📌 Measurement in Physics – Class 9
🔹 Introduction to Measurement
Physics heavily relies on measurements for describing and analyzing the physical world. Without measurement, it would be impossible to express scientific laws and principles in a quantitative form.
All measurements require two key components: a number (magnitude) and a unit. Together, these define the quantity being measured.
🔹 Fundamental Quantities & Their SI Units
Fundamental quantities are the basic physical quantities that are not derived from other quantities. The International System of Units (SI) defines 7 such fundamental quantities:
| Quantity | Symbol | SI Unit |
|---|---|---|
| Length | L | metre (m) |
| Mass | M | kilogram (kg) |
| Time | T | second (s) |
| Temperature | Θ | kelvin (K) |
| Electric Current | I | ampere (A) |
| Amount of Substance | n | mole (mol) |
| Luminous Intensity | Iv | candela (cd) |
Why Use SI Units?
SI units provide a standard method for communicating measurements across the world, ensuring consistency and clarity in scientific work.
🔹 Derived Quantities & Their Units
Derived quantities are formed by combining fundamental quantities through multiplication or division. They describe more complex physical properties:
- ✅ Area: square metre (m²)
- ✅ Volume: cubic metre (m³)
- ✅ Density: kilogram per cubic metre (kg/m³)
- ✅ Speed: metre per second (m/s)
- ✅ Force: newton (N) = kg·m/s²
Importance of Derived Units
Derived units simplify the representation of complex physical laws, making calculations more manageable in real-world applications.
🔹 Vernier Calipers
The Vernier Calipers is a device that can measure internal and external dimensions as well as depth with high accuracy.
- ✅ Least count: 0.01 cm or 0.1 mm
- ✅ Used to measure external diameter of cylinders or pipes
- ✅ Also used to measure internal diameter of hollow objects and depth of containers
- ✅ Formula: Measurement = Main Scale Reading + (Vernier Scale Division × Least Count)
🔹 Screw Gauge
The Screw Gauge is ideal for measuring very small lengths or thicknesses with high precision.
- ✅ Least count: 0.01 mm
- ✅ Commonly used for measuring wire thickness, diameters of small spheres, etc.
- ✅ Formula: Measurement = Main Scale Reading + (Circular Scale Division × Least Count)
Working Principle
Screw Gauge works on the principle of a screw. The rotation of a screw helps translate small distances accurately.
🔹 Simple Pendulum
A simple pendulum consists of a mass (bob) attached to a string and suspended from a fixed point, allowed to swing freely.
- ✅ Time Period (T): Time taken for one complete oscillation
- ✅ Formula: T = 2Ï€ √(L/g)
- ✅ The time period is directly proportional to the square root of length and independent of mass.
Applications
Simple pendulums are used in clocks, educational experiments, and to study gravitational variations.
🔬 Fun Experiment: Measuring the Time Period of a Pendulum
Perform this simple experiment to understand how the time period changes with length:
- Take a string and attach a small ball (bob).
- Fix the string so the bob can swing freely.
- Displace the bob slightly and release it to start oscillation.
- Use a stopwatch to time 10 oscillations.
- Divide the total time by 10 to find one oscillation’s time (T).
💡 Conclusion: The time period increases with the length of the string and does not depend on the bob’s mass.
📌 Final Thoughts
Measurement forms the foundation of all physical sciences. A strong grasp of fundamental and derived quantities, as well as instruments like Vernier Calipers, Screw Gauge, and the Simple Pendulum, prepares students for advanced study in physics.
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