Physics — Study Notes for SSC GD

Overview

Physics questions in SSC GD test your grasp of everyday phenomena explained through basic scientific principles. The exam expects you to recognise fundamental laws in mechanics (motion, force, energy), electricity (circuits, resistance, power), optics (light, mirrors, lenses) and general concepts like heat, sound and pressure. Questions are typically one-liners asking for definitions, units, formulas, laws or direct applications — no advanced calculus or derivations.

Most questions reward common sense paired with memorised facts: knowing that convex lenses converge light, that resistance increases with length of a wire, or that kinetic energy depends on mass and velocity. You will encounter 3–5 physics questions in the GK section. A solid grip on units (SI system), famous scientists and their contributions, and the physical meaning behind formulas will secure these marks. Avoid overthinking — the exam tests recall and simple reasoning, not problem-solving depth.

Prioritise the high-yield areas: Newton's laws, types of energy, Ohm's law, mirror and lens formulas, and units of physical quantities. Many questions repeat across years in slightly reworded forms.

Key Concepts

• **Motion and Force**: Displacement, velocity and acceleration describe how objects move. Newton's first law states inertia (objects resist change in motion). Newton's second law links force, mass and acceleration: F = ma. Newton's third law says every action has an equal and opposite reaction.
• **Energy and Work**: Work is force times displacement in the direction of force. Kinetic energy is energy of motion (½mv²). Potential energy is stored energy due to position or state. Energy cannot be created or destroyed, only transformed (conservation of energy).
• **Pressure and Buoyancy**: Pressure is force per unit area. Atmospheric pressure at sea level is about 1.01 × 10⁵ Pa. Archimedes' principle explains buoyancy: upthrust equals the weight of fluid displaced. Density is mass per unit volume.
• **Electricity Basics**: Current (I) is the flow of charge, measured in amperes. Voltage (V) is electrical potential difference, measured in volts. Resistance (R) opposes current flow, measured in ohms. Ohm's law: V = IR.
• **Circuits and Power**: In series circuits, current is the same everywhere; resistances add. In parallel circuits, voltage is the same across branches; reciprocal resistances add. Electrical power P = VI = I²R = V²/R, measured in watts.
• **Optics — Reflection**: Law of reflection: angle of incidence equals angle of reflection. Plane mirrors form virtual, upright, same-size images. Concave mirrors converge light and can form real or virtual images. Convex mirrors diverge light and always form smaller virtual images.
• **Optics — Refraction**: Light bends when entering a different medium. Convex lenses converge light; used in magnifying glasses and to correct farsightedness. Concave lenses diverge light; used to correct nearsightedness. Lens formula: 1/f = 1/v – 1/u.
• **Heat and Temperature**: Temperature measures hotness; thermometers use expansion of liquids or gases. Heat flows from hot to cold. Conduction, convection and radiation are modes of heat transfer. Specific heat capacity is heat needed to raise 1 kg by 1°C.

Formulas / Key Facts

• **Newton's Second Law**: F = ma (force in newtons, mass in kg, acceleration in m/s²)
• **Kinetic Energy**: KE = ½mv² (m in kg, v in m/s; energy in joules)
• **Potential Energy (gravitational)**: PE = mgh (m in kg, g ≈ 10 m/s², h in metres)
• **Work**: W = F × d (force in newtons, displacement in metres; work in joules)
• **Pressure**: P = F/A (force in newtons, area in m²; pressure in pascals)
• **Ohm's Law**: V = IR (voltage in volts, current in amperes, resistance in ohms)
• **Electric Power**: P = VI = I²R = V²/R (power in watts)
• **Mirror Formula**: 1/f = 1/v + 1/u (f = focal length, v = image distance, u = object distance; distances measured from mirror)
• **Lens Formula**: 1/f = 1/v – 1/u (same sign convention as mirrors)
• **Speed of Light in Vacuum**: c ≈ 3 × 10⁸ m/s
• **Acceleration due to Gravity**: g ≈ 9.8 m/s² (often approximated as 10 m/s² in calculations)
• **1 kilowatt-hour (kWh)**: Commercial unit of energy = 3.6 × 10⁶ joules
• **Resistance of a Wire**: R = ρL/A (ρ = resistivity, L = length, A = cross-sectional area)

Worked Examples

**Example 1 — Force and Acceleration** *Question*: A 5 kg object accelerates at 2 m/s². What force acts on it? *Solution*: Use F = ma. F = 5 kg × 2 m/s² = 10 N. The force is 10 newtons.

**Example 2 — Ohm's Law** *Question*: A resistor of 20 Ω has a current of 0.5 A passing through it. What is the voltage across it? *Solution*: V = IR = 0.5 A × 20 Ω = 10 V. The voltage is 10 volts.

**Example 3 — Kinetic Energy** *Question*: A 2 kg ball moves at 3 m/s. Calculate its kinetic energy. *Solution*: KE = ½mv² = ½ × 2 kg × (3 m/s)² = ½ × 2 × 9 = 9 J. The kinetic energy is 9 joules.

**Example 4 — Power Consumption** *Question*: A 100 W bulb runs for 5 hours. How much energy in kWh does it consume? *Solution*: Energy = Power × Time = 100 W × 5 h = 500 Wh = 0.5 kWh. The bulb consumes 0.5 kilowatt-hours.

Common Mistakes

• **Confusing mass and weight**: Mass is the amount of matter (kg); weight is the gravitational force on that mass (newtons). Weight = mg, not just m.
• **Mixing up concave and convex**: Concave mirrors and lenses curve inward (think "cave" inward). Convex curves outward. Concave mirrors converge light; convex mirrors diverge. Reverse is true for lenses relative to their shape.
• **Forgetting units in formulas**: Writing F = ma without checking units leads to errors. Force must be in newtons (kg·m/s²), not grams or other units.
• **Sign errors in mirror/lens formula**: Object distance u is typically negative (object on the same side as incoming light). Forgetting sign conventions scrambles image-distance calculations.
• **Assuming series and parallel rules interchangeably**: In series, resistances add directly; in parallel, reciprocals add. Mixing these rules gives wrong total resistance. Remember: series increases resistance, parallel decreases it.

Quick Reference

• **Newton's laws**: (1) Inertia, (2) F = ma, (3) Action-reaction pairs.
• **Energy cannot be created or destroyed** — only converted from one form to another.
• **Ohm's law**: V = IR. Voltage, current and resistance are directly linked.
• **Concave mirrors and convex lenses converge** light; convex mirrors and concave lenses diverge light.
• **SI unit of force** = newton (N); energy = joule (J); power = watt (W); pressure = pascal (Pa).
• **Electricity bill is measured in kWh** (kilowatt-hours), the commercial unit of energy.