Q.5 Physics ⭐ 360° Option Elimination Active

NEET 2025 Physics Question 5 Solution

Topic: Work, Energy and Power · Official NTA Booklet Code 45

Question

The kinetic energies of two similar cars A and B are 100 J and 225 J respectively. On applying breaks, car A stops after 1000 m and car B stops after 1500 m. If F_A and F_B are the forces applied by the breaks on cars A and B, respectively, then the ratio F_A/F_B is

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Verified Against NTA Final Answer Key

Option (2) : \frac{2}{3}

📚 Work, Energy and Power Easy

📋 Given Parameters & Problem Setup

Kinetic Energies KE_A = 100 J, KE_B = 225 J

Stopping Distances s_A = 1000 m, s_B = 1500 m

Target Ratio F_A / F_B (Ratio of braking forces)

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The 30-Second Exam Hall Shortcut: ⚡ Work-Energy Theorem Direct Ratio

⏱️ Save ~40 seconds

Work W = F · s = KE ⟹ F = KE / s ⟹ F_A / F_B = (100 / 1000) / (225 / 1500) = (1/10) / (3/20) = 2/3!

1 By Work-Energy Theorem: Work done by braking force = Initial kinetic energy dissipated (F · s = KE).

2 Braking force is F = KE / s.

3 Calculate F_A = 100 J / 1000 m = 1/10 N = 0.1 N.

4 Calculate F_B = 225 J / 1500 m = 15/100 N = 3/20 N = 0.15 N.

5 Ratio F_A / F_B = 0.1 / 0.15 = 2/3.

6 Pick Option (2) in under 15 seconds!

🛡️

Why You Can 100% Trust This Shortcut: Work-Energy Theorem (NCERT Physics · Work, Energy and Power: W_net = ΔK).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Apply Work-Energy Theorem

When a car of initial kinetic energy KE is brought to rest by a constant braking force F over distance s, the net work done by friction/brakes equals the loss in kinetic energy.

W_{net} = \Delta KE \implies F \cdot s = KE \implies F = \frac{KE}{s}

• Forces are assumed constant over the stopping distances.

Step 2

Compute the Ratio F_A / F_B

Set up the ratio of the braking forces for cars A and B:

\frac{F_A}{F_B} = \frac{KE_A / s_A}{KE_B / s_B} = \frac{100 / 1000}{225 / 1500} = \frac{0.10}{0.15} = \frac{2}{3}

• 225 / 1500 simplifies by dividing numerator and denominator by 75: 3 / 20 = 0.15.
• Ratio is exactly 2/3, corresponding to Option (2).

⭐

360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 A

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Inverted ratio F_B / F_A instead of F_A / F_B.

⚠️

Student Fallacy (Why candidates pick this trap): Calculating the reciprocal ratio.

2 B

✓ 100% Correct

Verdict & Scientific Reason:

100% correct. (100/1000) / (225/1500) = (1/10) / (3/20) = 2/3.

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Why this is airtight: None; straightforward algebra.

3 C

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Arithmetic miscalculation.

⚠️

Student Fallacy (Why candidates pick this trap): Dropping factor of 2 during fraction division.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Neglecting distance ratio.

⚠️

Student Fallacy (Why candidates pick this trap): Assuming distances were 1000 m and 2000 m.

⚠️

NTA Trap Alert: Inverted Ratio Trap

Option (1) is 3/2, which is the exact inverse (F_B / F_A). Always double-check whether the question asks for F_A / F_B or F_B / F_A.

🛡️ How to avoid the -1 mark: Write "A on top, B on bottom" before calculating.

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Core NEET Syllabus Standard

NEET Physics Core Syllabus · Work, Energy and Power Standard Benchmark

Core Scientific Principle Tested:

"Stopping force is proportional to kinetic energy divided by stopping distance: F = KE / s."

Syllabus Focus: Work-Energy Theorem

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Force = Energy / Distance: (100/1000) / (225/1500) = 2/3!

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NEET 2025 Physics Question 5 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Work-Energy Theorem Direct Ratio

Step-by-Step Rigorous Workflow & Mathematical Proof

Apply Work-Energy Theorem

Compute the Ratio F_A / F_B

360° Option Elimination Engine Key Feature

NTA Trap Alert: Inverted Ratio Trap

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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