Study Notes: Basic Physics

Overview

Basic Physics forms a critical component of the General Science section in UPSSSC PET, typically contributing 4-6 questions from the total science allocation. This topic tests your understanding of fundamental physical principles that govern everyday phenomena—from why objects fall to how electricity flows through circuits.

Success in this section requires mastering conceptual clarity rather than complex calculations. UPSSSC PET emphasizes direct applications, unit identification, and understanding cause-effect relationships in physical systems. Students must be comfortable with basic numerical problems involving standard formulas and be able to identify correct physical principles in real-world scenarios. The questions test your grasp of motion, forces, energy transformations, heat transfer, light behavior, electrical circuits, and magnetic effects—all at a fundamental level suitable for undergraduate science curriculum.

Focus on understanding the "why" behind physical laws, memorizing standard units and constants, and practicing one-step numerical problems. This topic integrates well with your everyday observations, making it both scoring and conceptually satisfying when approached systematically.

Key Concepts

• **Motion** is change in position over time; velocity includes direction (vector) while speed does not (scalar). Acceleration is the rate of change of velocity.

• **Newton's Laws** govern force and motion: (1) objects resist change in motion (inertia), (2) force equals mass times acceleration (F = ma), (3) every action has an equal and opposite reaction.

• **Gravitation** is the universal attractive force between masses; weight is gravitational force on an object (W = mg), while mass remains constant everywhere.

• **Work** is done when force causes displacement in its direction; energy is the capacity to do work. Both are measured in joules. Energy transforms but total energy remains conserved.

• **Heat** is thermal energy transfer from hot to cold bodies; temperature measures hotness. Heat flows via conduction (direct contact), convection (fluid movement), and radiation (electromagnetic waves).

• **Light** travels in straight lines at 3 × 10⁸ m/s in vacuum; it reflects (bounces off) and refracts (bends when entering different media). Mirrors form images by reflection, lenses by refraction.

• **Electric current** is the flow of charge; voltage (potential difference) drives current through resistance. Power is the rate of energy consumption in electrical devices.

• **Magnetism** arises from moving charges; magnetic field lines emerge from north pole and enter south pole. Electric current produces magnetic fields (electromagnetic effect).

Formulas / Key Facts

**Motion**

• Distance = Speed × Time; Velocity = Displacement/Time
• Acceleration = Change in velocity/Time = (v - u)/t
• v = u + at; s = ut + ½at²; v² = u² + 2as (equations of motion)

**Force & Gravitation**

• Force (F) = mass (m) × acceleration (a); SI unit: newton (N)
• Weight (W) = mg, where g = 9.8 m/s² (acceleration due to gravity on Earth)
• Universal gravitation: F = G(m₁m₂)/r², where G = 6.67 × 10⁻¹¹ Nm²/kg²

**Work, Energy & Power**

• Work (W) = Force × Displacement = F × s; SI unit: joule (J)
• Kinetic Energy (KE) = ½mv²; Potential Energy (PE) = mgh
• Power (P) = Work/Time; SI unit: watt (W); 1 HP = 746 watts

**Heat**

• Heat (Q) = mass × specific heat × temperature change = mcΔT
• 1 calorie = 4.2 joules; specific heat of water = 1 cal/g°C = 4200 J/kg°C

**Light**

• Speed of light (c) = 3 × 10⁸ m/s in vacuum
• Mirror formula: 1/f = 1/v + 1/u; magnification = v/u = height of image/height of object
• Lens formula: 1/f = 1/v - 1/u (sign convention matters)

**Electricity**

• Ohm's Law: V = IR (voltage = current × resistance)
• Electric Power: P = VI = I²R = V²/R; SI unit: watt
• Electrical Energy = Power × Time = VIt; commercial unit: kWh (1 kWh = 3.6 × 10⁶ J)

**Magnetism**

• Like poles repel, unlike poles attract
• Magnetic field due to current-carrying conductor (right-hand thumb rule)

Worked Examples

**Example 1: Motion** A car accelerates from rest to 20 m/s in 5 seconds. Find acceleration and distance covered.

*Solution:*

• Initial velocity (u) = 0, final velocity (v) = 20 m/s, time (t) = 5 s
• Acceleration: a = (v - u)/t = (20 - 0)/5 = 4 m/s²
• Distance: s = ut + ½at² = 0 × 5 + ½ × 4 × 5² = 0 + 2 × 25 = 50 m

**Example 2: Work-Energy** A 2 kg object is lifted to a height of 10 m. Calculate work done. (g = 10 m/s²)

*Solution:*

• Force required = Weight = mg = 2 × 10 = 20 N
• Work = Force × Displacement = 20 × 10 = 200 J
• This work is stored as potential energy: PE = mgh = 2 × 10 × 10 = 200 J

**Example 3: Electricity** A 100 W bulb operates for 10 hours daily. Find monthly energy consumption and cost at ₹5 per kWh.

*Solution:*

• Power = 100 W = 0.1 kW
• Daily energy = 0.1 kW × 10 h = 1 kWh
• Monthly energy (30 days) = 30 kWh
• Cost = 30 × 5 = ₹150

Common Mistakes

**Mass vs. Weight confusion** → Mass is the amount of matter (constant everywhere, measured in kg); weight is gravitational force (varies with location, measured in newtons). Remember: weight = mg.

**Distance vs. Displacement** → Distance is total path length (scalar); displacement is shortest distance between start and end points (vector). A person walking around a circular track covers distance but may have zero displacement.

**Heat vs. Temperature** → Heat is energy transfer (measured in calories or joules); temperature is degree of hotness (measured in °C, K, or °F). Adding heat raises temperature, but they're not the same thing.

**Power vs. Energy in electricity** → A 100 W bulb doesn't consume 100 units per hour. Power (watt) is rate; energy is power × time. 100 W for 10 hours = 1 kWh (1 unit).

**Sign convention in mirrors/lenses** → Distances measured against incident light direction are negative; with incident light are positive. Forgetting sign conventions leads to wrong focal length or image position.

Quick Reference

• **SI Units**: Force (newton), Work/Energy (joule), Power (watt), Charge (coulomb), Current (ampere), Resistance (ohm)
• **g = 9.8 m/s²** (often approximated as 10 m/s² for quick calculations)
• **Speed of light = 3 × 10⁸ m/s**; speed of sound in air ≈ 340 m/s
• **1 kWh = 1 unit of electricity** = 3.6 million joules
• **Ohm's Law: V = IR**; higher resistance means lower current for same voltage
• **Like charges/poles repel; unlike attract**—applies to both electricity and magnetism