Math Antics - Telling Time - Free Educational videos for Students in K-12 | Lumos Learning

Math Antics - Telling Time - Free Educational videos for Students in k-12


Math Antics - Telling Time - By Lumos Learning



Transcript
00:03 uh huh . Hi , I'm Rob . Welcome to
00:07 math antics . In this lesson , it's finally time
00:10 for us to learn about time like distance . Time
00:13 is something you can measure and represent as a number
00:16 , but we use different units to measure time and
00:19 also different devices to make the measurements . There are
00:23 lots of different units of time from really small ones
00:26 , like nanoseconds to really big ones like centuries .
00:29 But in this video , we're only going to learn
00:31 about four of the most common units of time seconds
00:35 , minutes , hours and days . You should be
00:38 really familiar with the day , you know , because
00:40 you encounter one every day . A day is the
00:43 amount of time it takes the Earth to rotate on
00:45 its axis one time . So it's pretty much the
00:48 fundamental unit of time for our planet . But as
00:51 important as the day is , we need smaller units
00:54 of time for planning daily activities , so people decided
00:58 to divide up each day into 24 equal chunks of
01:01 time called hours . Even though an hour is only
01:04 one 24th of a day , it's still a pretty
01:07 long time , so we divide hours up into even
01:10 smaller units called minutes specifically , each hour is divided
01:14 up into 60 minutes . So a minute is 1/60
01:18 of an hour . And to get even smaller units
01:21 of time , we divide up each minute into 60
01:23 equal parts called seconds . So a second is 1/60
01:27 of a minute , going from the smallest to the
01:30 biggest . You should memorize that . 60 seconds equals
01:33 one minute . 60 minutes equals one hour and 24
01:37 hours equals one full day . There are some common
01:40 abbreviations for these units that you should know too .
01:43 D for days . H four hours , men for
01:47 minutes and s for seconds . One of the earliest
01:51 devices for measuring time was called a sun dial .
01:54 It was basically a flat disk with a triangular pointer
01:57 in the middle As the earth rotated , the shadow
02:00 of the pointer would slowly change position and pointed different
02:03 markings on the edge of the circle , which represented
02:05 different times of the day . Are you tired of
02:08 the man telling you what time it is ? Now
02:10 ? You can get the time directly from the source
02:13 with the Sun O Matic 3000 . Just put it
02:15 in your yard and let the sun do the rest
02:18 for daytime use only not for use in cloudy conditions
02:21 . Times may not be accurate , depending on latitude
02:24 , some calibration required . The sun dial was a
02:27 really cool invention , but it had a lot of
02:29 limitations . Fortunately , there are much better modern devices
02:33 for keeping track of time . In fact , you
02:35 probably have one hanging on your wall at home or
02:37 in your classroom that looks something like this . This
02:40 traditional clock is designed to keep track of three different
02:44 units of time , seconds , minutes and hours .
02:48 That's why it has three different pointers or hands .
02:51 This thin red pointer is called the second hand because
02:54 it helps us keep track of seconds . This thicker
02:57 black pointer is called the minute hand because it helps
02:59 us keep track of minutes , and the shorter black
03:02 pointer is called the hour hand because it helps us
03:05 keep track of ours . Oh , and the circle
03:07 itself is often referred to as the clock face .
03:11 Oclock is actually kind of complicated , so to see
03:14 how it works , let's start with the version that
03:16 has everything removed except for the second hand and the
03:19 60 marks around the edge of the clock face .
03:21 Why are there 60 marks around the edge you'll see
03:24 in just a minute , starting with the second hand
03:27 , pointing straight up at the top of the clock
03:29 face . If the hand rotates to the right so
03:32 that it's pointing at the next mark , that means
03:34 that one second of time has passed . So we'll
03:37 label that mark one . The next mark will be
03:39 to the next three and so on until we get
03:42 all the way around and the hand is pointing straight
03:45 up again . That means exactly 60 seconds have passed
03:49 . So label that mark with a 60 . But
03:51 we could have labeled that top mark as a zero
03:54 , since that's where the sequence of numbers starts .
