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1.2: Base and Derived Physical Quantities

9th Physics Book PTCB (2025)

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Chapter 1: Physical Quantities and Measurements

1.2: Base and Derived Physical Quantities

25 views Chapter 1: Physical Quantities and Measurements

⚖️ Base & Derived Physical Quantities: The Building Blocks of Physics

🌟 Introduction

Imagine building with LEGO blocks 🧱.
You have basic blocks and special pieces made by combining those basics.
Physics works the same way!

  • Base quantities = basic building blocks

  • Derived quantities = made by combining base quantities

📐 What Are Base Quantities?

Base quantities are the fundamental physical quantities.
They cannot be broken down into simpler quantities.

🎯 Think of them as:

  • The alphabet of physics 🔤

  • Primary colors in art 🎨

  • Basic ingredients in cooking 🍳

✅ Seven Base Quantities (SI System)

Base Quantity Symbol Example Unit
Length 📏 L Height of a door meter (m)
Mass ⚖️ M Weight of a bag kilogram (kg)
Time ⏰ t 5 seconds second (s)
Temperature 🌡️ T Body temperature Kelvin (K)
Electric Current ⚡ I Current in a wire Ampere (A)
Amount of Substance 🧪 n Mole of particles mole (mol)
Luminous Intensity 💡 Iv Brightness of a lamp candela (cd)

🔧 What Are Derived Quantities?

Derived quantities are made by combining two or more base quantities mathematically.

🎯 Think of them as:

  • Words made from letters 📖

  • Mixed colors 🎨

  • Dishes made from ingredients 🍝

📘 Common Derived Quantities

Derived Quantity Formula Built From (Base Quantities) Unit
Speed 🚗 Length ÷ Time L / t m/s
Area 📦 Length × Length L × L m²
Volume 🧊 Length³ L × L × L m³
Density 🪨 Mass ÷ Volume M / L³ kg/m³
Force 💪 Mass × Acceleration M × (L/t²) Newton (N)
Pressure 🔽 Force ÷ Area (M·L/t²)/L² Pascal (Pa)

📊 Simple Understanding

  • Base Quantity → Cannot be broken down

  • Derived Quantity → Made from base quantities

Example:

  • Length (m) = Base

  • Speed (m/s) = Derived

📏 What Is Measurement?

Measurement = Comparing an unknown quantity with a standard unit.

❓ Why do we need standard units?

🔍 Activity:

Measure your desk with your hand span 🖐️.

  • You: 5 spans

  • Friend: 6 spans

  • Teacher: 4 spans

Problem: Different answers!
Solution: Use the meter — same for everyone.

🏛️ History of Measurement

⏳ Old Units:

  • Hand span 🖐️

  • Cubit 💪

  • Foot 👣

  • Step 🚶

❌ Problems:

  • Different sizes

  • No standard

  • Trade arguments

🌍 Solution:

Scientists created international standard units (SI units).

🌍 Two Parts of Every Measurement

Every measurement has:

1️⃣ Number 🔢
2️⃣ Unit 📏

Correct: 5 meters
Incorrect: 5 (5 what?)

👉 A measurement without a unit is meaningless.

🥭 Real-World Example 1: Making Mango Juice

▶️ Base Quantities:

  • Mass of mangoes ⚖️ → 2 kg

  • Time to blend ⏰ → 3 min

  • Temperature 🌡️ → 25°C

▶️ Derived Quantities:

  • Volume of juice 🧃 → liters

  • Density of juice 🥤 → kg/L

  • Blender speed 🌀 → rotations/min

Everyone gets the same recipe if everyone uses the same units!

🎒 Real-World Example 2: A Student’s School Day

▶️ Base Quantities:

  • Distance to school 📏 → 3 km

  • Time ⏰ → 20 min

  • Mass of bag ⚖️ → 5 kg

  • Temperature 🌡️ → 35°C

▶️ Derived Quantities:

  • Speed 🚶 → distance ÷ time

  • Area of classroom 📐 → 48 m²

  • Density of water bottle 💧 → 1 kg/L

🎯 Quick Summary

Base Quantities

✔️ Fundamental
✔️ Cannot be broken down
✔️ 7 SI base quantities
✔️ Examples: length, mass, time

Derived Quantities

✔️ Formed by combining base quantities
✔️ Have formulas
✔️ Examples: speed, volume, density

Measurement

✔️ Has number + unit
✔️ Must use standard units
✔️ Ensures correct & universal results

💡 Memory Trick

“Base quantities = ingredients 🍚
Derived quantities = dishes 🍕”

📝 Quiz Time

Question: Which of the following is a base quantity?

A) Speed
B) Mass
C) Density
D) Area

✅ Correct Answer: B) Mass ⚖️

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