Section outline
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In this course you will learn about:
- The 12-hour clock.
- The 24-hour clock.
- Units of time.
- How to convert between the different units of time.
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Time! This is something which structures our lives on a daily basis and even gives us a term of reference to events in our lives, e.g.
- School starts at 8am
- Soccer practice is at 3pm
- You ran the 100 m sprint in 16 seconds
But can you actually define what time is?
In simpler words time is "the measured or measurable period during which an action, process, or condition exists or continues", e.g. how long does it take you to brush your teeth? How long are your school holidays? What time does your soccer practice start?
In this course we will explore the different time clocks, how to convert between one and the other, various time units and then we will progress on to how to add and subtract units of time as well as how to calculate elapsed time.
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How to read / write the time using the 12-hour clock
Have you heard of the 12-hour clock? This clock provides one of the ways in which to read and write the time. As its name suggests, it works on a 12-hour basis - but there are 24 hours in one day so therefore the clock goes past the 12 o' clock number twice!
So essentially, the 24-hour day is divided into two, namely:
- AM: Ante meridiem. The easy way to remember what AM stands for is BEFORE midday / noon
- PM: Post meridiem. An easy way to differentiate PM from AM is that PM is AFTER, or POST midday
Watch the YouTube video below which explains this concept very clearly.MooMooMath and Science. (2019). Difference between AM and PM (Standard YouTube licence)
Test your understanding of what has been covered so far by answering the questions in the short quiz below.
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What is the 24-hour clock?
The 24-hour clock is literally what the name depicts i.e. it runs for 24 hours, unlike the 12-hour clock which runs for two 12 hour lengths.
Since we know that there are 24 hours in one day and it's a 24-hour clock, it makes sense that the 24-hour clock will run for one 24 hour session starting and ending at midnight. The 24-hour clock starts at 00:00 and ends at 23:59. There are no AMs or PMs involved! Some people prefer the following display: 00h00 and 23h59 - both are correct.
Example
What is the time on the above clock? (Note: remember that the long clock hand refers to the minutes, the short hand to the hours and the very thin long hand refers to the seconds. Not all clocks will have a second hand.)
Using the 12-hour clock do you agree that the time could read either:
- 10.10am
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- 10.10pm
We would only know whether it's am or pm by whether it is light or dark outside!Using the 24-hour clock we will tell the above time as follows:
- 10h10 if it was in the morning
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- 22h10 if it was during the night
Do you see where we got the 22 from? 10 o'clock at night is the 22nd hour after midnight.Watch the YouTube video below which clearly describes the 24-hour clock.
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Now that you understand the 12-hour and 24-hour clocks, let's look at the units of time, starting with the smallest:
- seconds
- minutes
- hours
Watch the video below which very clearly discusses the second, minute and hour.
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When working with these smaller units of time, it is important to remember the following rule:
When converting from larger to smaller units, multiply. When converting from smaller to larger units, divide! Now you just need to remember that there are 60 seconds in one minute and 60 minutes in one hour.
Remembering the above rule will assist you when you are required to convert from one unit of time to another!
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There are some basic principles that you should follow when adding, or subtracting, measures of time.
Always:
- add the 'like' units first
- start with the smallest unit
- add seconds to seconds, minutes to minutes, and hours to hours
- then simplify the answer when necessary by converting the units.
Example: Adding time units
Find the sum of 1 hour 55 minutes and 45 minutes.
1 hr 55 min
45 min
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1 hr 100 minYou know that 60 minutes = 1 hour. Simplify 100 minutes by converting it in terms of hours and minutes:
- 100 minutes = ____ hours ___ minutes
- divide the number of minutes by 60
- 100 ÷ 60 = 1 with a remainder of 40
- 100 minutes = 1 hour 40 minutes.
Rewrite 100 minutes as 1 hour 40 minutes. This will add 1 hour to the total number of hours:
- 1 hr 100 min = 1 hr + 1 hr 40 min = 2 hr 40 min
Example: Subtracting time units
To subtract a measure of time, start by subtracting the smallest unit first. You may need to borrow from the larger unit. Then, simplify the answer when necessary by converting the units.
Find the difference between 2 minutes 30 seconds and 1 minute 40 seconds.
2 min 30 sec
You cannot subtract 40 from 30. Borrow from the minute unit. 1 minute equals 60 seconds. Add 60 seconds to 30 seconds.
- 1 min 40 sec
The difference between 2 minutes 30 seconds and 1 minute 40 seconds is 50 seconds.
Time can also be described as fractional units of time, for example: "a quarter of an hour" or "half an hour." You know that 1 hour equals 60 minutes. To find a fraction of an hour, in this example a quarter of an hour, multiply the fraction by 60 minutes.
\( 1/4 (60) = 60 \div4 = 15 \)
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Elapsed time
Elapsed time tells us how much time passes between two events. We can calculate the elapsed time by figuring out the difference between a start and a stop time.
Example
School starts at 8am. The first break is at 10am. How many hours until the first break?This is an easy example to answer since both of the times are 'am' so you can just subtract the two, i.e. 10am - 8am = 2 hr.
However, events can also occur before noon (am) and end after noon (pm). When that happens, finding the elapsed time requires an additional step, if you use the 12-hour clock:
Example
Hajierah goes to school at 8am and returns home at 3pm. How long was she at school? You might, incorrectly think, that Hajierah has been at school for five (5) hours. But this is not the case. Before finding the difference, convert the times to the 24-hour clock:
8am = 08h00
3pm = 15h00 (3pm is 15 hours into the day, i.e. it is 15 hours after midnight)
Now it is easy to work out how long Hajierah was at school. Simply subtract 08h00 from 15h00 and you will work out that she was at school for seven (7) hours.Now watch the video below which provides further examples on how to add, and subtract, time units.
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