Useful Info

# 47 Metric units

### Metric units

Throughout this book, I will use the metric system. The metric system uses base units of measure (like seconds, meters and grams) and standard prefixes (like kilo-, milli-and micro-). The prefixes are based on powers of ten, so that unit conversion within the metric system is easy.

The basic time unit in the metric system is the second (symbol: s). Other everyday time units (like days, hours and minutes) are not metric. The basic unit for distance is the meter (symbol: m). One meter is about a yard. The basic unit for mass is the gram. The gram is a small amount of mass- about the mass of a raisin. (For comparison, one pound is roughly 454 grams).

Other metric units can be made by combining basic metric units. Speeds are measured in m/s (meters per second), areas in m^{2} (square meters) and so on. Special combinations of metric units are given their own names- like the Newton (symbol: N), Pascal (Pa) or Hertz (Hz).

### Prefixes

The metric system uses prefixes to deal with very large and very small numbers. These prefixes are based on powers of ten. The same prefixes work with all base units. The tables below show the most common prefixes and their meanings.

Abbreviation | Long Name | Meaning |

1 kHz | 1 kilohertz | 1,000 Hz |

1 MHz | 1 megahertz | 1,000,000 Hz |

1 GHz | 1 gigahertz | 1,000,000,000 Hz |

Abbreviation | Long name | Meaning |

1 cm | 1 centimeter | 0.01 m |

1 mm | 1 millimeter | 0.001 m |

1 μm | 1 micrometer | 0.000001 m |

1 pm | 1 picometer | 0.000000001 m |

### Stop to Think

Use metric prefixes to simplify the following numbers:

- 4,000 Hz
- 1,000,000 Hz
- 0.03 sec
- 0.0002 sec

### Stop to think answers

- 4 kHz; 1 MHz; 30 ms; 0.2 ms or 200 μs

### Some resources:

Some practice at converting units and metric system is here: https://www.ixl.com/math/grade-4