Class 9 > Unit # 01: Physical Quantities & Measurement > Basic and Derived Quantities

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Chapter # 1: Physical Quantities and Measurements — Fundamental and Derived Quantities

Lecture Overview

This lecture from Talha's Physics Academy provides a comprehensive exploration of physical quantities, distinguishing them from non-physical concepts, and covers the two main categories: Fundamental Quantities and Derived Quantities (Class 9 Physics, Chapter 1).

Source Video: Fundamental and Derived Quantities | Chapter 01 | Physical Quantities and Measurements | Class 9

Physical vs. Non-Physical Quantities

A physical quantity is any quantity that can be measured accurately and expressed using a numerical magnitude and a unit.

• Components of Measurement: A complete physical measurement requires both a numerical value (magnitude) and a unit (e.g., \(2.5 \text{ hours}\), \(10 \text{ kg}\), \(2.1 \text{ km}\)).
• Non-Physical Quantities: Human feelings, emotions, and psychological states (such as happiness, anger, or sorrow) cannot be measured with an instrument or quantified with standard units, and thus are not studied as physical quantities.

$$\text{Physical Quantity} = \text{Magnitude (Numerical Value)} \times \text{Unit}$$

1. Fundamental (Base) Quantities

Fundamental quantities are the basic, independent physical quantities that do not depend on other quantities and serve as the foundation to derive other quantities. There are exactly seven fundamental quantities in the International System of Units (SI):

• Mass: Quantity of matter contained in an object. Measured in Kilograms (\(\text{kg}\)).
• Length: Distance between two points. Measured in Meters (\(\text{m}\)).
• Time: Duration of events. Measured in Seconds (\(\text{s}\)).
• Temperature: Degree of hotness or coldness. Measured in Kelvin (\(\text{K}\)).
• Electric Current: Flow of electric charge. Measured in Ampere (\(\text{A}\)).
• Amount of Substance: Measure of elementary entities. Measured in Mole (\(\text{mol}\)).
• Luminous Intensity: Wavelength-weighted power emitted by a light source. Measured in Candela (\(\text{cd}\)).

2. Derived Quantities

Derived quantities are those physical quantities that are expressed in terms of fundamental (base) quantities through multiplication or division.

• Area: Length \(\times\) Width — Unit: Square Meter (\(\text{m}^2\))
• Volume: Area \(\times\) Height — Unit: Cubic Meter (\(\text{m}^3\))
• Speed: Distance / Time — Unit: Meter per Second (\(\text{m/s}\))
• Acceleration: Velocity / Time — Unit: Meter per Second squared (\(\text{m/s}^2\))
• Force: Mass \(\times\) Acceleration — Unit: Newton (\(\text{N}\) or \(\text{kg}\cdot\text{m/s}^2\))
• Pressure: Force / Area — Unit: Pascal (\(\text{Pa}\) or \(\text{N/m}^2\))

$$\text{Derived Quantity} = \text{Combination of Fundamental Quantities (via Multiplication/Division)}$$

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