Key Concepts & Formulas

10 Practice MCQs

Q1. A 1000 kg train coach is moving at 10 m/s. What is its momentum? A) 100 kg·m/s B) 1000 kg·m/s C) 10,000 kg·m/s D) 10 kg·m/s

Answer: C) 10,000 kg·m/s

Solution: Momentum p = mv Given: m = 1000 kg, v = 10 m/s p = 1000 × 10 = 10,000 kg·m/s

Shortcut: Just multiply mass by velocity directly

Concept: Physics Mechanics - Momentum calculation

Q2. A railway porter lifts a 20 kg bag vertically by 2 meters. What work is done? (g=10 m/s²) A) 40 J B) 400 J C) 200 J D) 4 J

Answer: B) 400 J

Solution: Work W = mgh Given: m = 20 kg, h = 2 m, g = 10 m/s² W = 20 × 10 × 2 = 400 J

Shortcut: W = weight × height = (20×10) × 2

Concept: Physics Mechanics - Work against gravity

Q3. A train accelerates from 0 to 20 m/s in 10 seconds. Find acceleration. A) 2 m/s² B) 0.5 m/s² C) 200 m/s² D) 5 m/s²

Answer: A) 2 m/s²

Solution: a = (v-u)/t Given: u = 0, v = 20 m/s, t = 10 s a = (20-0)/10 = 2 m/s²

Shortcut: Acceleration = velocity change ÷ time

Concept: Physics Mechanics - Basic kinematics

Q4. A 500 kg railway cart moving at 5 m/s stops in 25 meters. Find braking force. A) 250 N B) 500 N C) 125 N D) 50 N

Answer: A) 250 N

Solution: Using v² = u² + 2as 0 = 5² + 2a(25) -25 = 50a, so a = -0.5 m/s² F = ma = 500 × 0.5 = 250 N (magnitude)

Shortcut: First find deceleration, then multiply by mass

Concept: Physics Mechanics - Force and deceleration

Q5. A 2000 kg engine exerts 4000 N force. Find its acceleration. A) 2 m/s² B) 0.5 m/s² C) 8 m/s² D) 20 m/s²

Answer: A) 2 m/s²

Solution: F = ma, so a = F/m Given: F = 4000 N, m = 2000 kg a = 4000/2000 = 2 m/s²

Shortcut: Acceleration = Force ÷ Mass

Concept: Physics Mechanics - Newton’s Second Law

Q6. A train’s kinetic energy is 50,000 J at 10 m/s. Find its mass. A) 500 kg B) 1000 kg C) 250 kg D) 200 kg

Answer: B) 1000 kg

Solution: KE = ½mv², so m = 2KE/v² Given: KE = 50,000 J, v = 10 m/s m = 2×50,000/100 = 1000 kg

Shortcut: m = 2KE ÷ v²

Concept: Physics Mechanics - Kinetic energy calculation

Q7. Two 1000 kg railway buffers compress 0.1 m during coupling. Find average force if initial speed is 2 m/s. A) 20,000 N B) 10,000 N C) 40,000 N D) 2000 N

Answer: A) 20,000 N

Solution: KE = ½mv² = 0.5×1000×4 = 2000 J Work done = Force × distance 2000 = F × 0.1, so F = 20,000 N

Shortcut: Convert KE to work, then divide by distance

Concept: Physics Mechanics - Work-energy principle

Q8. A 1000 kg train moves up a 1:100 slope against 2000 N friction. Find total force needed for constant velocity. (g=10 m/s²) A) 3000 N B) 1200 N C) 3000 N D) 12000 N

Answer: C) 3000 N

Solution: Slope force = mg × sinθ ≈ mg × (1/100) = 1000×10×0.01 = 1000 N Total force = slope force + friction = 1000 + 2000 = 3000 N

Shortcut: For small slopes, slope force ≈ weight × gradient

Concept: Physics Mechanics - Forces on inclined planes

Q9. A 2000 kg railway wagon moving at 5 m/s couples with stationary 3000 kg wagon. Find final velocity. A) 2 m/s B) 3 m/s C) 1.5 m/s D) 4 m/s

Answer: A) 2 m/s

Solution: Conservation of momentum: m₁v₁ = (m₁+m₂)v 2000×5 = (2000+3000)v 10,000 = 5000v, so v = 2 m/s

Shortcut: Final velocity = (m₁×v₁) ÷ (total mass)

Concept: Physics Mechanics - Conservation of momentum

Q10. A train engine (5000 kg) pulls 4 coaches (each 2000 kg) with 2 m/s² acceleration. Find tension in coupling between engine and first coach. A) 16,000 N B) 8000 N C) 24,000 N D) 12,000 N

Answer: A) 16,000 N

Solution: Tension pulls 4 coaches: T = m×a Total mass of coaches = 4×2000 = 8000 kg T = 8000 × 2 = 16,000 N

Shortcut: Tension = mass being pulled × acceleration

Concept: Physics Mechanics - Tension in connected systems

5 Previous Year Questions

PYQ 1. A 1000 kg car and 2000 kg truck have same momentum. If car’s speed is 20 m/s, find truck’s speed. [RRB NTPC 2021 CBT-1]

Answer: 10 m/s

Solution: Momentum conservation: m₁v₁ = m₂v₂ 1000×20 = 2000×v₂ v₂ = 20,000/2000 = 10 m/s

Exam Tip: When momentum is equal, heavier object moves slower

PYQ 2. Work done to lift 50 kg person through 3 m height? (g=10 m/s²) [RRB Group D 2022]

Answer: 1500 J

Solution: W = mgh = 50×10×3 = 1500 J

Exam Tip: Remember g=10 m/s² for railway exams

PYQ 3. A train reduces speed from 72 km/h to 36 km/h in 10 seconds. Find retardation. [RRB ALP 2018]

Answer: 1 m/s²

Solution: Convert: 72 km/h = 20 m/s, 36 km/h = 10 m/s a = (v-u)/t = (10-20)/10 = -1 m/s² Retardation = 1 m/s²

Exam Tip: Convert km/h to m/s first: multiply by 5/18

PYQ 4. Power of pump lifting 1000 kg water to 10 m height in 5 seconds? (g=10 m/s²) [RRB JE 2019]

Answer: 20,000 W

Solution: Work = mgh = 1000×10×10 = 100,000 J Power = Work/time = 100,000/5 = 20,000 W

Exam Tip: Power = (weight × height) ÷ time

PYQ 5. A 10 g bullet fired at 400 m/s embeds in 2 kg wooden block. Find block’s velocity. [RPF SI 2019]

Answer: 2 m/s

Solution: m₁v₁ = (m₁+m₂)v 0.01×400 = (0.01+2)×v 4 = 2.01v ≈ 2 m/s

Exam Tip: Convert grams to kg, approximate for small bullet mass

Speed Tricks & Shortcuts

Common Mistakes to Avoid

Quick Revision Flashcards

Topic Connections

• Direct Link: Kinematics connects to Speed, Time & Distance problems; Energy concepts appear in Electricity (power calculations)
• Combined Questions: Mechanics + Trigonometry (inclined planes); Mechanics + Percentage (efficiency calculations)
• Foundation For: Understanding friction helps in Brake systems; Momentum concepts essential for Collision problems in safety systems