Time is measured in seconds (s).

SI prefixes for large time intervals such as kilosecond  (ks) and megasecond (Ms) are seldom used and we usually see the common units such as minutes and hours used instead.

However for smaller intervals of time we do see millisecond (ms).

 

Caution
It is common to see the abbreviation “sec” written in newspapers etc. This is not an accepted S.I. abbreviation of second and must not be used. Correct: 37 seconds 37 s Wrong: 37                      (no units) 37 sec             (not an acceptable abbreviation)

 

The precision of the electronic stopwatch (which is 0.01 s) is much shorter than the human reaction time. Thus we will only record stopwatch readings to 0.1 s.

Example
How should the following stopwatch reading be recorded? t = 19.5 s Show Answer

 

Because there is likely to be human reaction time in starting/stopping the electronic stopwatch we will always take several readings of time and determine an average. This process will minimise the random element of such human reaction timing errors.

 

Additionally, for a longer time interval (eg a minute rather than a second) then the human reaction error as a percentage of the whole reading is a smaller percentage. Thus we try to take longer time readings whenever possible. i.e. we will record the time for 20 oscillations of a pendulum rather than trying to record just a single oscillation of the pendulum bob.

 

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