Key concepts of physics

Welcome to Your Physics Toolkit!

Welcome to the first chapter of your Paper 2 studies! Think of Key Concepts of Physics as your "scientific toolkit." Before we can dive into the big mysteries of the universe, we need to make sure we know how to measure things correctly and speak the universal language of scientists. Don't worry if some of the math feels a bit new; we'll take it one step at a time!

1. SI Units: The Universal Language

Imagine trying to bake a cake where the recipe used "handfuls" or "a few pinches." It would be a disaster! In Physics, we use the SI Unit system (International System of Units) so that a scientist in London and a scientist in Tokyo mean exactly the same thing when they say "one meter."

What are the main SI Units?

Every physical quantity (something you can measure) has a specific unit. Here are the ones you need to remember most often:

• Length: metre (m)
• Mass: kilogram (kg)
• Time: second (s)
• Current: ampere (A)
• Temperature: kelvin (K)
• Amount of substance: mole (mol)

Quick Review Box:
Whenever you finish a calculation, always check if you've included the unit. A number without a unit in Physics is just a lonely number!

2. Multiples and Sub-multiples (Prefixes)

Sometimes, the standard unit is too big or too small. For example, measuring the distance to the next city in metres would involve huge numbers, and measuring the thickness of a hair in metres would involve tiny decimals. We use prefixes to make these numbers easier to manage.

The "Ladder" of Prefixes

You need to recall these specific prefixes from the largest to the smallest:

• Giga (G): \(10^9\) (1,000,000,000)
• Mega (M): \(10^6\) (1,000,000)
• kilo (k): \(10^3\) (1,000)
• centi (c): \(10^{-2}\) (0.01)
• milli (m): \(10^{-3}\) (0.001)
• micro (\(\mu\)): \(10^{-6}\) (0.000001)
• nano (n): \(10^{-9}\) (0.000000001)

Memory Aid:
Think of Giga and Mega like computer storage (Gigabytes and Megabytes)—they are huge! Think of milli, micro, and nano as "miniature"—they are very small.

Did you know?
The symbol for micro is the Greek letter 'mu' (\(\mu\)). It looks a bit like a 'u' with a long tail at the front!

3. Converting Between Units

One of the most common places students lose marks is by forgetting to convert units before using an equation. Most Physics equations require the "standard" unit (like seconds or metres).

How to Convert: The Golden Rule

• To go from a larger unit to a smaller unit (e.g., kg to g), you multiply.
• To go from a smaller unit to a larger unit (e.g., mm to m), you divide.

Converting Time: Hours to Seconds

This is a classic exam trick! To turn hours into seconds, you have to go through minutes first.
1. Multiply by 60 to get minutes.
2. Multiply by 60 again to get seconds.

\(1 \text{ hour} \times 60 \times 60 = 3600 \text{ seconds}\)

Common Mistake:
Many students think there are 100 seconds in a minute because of the metric system. Remember: Time still works in 60s!

Key Takeaway: Always ensure your units "match" the equation requirements before you start your calculation.

4. Standard Form and Significant Figures

In Physics, we deal with the massive (like the mass of a planet) and the tiny (like the size of an atom). Standard Form helps us write these without drawing dozens of zeros.

Standard Form

Standard form always looks like this: \(A \times 10^n\)
Where \(A\) is a number between 1 and 10, and \(n\) is the power of 10.

• Example (Big number): \(300,000,000\) becomes \(3 \times 10^8\)
• Example (Small number): \(0.0005\) becomes \(5 \times 10^{-4}\)

Significant Figures (Sig Figs)

Using significant figures is about being honest about how precise your measurement is. If your ruler only measures in millimetres, you can't give an answer to ten decimal places!

Step-by-Step for Sig Figs:
1. Start counting from the first non-zero digit.
2. Count the number of digits required.
3. Look at the next digit to decide whether to round up (if it's 5 or more) or stay the same.

Example: Round 0.004562 to 2 significant figures.
- The first non-zero is the '4'.
- The second is the '5'.
- The next digit is '6', so we round the '5' up to a '6'.
- Answer: 0.0046

Pro-Tip:
If the exam question doesn't specify, giving your answer to 2 or 3 significant figures is usually a safe bet.

Summary of Key Concepts

Checklist for your next Physics problem:

1. Identify the SI Units for the quantities given.
2. Check for Prefixes (like kilo or milli) and convert them to the base unit.
3. If time is in hours or minutes, convert it to seconds.
4. Perform your calculation carefully.
5. Write your final answer in Standard Form and the correct number of Significant Figures.
6. Don't forget to add the unit at the end!