Welcome to the Language of Physics!
Ever tried to follow a recipe that just said "add 5 flour"? 5 grams? 5 cups? 5 kilograms? You’d probably end up with a mess! Physics is exactly the same. To understand the universe, we need to measure things, and to share those measurements, we need a universal language. That’s where S.I. Units come in. In this chapter, we’ll learn the "alphabet" of Physics and how to combine these units to describe everything from the tiny electron to the vastness of space.
2.1.1 Physical Quantities
Every measurement in physics is a physical quantity. A physical quantity must always have two parts: a numerical value and a unit.
Example: If you say a car is traveling at "30", it means nothing. But "30 m s⁻¹" tells us exactly how fast it is going.
Estimation: The "Physicist’s Superpower"
Sometimes we don't need a perfect answer; we just need to know if an answer is sensible. Estimation involves making a reasonable guess at a value, usually to the nearest order of magnitude (power of ten).
Don't worry if this seems tricky at first! You just need to relate things to what you know. For example, the height of a door is about 2 meters, and the mass of an apple is about 100 grams (0.1 kg).
Quick Review:
• Physical Quantity = Value + Unit
• Estimation = Making an educated guess to check if an answer "makes sense."
2.1.2 S.I. Units: The "Base" of Everything
The Système Internationale (S.I.) is the standard system of units used worldwide. There are six base units you need to know for your AS level. Think of these as the "primary colors" of Physics; every other unit is just a mix of these six.
The Six S.I. Base Units
1. Mass: kilogram (kg)
2. Length: metre (m)
3. Time: second (s)
4. Current: ampère (A)
5. Temperature: kelvin (K)
6. Amount of substance: mole (mol)
Did you know? Even though we use "grams" in chemistry, the kilogram is the only base unit that already has a prefix (kilo-). In Physics calculations, always try to convert your mass to kg first!
Derived Units
Most other units, like the Newton (N) or the Joule (J), are derived units. This means they are made by multiplying or dividing base units together.
Example: Density is mass divided by volume. Its S.I. base units are \( \text{kg m}^{-3} \).
Common Mistake to Avoid: Many students forget the negative sign in units. \( \text{m/s} \) is written as \( \text{m s}^{-1} \) in A-Level Physics. The \( ^{-1} \) just means "divided by".
Key Takeaway: All units in the universe can be broken down into the six base units (kg, m, s, A, K, mol).
Checking Homogeneity: Does the Equation Work?
For a Physics equation to be correct, it must be homogeneous. This is a fancy way of saying that the units on the left side of the equals sign must be exactly the same as the units on the right side.
Step-by-Step: Checking an Equation
Let's check if \( \text{Speed} = \text{Distance} / \text{Time} \) is homogeneous:
1. Left side units: Speed is \( \text{m s}^{-1} \).
2. Right side units: Distance (m) / Time (s) = \( \text{m s}^{-1} \).
3. Both sides match! The equation is homogeneous.
Memory Aid: Think of homogeneity like a balanced scale. You can't have "3 kilograms = 5 meters". It just doesn't make sense!
Prefixes: Making Numbers Manageable
Physics deals with the very large (galaxies) and the very small (atoms). We use prefixes to avoid writing too many zeros. You need to memorize these symbols and their multipliers:
Multiples (Big things)
• Tera (T): \( 10^{12} \)
• Giga (G): \( 10^9 \)
• Mega (M): \( 10^6 \)
• Kilo (k): \( 10^3 \)
Sub-multiples (Small things)
• Deci (d): \( 10^{-1} \)
• Centi (c): \( 10^{-2} \)
• Milli (m): \( 10^{-3} \)
• Micro (μ): \( 10^{-6} \)
• Nano (n): \( 10^{-9} \)
• Pico (p): \( 10^{-12} \)
Quick Review Box:
When converting, if you go from a big unit to a small unit, the number gets bigger.
Example: \( 1 \text{ km} = 1000 \text{ m} \).
Conventions for Graphs and Tables
When you draw a table or a graph in Physics, there is a very specific way to label your headers and axes. We use the "Quantity / Unit" convention.
Example: Speed / m s⁻¹ or Time / s.
The forward slash (/) actually means "divided by". This is useful because it means the numbers written on the axis are just pure numbers with no units attached to them.
Key Takeaway Summary:
• Always include a unit with your numbers.
• Learn the 6 base units (kg, m, s, A, K, mol) by heart.
• Use prefixes to handle very large or very small numbers.
• Always label graph axes as "Quantity / Unit".