Physics: Notes और MCQ Practice

आसान भाषा में:

Physics SSC CGL में deeply theoretical नहीं, बल्कि mostly daily life applications और basic concepts पर आधारित होती है। Mechanics, Optics, Sound, और Electricity से direct standard questions पूछे जाते हैं। Formulas के basic relations और SI Units को पकड़कर चलो, 2-3 marks पक्के हो जाएंगे!

Most Important Formulas & Rules:

• Work & Energy: Work = Force × Displacement × cos(θ) | Kinetic Energy = ½ × m × v² | Potential Energy = m × g × h
• Power: Power = Work ÷ Time (Unit: Watt; 1 Horsepower = 746 Watts)
• Ohm's Law: V = I × R | Resistance formula: R = ρ × L ÷ A (Wire लंबा होगा तो Resistance बढ़ेगा, मोटा होगा तो घटेगा)
• Mirrors:
• Concave Mirror: Converging (Headlights, Shaving mirror, Dentist tool)
• Convex Mirror: Diverging, always Virtual & Diminished image (Rear-view mirror)
• Speed of Sound: Solids > Liquids > Gases (Sound Vacuum में travel नहीं कर सकती; 0 m/s)

2 Quick Exam Tricks:

1. Eye Defects Trick: Myopia (छोटा शब्द) = Concave Lens (Near-sightedness); Hypermetropia (बड़ा शब्द) = Convex Lens (Far-sightedness).
2. Primary Colors Trick: याद रखो RGB (Red, Green, Blue). बाकी सब Secondary Colors हैं।

2 Common Mistakes:

1. Mass vs Weight: Mass हमेशा constant रहता है, चाहे Moon पर जाओ या Jupiter पर। Weight (W = m × g) gravity के बदलने से बदलता है (Moon पर Weight = Earth Weight ÷ 6).
2. Sound vs Light in Medium: Light की speed Denser medium (जैसे Glass) में घटती है, लेकिन Sound की speed Denser medium (Solids) में सबसे fast होती है!

Physics: Core Notes for SSC CGL

Physics in the SSC CGL General Awareness section focuses on practical applications, fundamental laws, SI units, and basic numerical relationships. You do not need deep mathematical derivations; instead, focus on conceptual clarity, real-world examples, and foundational formulas.

1. Concept: Key Fundamental Areas

A. Mechanics (Motion, Force, and Energy)

• Scalar vs. Vector Quantities:
• Scalar has magnitude only (e.g., Mass, Distance, Speed, Work, Energy, Time, Pressure).
• Vector has both magnitude and direction (e.g., Displacement, Velocity, Acceleration, Force, Momentum).
• Newton's Laws of Motion:
• First Law (Law of Inertia): An object remains at rest or in uniform motion unless acted upon by an external unbalanced force. Example: Passengers jerk backward when a bus starts suddenly.
• Second Law: Rate of change of momentum is proportional to the applied force (Force = Mass × Acceleration). Example: A cricket player pulls his hands back while catching a ball to increase time and reduce impact force.
• Third Law: Every action has an equal and opposite reaction. Example: Recoil of a gun, propulsion of a rocket.

B. Gravitation and Pressure

• Gravity (g): Acceleration due to gravity on Earth is approximately 9.8 m/s².
• Maximum at poles, minimum at the equator, zero at the center of the Earth.
• Moon's gravity is 1/6th of Earth's gravity.
• Mass vs. Weight:
• Mass is constant everywhere (measured in kg).
• Weight is a force (Weight = Mass × g) and varies with location (measured in Newtons).
• Atmospheric Pressure: Decreases with increasing altitude. Measured using a barometer (invented by Torricelli). Sudden fall in barometer reading indicates a storm.

C. Optics (Light)

• Reflection: Angle of incidence = Angle of reflection.
• Concave Mirror: Converging mirror. Used in shaving mirrors, dentist head mirrors, car headlights.
• Convex Mirror: Diverging mirror, forms virtual, erect, and diminished images. Used as rear-view mirrors in vehicles because it provides a wider field of view.
• Refraction: Bending of light when passing from one medium to another.
• Twinkling of stars and apparent bending of a stick in water are due to refraction.
• Total Internal Reflection (TIR): Occurs when light travels from a denser to a rarer medium and the angle of incidence is greater than the critical angle.
• Applications: Optical fibers, mirages in deserts, sparkling of diamonds.
• Human Eye Defects:
• Myopia (Short-sightedness): Can see near objects clearly; corrected using a Concave lens.
• Hypermetropia (Far-sightedness): Can see distant objects clearly; corrected using a Convex lens.

D. Heat, Sound, and Electricity

• Sound Waves: Longitudinal mechanical waves; cannot travel through a vacuum. Speed is highest in solids, lower in liquids, lowest in gases.
• Electricity: Electric current is the rate of flow of charge (Current = Charge ÷ Time). Resistance opposes current flow.