03:56 That's one of the cool things about o'clock being a
03:58 circle . The starting point is also the ending point
04:01 . So as soon as you hit the end 60
04:04 seconds , you're also back at the beginning zero seconds
04:08 . The reason that there's exactly 60 marks around the
04:10 edge of a clock is that 60 seconds equals one
04:13 minute . That means when the second hand makes one
04:16 full rotation around the clock face , exactly one minute
04:20 of time has passed , and if it makes two
04:22 full rotations , two minutes of pest and if it
04:25 makes three rotations , three minutes of pest and so
04:28 on . So even though the second hand is counting
04:31 seconds , it also helps us know how many minutes
04:33 have passed . But how are we going to keep
04:35 track of those minutes ? Well , that's where the
04:37 minute hand comes in handy , so let's add it
04:40 back to our simple clock . The minute hand works
04:43 a lot like the second hand . The main difference
04:45 is that it travels slower 60 times slower to be
04:48 exact , since a minute is 60 times longer than
04:52 a second . While the secondhand travels all the way
04:55 around the clock , the minute hand only moves from
04:58 one mark to the next . For example , if
05:01 both hands start straight up at the top of the
05:04 clock , while the secondhand travels all the way around
05:07 the clock , counting out 60 seconds as it goes
05:10 , the minute hand will only travel from 0 to
05:13 1 , because only one minute has passed . Then
05:16 , while the secondhand travels around the clock is second
05:19 time , the minute hand will travel from 1 to
05:22 2 because two minutes have passed notice that we use
05:25 the exact same set of 60 marks to measure both
05:28 seconds and minutes . But the different hands tell you
05:32 how to interpret those marks . If the second hand
05:34 is pointing at the fifth mark , it means five
05:37 seconds . But if the minute hand is pointing at
05:39 the fifth mark , it means five minutes . Of
05:42 course , being able to quickly tell which market hand
05:44 is pointing at is tricky , since clocks don't usually
05:48 number every mark . So to make it easier ,
05:50 the inventors of the clock decided to make every fifth
05:53 mark bigger or bolder , so you could skip count
05:56 by fives , which helps you quickly tell where the
05:58 hands are pointing . For example , if the second
06:01 hand is pointing right here , you can tell right
06:04 away that it's just two marks passed the 15th mark
06:07 . So it's pointing at 17 seconds since 15 plus
06:10 two equals 17 . And if the minute hand is
06:13 pointing here , you'll quickly see that it's just two
06:16 marks before the 40th mark . So it's pointing to
06:19 38 minutes since 40 minus two equals 38 . These
06:23 bigger marks are so helpful for reading a clock that
06:26 you should memorize the values they represent when the minute
06:29 or second hands are pointing at , especially the ones
06:32 that break the clock face up into quarters . Okay
06:36 , so now you know how the second and minute
06:38 hands work . When the second hand goes all the
06:40 way around , the minute hand increases by one .
06:43 But what happens when the minute hand eventually goes all
06:46 the way around and has counted out 60 minutes ?
06:49 If you paid close attention at the beginning of the
06:51 video , you probably know the answer . 60 minutes
06:55 equals one hour . So when the minute hand goes
06:57 all the way around , it means one hour of
07:00 time has passed . That means we're going to need
07:02 to add the hour hand back to our clock to
07:04 keep track of ours . But the hour hand doesn't
07:07 use the 60 marks on the edge of the clock
07:09 in the same way that the other hands do .
07:11 Why not ? Because there aren't 60 hours in a
07:14 day . There's only 24 . If the hour hand
07:16 just moved one mark for each hour , it wouldn't
07:19 make it all the way around the clock face in
07:21 a single day . Ideally , it would be nice
07:23 if we could have the hour hand go around the
07:25 clock once per day . But unfortunately , the 60
07:29 marks on the clock don't divide evenly by 24 .
