SI Units and Measurement

Learning Objectives

Identify the seven base units of the International System of Units (SI).

Use common metric prefixes (kilo-, centi-, milli-) to convert between units.

Explain the difference between accuracy and precision.

Why Standard Units Matter

Imagine you're trying to bake a cake, and the recipe says 'add two handfuls of flour.' Your handful might be very different from someone else's! This is why scientists, and people all over the world, use a standard system of measurement. The most widely used system is the International System of Units, abbreviated as SI from its French name, Système International d'Unités.

The Seven Base SI Units

The entire SI system is built upon seven fundamental units, called base units. Every other unit can be derived from these seven.

1.Length: meter (m)

2.Mass: kilogram (kg) - Note: Even though 'kilo' is a prefix, the kilogram is the base unit for mass, not the gram.

3.Time: second (s)

4.Temperature: Kelvin (K) - In everyday science, Celsius (°C) is often used, but Kelvin is the official SI base unit.

5.Electric Current: ampere (A)

6.Amount of Substance: mole (mol)

7.Luminous Intensity: candela (cd)

For middle school science, you'll most often work with the meter, kilogram (or gram), second, and Celsius/Kelvin.

Metric Prefixes

The SI system is a decimal system, meaning it's based on powers of 10. This makes conversions very easy! We use prefixes to show multiples or fractions of the base units.

Here are the most common ones:

kilo- (k): 1,000 (one thousand) -> 1 kilometer (km) = 1,000 meters (m)

hecto- (h): 100 (one hundred)

deka- (da): 10 (ten)

(base unit): 1 (meter, gram, liter)

deci- (d): 0.1 (one-tenth)

centi- (c): 0.01 (one-hundredth) -> 1 meter = 100 centimeters (cm)

milli- (m): 0.001 (one-thousandth) -> 1 gram = 1,000 milligrams (mg)

To convert, you just move the decimal point. To go from a larger unit to a smaller unit (like meters to centimeters), you multiply by 10 for each step, moving the decimal to the right. To go from a smaller unit to a larger one (like milligrams to grams), you divide by 10 for each step, moving the decimal to the left.

Accuracy vs. Precision

These two words are often used interchangeably in everyday language, but in science, they have very specific meanings.

Accuracy: How close a measurement is to the true or accepted value. If a bag of sugar is known to weigh exactly 5 kg, and you measure it as 4.99 kg, your measurement is very accurate.

Precision: How close a series of measurements are to each other. It describes the reproducibility of a measurement. If you weigh the same bag of sugar three times and get 4.75 kg, 4.76 kg, and 4.74 kg, your measurements are very precise (they are all very close to each other), but they are not accurate (they are far from the true value of 5 kg).

An ideal scientific instrument is both accurate and precise. It gives you the correct reading every single time.

Key Terms

**SI (International System of Units): The modern form of the metric system and the most widely used system of measurement.~|~Meter (m): The SI base unit for length.~|~Kilogram (kg): The SI base unit for mass.~|~Second (s): The SI base unit for time.~|~Accuracy: How close a measured value is to the actual (true) value.~|~Precision: How close a series of measurements are to one another, regardless of their accuracy.