The unit of time

The SI unit of time is the second, with the symbol s. Bigger units are the minute (min) and the hour (h):

• 60 s = 1 min
• 60 min = 1 h

The book is careful about writing these correctly:

• The names second, minute, hour start with a small letter (except at the start of a sentence).
• The symbols s, min, h are small letters too, and never get an extra 's' for plural: 5 min, not 5 mins.
• No full stop after the symbol, except at the end of a sentence.
• Always leave a space between the number and the unit: 20 s, not 20s.

Two common mistakes: writing 'sec' for second and 'hrs' for hour are both wrong.

Activity 8.3 asks: what is the smallest interval of time you can measure with a wall clock? One second, the jump of the seconds hand.

But the modern world often needs much smaller bits of a second:

• Sports: timers record races to one-hundredth or even one-thousandth of a second (a millisecond) to decide the winner.
• Medicine: ECG machines measure millisecond changes in heartbeats to find health problems.
• Music: digital recordings capture sound thousands of times a second.
• Phones and computers handle signals in microseconds, one-millionth of a second.