Class 9 Physics Unit 1 Notes: Physical Quantities and Measurement

 

Class 9 Physics: Unit 1

Physical Quantities and Measurement - Comprehensive Study Guide

1. Physical Quantities

Introduction: Physics ki bunyaad naap-tol par hai. Jab hum kisi cheez ko accurately measure karte hain, tabhi physics ke laws bante hain.

Definition: "All measurable quantities are called physical quantities."

Formula: Physical Quantity = Numerical Magnitude × Unit

Example: Ek student ki height 1.6 meters hai. 1.6 magnitude hai aur meter unit hai.

Explanation: Inki do types hain: Base Quantities (7 bunyadi quantities) aur Derived Quantities (jo base se nikalti hain).

2. Vernier Callipers

Introduction: Meter rule se zyada accuracy ke liye Vernier Callipers use hota hai.

Definition: It is a device used to measure small lengths and diameters up to 0.1 mm.

Formula (Least Count): L.C = (Smallest Main Scale Div) / (Total Vernier Div)

Solution: L.C = 1 mm / 10 = 0.1 mm = 0.01 cm.

Explanation: Is mein ek fixed main scale aur ek sliding vernier scale hota hai.

3. Screw Gauge (Micrometer)

Introduction: Ye Vernier Callipers se bhi barik (more precise) measurement karta hai.

Definition: Used to measure very small diameters like wire thickness up to 0.01 mm.

Formula: L.C = Pitch / Total Divisions on Circular Scale.

Example: Pitch = 1mm, Div = 100 → L.C = 0.01 mm = 0.001 cm.

Explanation: Is mein ek circular scale hota hai jo thimble ke sath rotate karta hai.

4. Measuring Cylinder

Definition: A graduated cylinder used to measure the volume of liquids and irregular solids.

Formula: V = V₂ - V₁ (Displacement method)

Explanation: Jab kisi irregular solid ko pani mein dala jaye, toh jitna pani upar jaye wo us solid ka volume hota hai.

5. Significant Figures

Definition: "All accurately known digits and the first doubtful digit."

Rules Table:

Rule	Example
Non-zero digits are significant.	45.3 (3 Sig-figs)
Zeros between non-zeros are significant.	5005 (4 Sig-figs)
Final zeros after decimal are significant.	2.50 (3 Sig-figs)

🧠 Quick Revision Quiz

1. How many sig-figs in 0.005?
(A) 1 (B) 3

See Answer

(A) 1 (Leading zeros count nahi hote)

UNIT 1 COMPLETE ✅

Aapka Unit 1 ab har lehaz se mukammal hai. Study jari rakhein!