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Measurement of Time

Physics is concerned not only with the measurement of length but also with the motion of objects. The idea of motion involves the concept of time. The separation between the occurrences of two events is called the time interval.

Any phenomenon that repeats itself can be used as a measure of time interval. For example, a pendulum can be used to measure time interval. The rotation of the earth on its axis, which determines the length of the day, has been used as a time standard from the earliest times. The day is subdivided into hours, minutes and seconds.

There are direct and indirect methods for measuring time intervals. Direct methods make use of a watch or a clock.

Indirect methods are used to measure very short and very long time intervals.

The laws of radioactive decay have been used to measure very long as well as very short time intervals.

Some radioactive substances, such as carbon 14, take millions of years to decay whereas others decay in a millionth fraction of a second. Carbon dating is used to estimate the age of rocks.

Table below gives the orders of magnitude of some measured time intervals. Measurements of time intervals can be made can be made over a vast range.



Age of the earth

1 × 1017

Human life expectancy

2 × 109

Time of earth’s orbit around Sun (1 year)

3 × 107

Time of earth’s rotation about its axis (1 day)

8.6 × 104

Time interval between normal heartbeats

1 × 100

Time period of sound waves

1 × 10-3

Time period of radio waves

1 × 10-7

Time period of rotation of a molecule

1 × 10-12

Half-life of neutral p ion

2.2 × 10-16

Period of nuclear vibrations

1 × 10-21

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