Physics is a global conversation, and every conversation needs a shared language. For scientists, that language is the International System of Units, or SI units. It’s the modern form of the metric system and ensures that a meter in one lab is the same length as a meter in a lab across the world.

These seven base units are the building blocks for all other measurements. Everything from the force of a rocket launch to the energy from a nuclear reaction can be described using combinations of these fundamental units. For example, force is measured in newtons (N), where $1 \, \text{N} = 1 \, \text{kg} \cdot \text{m/s}^2$. This standardization is what allows scientists to build upon each other's work with confidence.

No measurement is perfect. If you measure the width of a book with a standard ruler, you might say it's 20.3 centimeters. But could it be 20.31? Or 20.29? The digits you write down communicate how precise your measurement is. This is the idea behind significant figures.

When you use measurements in calculations, their precision carries through. For multiplication and division, your answer should have the same number of significant figures as your least precise measurement. For addition and subtraction, the answer should have the same number of decimal places as the measurement with the fewest.

In the example above, the width (4.5 cm) has only two significant figures, so the final answer for the area is rounded to two significant figures.

Beyond the limits of our tools, errors can creep into our measurements in two main ways. It’s important to know the difference so we can account for them.

Systematic errors are like a biased referee. They might come from a faulty instrument or a flawed experimental setup. Because they are consistent, they can be hard to spot, but once identified, they can often be corrected.

Random errors are unavoidable. They can be caused by slight variations in the environment or limitations in the observer's ability to read an instrument. While you can't eliminate random error, you can minimize its effect by taking many measurements and averaging them. The average will be a better estimate of the true value than any single measurement.

Some quantities in physics can be described by a number alone. Others need a number and a direction. This distinction is fundamental.

Telling someone your speed is 60 miles per hour is useful. But telling them your velocity is 60 miles per hour due north gives them the full picture of your motion. In physics, knowing the direction is often just as important as knowing the amount.

This diagram shows the difference between distance and displacement. You might walk 10 km along a winding path, but your displacement is the straight-line path from your start to your finish. One is a scalar, the other a vector.

Understanding these core concepts—standard units, precision, uncertainty, and the nature of physical quantities—is the first step in thinking like a physicist. They are the tools you'll use in every experiment and calculation that follows.