Q.35 Physics · Section A ⭐ 360° Option Elimination Active
NEET 2024 Physics Question 35 Solution
Topic: Laws of Motion · Official NTA Booklet Code T1
Question
kbar, respectively. The solubility ofthese gases
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Verified Against NTA Final Answer Key
Option (1) : 6 N
📚 Laws of Motion Easy
📋 Given Parameters & Problem Setup
Mass of block A m_A = 2 kg
Mass of block B m_B = 3 kg
Applied horizontal force on A F = 10 N
Surface Frictionless horizontal surface
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⏱️ Save ~40 seconds The 30-Second Exam Hall Shortcut: ⚡ Isolated Block B Acceleration Shortcut
System a = F / (m_A + m_B) = 10 / (2 + 3) = 2 m/s²; Force on B is purely m_B × a = 3 × 2 = 6 N!
1 Combine blocks into one system: total mass M = 2 + 3 = 5 kg.
2 Common acceleration a = F / M = 10 / 5 = 2 m/s².
3 Block B accelerates at 2 m/s² solely due to the normal contact force N from block A.
4 By Newton’s second law for block B: N = m_B × a = 3 kg × 2 m/s² = 6 N.
5 Option (1) in under 5 seconds!
🛡️
Why You Can 100% Trust This Shortcut: Newton’s Second and Third Laws of Motion (Internal normal contact force: N_AB = m_B · a_system).
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Step-by-Step Rigorous Workflow & Mathematical Proof
Step 1
Acceleration of the Combined System
Since both blocks slide together on a frictionless surface, they share a common horizontal acceleration a. Treating both blocks as a single system of total mass M = m_A + m_B:
Step 2
Free Body Diagram of Block B
Block B of mass m_B = 3 kg experiences only one horizontal force: the normal push force N exerted by block A on block B acting to the right. Applying Newton’s second law:
Step 3
Calculation of Normal Contact Force
Substitute m_B = 3 kg and a = 2 m/s²:
Step 4
Cross-Verification on Block A
For block A: F - N = m_A · a => 10 - 6 = 2 kg × 2 m/s² = 4 N, which confirms perfect dynamic equilibrium.
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4-Option Deep Scan
360° Option Elimination Engine Key Feature
Diagnose every choice: Know exactly why the right option works and why distractors fail.
1 (1)
✓ 100% Correct Verdict & Scientific Reason:
Exact contact force: N = m_B · a = 3 × 2 = 6 N.
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Why this is airtight: None.
2 (2)
✗ Incorrect Verdict & Scientific Reason:
This is the full external force applied to block A, ignoring inertia of block A.
⚠️
Student Fallacy (Why candidates pick this trap): Assuming the external force transmits 100% undiminished to block B.
3 (3)
✗ Incorrect Verdict & Scientific Reason:
If contact force were zero, block B could not accelerate.
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Student Fallacy (Why candidates pick this trap): Thinking normal reaction exists only on vertical surfaces.
4 (4)
✗ Incorrect Verdict & Scientific Reason:
This is the net force accelerating block A (m_A · a = 2 × 2 = 4 N), NOT the force on block B!
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Student Fallacy (Why candidates pick this trap): Calculating m_A · a instead of m_B · a.
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NTA Trap Alert: Calculating Force on Block A (4 N) Instead of Block B (6 N)
Option (4) offers 4 N, which is m_A · a. Students often mix up which block’s mass multiplies the acceleration.
🛡️ How to avoid the -1 mark: The contact force pushes block B forward, so it must accelerate block B: N = m_B × a = 3 × 2 = 6 N.
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Core NEET Syllabus Standard
Physics Core Mechanics · Laws of Motion Standard Benchmark
Core Scientific Principle Tested:
"In a multi-block system pushed from one side, the contact force between blocks equals the total trailing mass multiplied by the common acceleration."
Syllabus Focus: Newton’s Second Law and Systems of Interacting Bodies
✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗
Push from A means B must ride: Mass of B times a on the other side!
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