# Physics Lesson 1.2.3 - Time

Welcome to our Physics lesson on Time, this is the third lesson of our suite of physics lessons covering the topic of Length, Mass and Time, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.

## Time

By definition, Time is the indefinite continued progress of existence and events that occur in an apparently irreversible succession from the past, through the present, into the future. Time is measured by using clocks and it is particularly important when we want to compare the duration of events or the intervals between them. It is also used to quantify the rates of change of several physical quantities. Time is considered as the fourth dimension, along with three spatial dimensions (length, width and height).

The unit of time in all systems of units is second (s). In scientific terms, the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of cesium-133 atoms is called one second.

Time is different from the other units of the SI system as its unit multiples do not change 10 by 10 (they do not rely on the base 10 system) but they change by multiples of 12 (the base 12 numerical system was very popular in the old times). Hence, we have:

1 minute (min) = 60s (5×12)
1hour (hr) = 60min (5×12) = 3600s (5×60×60)
1 day (d) = 24hr (2×12) = 1440min (24×60) = 86400s (24×60×60)
1 week (wk) = 7d = 168hr (7×24) = 10,800min (7×24×60) = 604,800s (7×24×60×60)

Also, the following submultiples of second are widely used:

millisecond (ms) = 10-3 s
microsecond (μm) = 10-6 s
nanosecond (ns) = 10-9 s

## More Length, Mass and Time Lessons and Learning Resources

Units and Measurements Learning Material
Tutorial IDPhysics Tutorial TitleTutorialVideo
Tutorial
Revision
Notes
Revision
Questions
1.2Length, Mass and Time
Lesson IDPhysics Lesson TitleLessonVideo
Lesson
1.2.1Length
1.2.2Mass
1.2.3Time
1.2.4Dimensional Analysis

## Whats next?

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2. Units and Measurements Physics tutorial: Length, Mass and Time. Read the Length, Mass and Time physics tutorial and build your physics knowledge of Units and Measurements
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7. Continuing learning units and measurements - read our next physics tutorial: Significant Figures and their Importance