PART–4 (iii) : IIT / JEE Previous Year Questions – Set 3
This set focuses on multi-force systems, constraints, and method selection—areas where IIT frequently tests students.
PYQ–11 (IIT–JEE)
Question:
Two blocks of masses m and 2m are connected by a light string and placed on a rough horizontal surface (coefficient of friction μ). A force F pulls the system. Find the acceleration.
Method Selection:
Treat both blocks as a single system. Friction acts on both blocks.
Solution:
Total mass = 3m
Total friction = μmg + 2μmg = 3μmg
Net force = F − 3μmg
Using F = ma:
a = (F − 3μmg) / 3m
Examiner’s Intention:
To test system approach and correct summation of friction forces.
Common Mistake:
Considering friction on only one block.
PYQ–12 (IIT–JEE)
Question:
A block of mass m is attached to a string passing over a smooth pulley. The other end carries a mass 2m. Find the acceleration of the system.
Method Selection:
Use Newton’s Second Law separately for both masses.
Solution:
For mass m (moving up):
T − mg = ma
For mass 2m (moving down):
2mg − T = 2ma
Adding equations:
mg = 3ma
a = g / 3
Examiner’s Intention:
To check correct direction choice and equation setup.
Common Mistake:
Taking same direction for both accelerations.
PYQ–13 (IIT–JEE)
Question:
A particle is moving in a straight line. Its velocity-time graph is a straight line parallel to the time axis. What is the net force acting on the particle?
Method Selection:
Graph-based conceptual question using Newton’s laws.
Solution:
Velocity constant ⇒ acceleration = 0
Net force = m × 0 = 0
Examiner’s Intention:
To test understanding of graphs and acceleration.
Common Mistake:
Assuming force exists because velocity is non-zero.
PYQ–14 (IIT–JEE)
Question:
A block of mass m rests on a rough inclined plane at angle θ. Find the condition for equilibrium.
Method Selection:
Use force balance along and perpendicular to the incline.
Solution:
Along incline: mg sinθ ≤ μmg cosθ
tanθ ≤ μ
Examiner’s Intention:
To test limiting friction and equilibrium conditions.
Common Mistake:
Ignoring inequality condition.
PYQ–15 (IIT–JEE)
Question:
A man standing on a weighing machine inside a lift finds his weight reduced. What can you conclude about the motion of the lift?
Method Selection:
Conceptual question using apparent weight.
Solution:
Reduced apparent weight means:
N = m(g − a)
Therefore, the lift is accelerating downward.
Examiner’s Intention:
To test understanding of apparent weight.
Common Mistake:
Confusing downward acceleration with upward motion.
High-Level Directional Tips
- Always check whether to use system approach or individual bodies.
- For pulleys, directions must be chosen carefully.
- Graph questions always test acceleration, not velocity.
- Equilibrium problems require inequality conditions.
PART–4 (iii) Completed.
PYQs covered so far: 15. We will continue to reach 30+ years coverage.
📚 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.
- Part 1 – Introduction to Topper Thinking
- Part 2 – Understanding the Problem Statement
- Part 3 – Visualization & Physical Sense
- Part 4 – Role of Acceleration & Forces
- Part 5 – Free Body Diagram Thinking
- Part 6 – Method Selection Strategy
- Part 7 – Avoiding Formula Traps
- Part 8 – System Approach Mindset
- Part 9 – Friction & Constraint Thinking
- Part 10 – Time vs Accuracy Balance
- Part 11 – Common Thinking Mistakes
- Part 12 – Decision Flow in Tough Problems
- Part 13 – Eliminating Wrong Options
- Part 14 – Handling Multi-Concept Questions
- Part 15 – Pressure Handling in Exam Hall
- Part 16 – Speed Building Without Guessing
- Part 17 – Extreme Case Thinking
- Part 18 – Logical Shortcuts vs Blind Tricks
- Part 19 – Advanced Reasoning Techniques
- Part 20 – Rank-Deciding Thought Patterns
- Part 21 – Physics Intuition Development
- Part 22 – Eliminating Panic in Tough Questions
- Part 23 – Last-Minute Thinking Strategy
- Part 24 – Mistake-Proof Problem Solving
- 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
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