2. Important Formulas, Rules, and Facts

Key SI Units

• Force: Newton (N) or kg·m/s²
• Work and Energy: Joule (J)
• Power: Watt (W) or J/s (1 Horsepower = 746 W)
• Pressure: Pascal (Pa) or N/m²
• Frequency: Hertz (Hz)
• Electric Current: Ampere (A)
• Electric Resistance: Ohm (Ω)
• Power of a Lens: Dioptre (D)

Important Formulas

• Speed and Velocity: Speed = Distance ÷ Time; Velocity = Displacement ÷ Time
• Acceleration: a = (v − u) ÷ t (where v = final velocity, u = initial velocity)
• Force: Force = Mass × Acceleration (F = m × a)
• Linear Momentum: p = Mass × Velocity (p = m × v)
• Work Done: Work = Force × Displacement (W = F × s)
• Kinetic Energy (KE): KE = (1 ÷ 2) × Mass × (Velocity)²
• Potential Energy (PE): PE = Mass × g × Height (PE = m × g × h)
• Power: Power = Work Done ÷ Time
• Ohm's Law: Voltage = Current × Resistance (V = I × R)
• Power of a Lens: Power (in Dioptres) = 1 ÷ Focal Length (in meters)

3. Solved Examples (Exam Style)

Example 1: Kinetic Energy and Velocity

Question: If the velocity of a moving object is tripled while its mass remains constant, by how many times will its kinetic energy increase?

Step-by-step Solution:

1. Formula: KE₁ = (1 ÷ 2) × m × v²
2. New velocity v₂ = 3 × v
3. New Kinetic Energy KE₂ = (1 ÷ 2) × m × (3 × v)²
4. KE₂ = (1 ÷ 2) × m × (9 × v²) = 9 × [(1 ÷ 2) × m × v²] = 9 × KE₁

Answer: The kinetic energy increases by 9 times.

Example 2: Weight Calculation

Question: An object has a mass of 60 kg on Earth. What will be its mass and weight on the surface of the Moon? (Take g on Earth = 10 m/s²)

Step-by-step Solution:

1. Mass is an intrinsic property and remains constant everywhere.

• Mass on Moon = 60 kg.

1. Weight on Earth = Mass × g = 60 × 10 = 600 N.
2. Moon's gravity is 1/6th of Earth's gravity:

• Weight on Moon = (Weight on Earth) ÷ 6 = 600 ÷ 6 = 100 N.

Answer: Mass = 60 kg, Weight = 100 N.

Example 3: Work Done

Question: A porter lifts a luggage of 15 kg from the ground and puts it on his head 1.5 m above the ground. Calculate the work done by him on the luggage. (Take g = 10 m/s²)

Step-by-step Solution:

1. Work done against gravity equals the gravitational potential energy gained: Work = m × g × h
2. Given: m = 15 kg, g = 10 m/s², h = 1.5 m
3. Work = 15 × 10 × 1.5 = 225 J

Answer: 225 Joules.

Example 4: Lens Power

Question: What is the power of a convex lens having a focal length of 50 cm?

Step-by-step Solution:

1. Convert focal length to meters: f = 50 cm = 0.5 m
2. For a convex lens, focal length is positive (+0.5 m).
3. Formula: Power (P) = 1 ÷ f
4. P = 1 ÷ 0.5 = +2 D

Answer: +2 Dioptres (+2 D).

4. Shortcuts and Memory Tricks

• Eye Defects Lens Rule (Word-Length Method):
• Short word pairs with short word: Myopia (6 letters) → Concave (7 letters)
• Long word pairs with long word: Hypermetropia (13 letters) → Convex (6 letters is shorter, but recall: Hyper = extra/plus = Convex brings rays together).
• Alternatively: "M-Cave, H-Vex" (Myopia uses Cave/Concave).
• Mirror Uses Shortcut:
• VEX = Driver's View: Convex is used as a rear-view mirror.
• CAVE = Close-up / Shaving: Concave magnifies when object is close.
• Sound Speed Order:
• S > L > G (Solid > Liquid > Gas). Sound is fastest in metals (like steel) and zero in vacuum.
• Barometer Trend:
• Sudden drop = Storm
• Slow drop = Rain
• Slow rise = Clear / Fair weather

5. Common Mistakes to Avoid

• Confusing Mass with Weight: Mass is measured in kg and never changes with location. Weight is a force, measured in Newtons, and changes when gravity changes.
• Focal Length Units in Lens Power: Students often plug in focal length in centimeters directly into P = 1 ÷ f. Always convert cm into meters first (P = 100 ÷ f in cm).
• Sign of Lens Power: Concave lenses always have negative power (−D); convex lenses always have positive power (+D).
• Light vs. Sound Waves: Light is an electromagnetic, transverse wave that does not need a medium (can travel in vacuum). Sound is a mechanical, longitudinal wave that strictly requires a material medium to propagate.