07:32 That means you can't find a nice hole . Number
07:35 two . Skip count to 24 by , for example
07:38 , if you had the hour hand move to marks
07:40 for each hour , it's still want to go far
07:42 enough to make it all the way around the clock
07:44 in a day . And if you had it moved
07:46 three marks for each hour , it would go too
07:48 far . Fortunately , the inventors of the clock came
07:51 up with a clever compromise . They realized that even
07:54 though 24 hours doesn't divide evenly into 60 half of
07:58 a day , which is 12 hours , does 60
08:02 divided by 12 equals five and five is a nice
08:05 hole . Number two skip count by in fact ,
08:07 we already have bigger marks on our clock face every
08:10 five marks , which makes this idea really convenient If
08:14 the hour and minute hands start pointing straight up as
08:17 the minute hand goes all the way around the clock
08:19 , counting out 60 minutes , the hour hand will
08:22 slowly move past the first five marks , which means
08:25 one hour has passed then , as the minute hand
08:28 goes all the way around again . The hour hand
08:31 will move past the next five marks , which means
08:33 that two hours will have passed . This continues on
08:36 until the hour hand makes it all the way around
08:38 the clock and gets to the 12 hour mark back
08:40 at the top , just like with minutes and seconds
08:43 . The top position on the clock represents both the
08:46 beginning and the end of the counting sequence for minutes
08:49 and seconds . That means the top position represents both
08:52 60 and zero before the hour hand . That top
08:55 spot represents both 12 and zero . Of course ,
08:59 12 hours is only half the day . That means
09:01 in order for this clock to keep track of one
09:03 whole day , the hour hand needs to make to
09:06 full trips around the clock face . It counts out
09:09 12 hours the first half and then 12 more for
09:12 the second half . That means that each hour mark
09:14 is passed twice during the day , so we need
09:16 a way to distinguish them . And that's where I
09:19 am and PM can help us out . AM stands
09:22 for Anti Meridian , which just means before Noon and
09:26 PM stands for post Meridian , which just means afternoon
09:30 to see how the AM and PM labels work with
09:32 the 12 hours on our clock face to keep track
09:35 of a full day . Let's compare them to a
09:37 simpler modern timekeeping system called the 24 hour clock Mhm
09:41 . In that system , the 24 hours of the
09:43 day are just labeled zero through 23 . In other
09:47 words , at the beginning of the day , which
09:49 happens to be midnight , the time is zero hours
09:52 and zero minutes . As the hours passed by ,
09:55 you eventually get to the time 23 hours and zero
09:58 minutes , which means you're starting the 24th hour of
10:00 the day . After that last hour passes , it
10:03 starts back over at zero for the next day on
10:06 the traditional clock face at midnight , the hour hand
10:09 is pointing straight up at 12 . So instead of
10:12 calling at zero o'clock , we call it 12 midnight
10:15 or 12 a.m. Then the 1st 12 hours of the
10:18 day pass one by one , using the A M
10:20 label , 1 a.m. Two a . M . 3
10:23 a.m. on up to 11 a.m. When the hour hand
10:27 gets to 11 a.m. you're beginning the 12th hour of
10:29 the day , and as soon as the 60 minutes
10:32 of that , our passed by the hour hand will
10:34 be back up at the top of the clock ,
10:36 pointing at 12 again . But this time we're going
10:38 to call that position 12 noon or 12 PM Then
10:42 the clock continues on counting out the last 12 hours
10:45 of the day using the PM label . One PM
10:48 two PM three PM on up to 11 p.m. When
10:52 the hour hand gets to 11 p.m. It means you're
10:54 starting the last or 24th hour of the day .
