94
Parts of a circle: hours and minutes
Textbook: Part 3, lesson 94, pp. 78–80
GoalThe clock face is a circle: 1 hour = 60 minutes, 1 minute = 1/60 of an hour; how hours, minutes and seconds are related; adding and subtracting time.
New words
clock face · siferblathour hand · soat miliminute · minutsecond · sekund
Explanation
The clock face is a circle. In one hour the minute hand makes one full turn. 1 hour = 60 minutes, so 1 minute is 1/60 of an hour. Half an hour is 30 minutes (1/2), a quarter of an hour is 15 minutes (1/4), a third of an hour is 20 minutes, a sixth is 10 minutes. 1 minute = 60 seconds, 1 hour = 60 × 60 = 3 600 seconds. A day has 24 hours and a week has 7 days. When adding and subtracting times we work with hours, minutes and seconds separately and carry to the next unit at 60.
Worked examples
7 min = 7 × 60 = 420 s. 540 s = 540 : 60 = 9 min. 2 days = 2 × 24 = 48 hours.
4 h 38 min + 6 h 47 min: 4 + 6 = 10 h, 38 + 47 = 85 min = 1 h 25 min; total 11 h 25 min. Subtraction: 9 h 15 min − 2 h 40 min = 8 h 75 min − 2 h 40 min = 6 h 35 min.
Class activity
“Living clock”: one child is the minute hand, another the hour hand. The teacher names a time (“a quarter of an hour”, “half an hour”, “a sixth of an hour”) and the children place their arms accordingly.
Practice
1
What fraction of the clock face does the minute hand cover in 15 minutes?
1/4
2
How many seconds are there in 12 minutes?
720
3
How many minutes are there in 5 400 seconds?
90
4
A class began at 14:15 and ended at 15:50. How many minutes did it last?
95
5
How many hours is 1 week + 10 hours?
178