IIT Toppers Think

Welcome to the Physics Thinking Library by Mind Grow Magazine. This library is specially designed for IIT–JEE & Intermediate students to understand how toppers think, decide, and solve Physics problems.

---

🎯 What You Will Learn From This Library

• How IIT toppers approach Physics questions
• Decision-making during problem solving
• Common traps & how to avoid them
• Exam-hall mindset and logic flow
• Step-by-step thinking framework

---

📘 Complete Series: How IIT Toppers Think (Part 1–25)

Click on any part below to start learning. It is recommended to read in order.

1. Part 1 – Introduction to Topper Thinking
2. Part 2 – Understanding the Problem Statement
3. Part 3 – Visualization & Physical Sense
4. Part 4 – Role of Acceleration & Forces
5. Part 5 – Free Body Diagram Thinking
6. Part 6 – Method Selection Strategy
7. Part 7 – Avoiding Formula Traps
8. Part 8 – System Approach Mindset
9. Part 9 – Friction & Constraint Thinking
10. Part 10 – Time vs Accuracy Balance
11. Part 11 – Common Thinking Mistakes
12. Part 12 – Decision Flow in Tough Problems
13. Part 13 – Eliminating Wrong Options
14. Part 14 – Handling Multi-Concept Questions
15. Part 15 – Pressure Handling in Exam Hall
16. Part 16 – Speed Building Without Guessing
17. Part 17 – Extreme Case Thinking
18. Part 18 – Logical Shortcuts vs Blind Tricks
19. Part 19 – Advanced Reasoning Techniques
20. Part 20 – Rank-Deciding Thought Patterns
21. Part 21 – Physics Intuition Development
22. Part 22 – Eliminating Panic in Tough Questions
23. Part 23 – Last-Minute Thinking Strategy
24. Part 24 – Mistake-Proof Problem Solving
25. Part 25 – Final Topper Framework

---

🧠 How to Use This Library Effectively

• Read 1–2 parts daily
• Apply thinking to real Physics problems
• Revisit library before exams
• Use as a mindset guide, not memorization

---

This library is a long-term asset for serious Physics learners.
© Mind Grow Magazine