10:57 After that . Last hour passes the hour hand points
11:00 straight up at 12 again , which is now 12
11:02 midnight or 12 a.m. and the cycle starts all over
11:06 for the next day . Let's briefly summarize what we've
11:09 learned about the clock so far before moving on to
11:11 a few examples of telling time . Each time the
11:15 second hand rotates all the way around the clock ,
11:17 one minute has passed and the minute hand advances one
11:20 mark each time the minute hand goes all the way
11:23 around the clock , one hour has passed and the
11:25 hour hand advances five marks each time the hour hand
11:29 goes around the clock , 12 hours have passed ,
11:31 which is half a day . The 1st 12 hours
11:34 are called the A M hours , and the 2nd
11:36 12 hours are called the P M hours . If
11:39 this is your first time learning how Oclock works ,
11:41 it might seem kind of confusing . But don't worry
11:45 . The more you practice looking at a clock and
11:47 noticing which hands are pointing to , which marks the
11:49 faster you'll be able to tell what time it is
11:51 . In fact , let's practice that a few times
11:54 now so you can get the hang of it before
11:55 doing more practice on your own . Let's start with
11:58 this clock face . What time of day is it
12:00 displaying ? The hour hand is past the seven ,
12:03 but not to eight yet , so the hour is
12:05 seven . The minute hand is pointing directly at the
12:08 three , but remember , that's the third group of
12:11 five marks for counting minutes or seconds . So it's
12:13 pointing at the 15 minute mark , and the second
12:16 hand is pointing straight up , which is the zero
12:18 second mark . So the time is seven hours ,
12:21 15 minutes and zero seconds . Oh , and that's
12:25 how you write time . Using numbers . You write
12:27 the hours first , then minutes and then seconds if
12:29 necessary , all separated by colons . What is this
12:33 ? 7:15 a.m. or P M . To tell that
12:36 you need to know if it's before noon or afternoon
12:39 . The halfway point of the day Books or tests
12:41 sometimes give you a clue about whether the time is
12:44 before noon or afternoon so you can figure it out
12:46 . Or they may just tell you if it's a
12:48 M or P . M , in this case ,
12:50 were given the clue . Soon the sun will set
12:53 , so we know it's afternoon or 7 15 PM
12:57 How about this example ? What time is this clock
13:00 displaying the hour hand is passed the 10 , but
13:03 not to 11 yet , so the hour is 10
13:06 . The minute hand is just past the fourth ,
13:08 which is the 20 minute mark , but it's not
13:10 too 21 yet , so the minute is 20 and
13:13 the second hand is pointing at the nine , which
13:15 is the 45 2nd mark . So the time is
13:18 10 hours , 20 minutes and 45 seconds , and
13:21 this time you're given the clue . It's a beautiful
13:24 morning . So you know that it's 10:20 a.m. Since
13:27 morning comes before noon . And finally , what time
13:30 of day is this clock display ? The hour hand
13:32 has passed the 12 , but not yet to the
13:34 one . So it's 12 something . The minute hand
13:37 is pointing just a hair past the six , which
13:40 is the 30 minute mark . So the minute is
13:42 30 and there's no second hand on this clock .
13:44 Not all clocks had them , so we're only concerned
13:47 about the time to the nearest minute . In this
13:49 example . Lastly , this clock faces actually labeled a
13:53 m . So we know that the time of day
13:55 is 12:30 a.m. which is 30 minutes past midnight .
13:59 All right , so now you know how seconds ,
14:01 minutes , hours and days all relate to each other
14:05 . You also know how a traditional clock works to
14:07 help you keep track of these units of time as
14:09 they pass . By and importantly , you've learned how
14:13 to read the hands of the clock face so that
14:15 you can tell what time of day it is .
14:16 Remember that the way to get good at anything in
14:19 math is to practice , So be sure to practice
14:21 telling time on your own as always . Thanks for
14:24 watching math antics , and I'll see you next time
14:27 . Learn more at math antics dot com .
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