Video Transcript
Time in Seconds
In this video, we’re going to learn
how to measure time in seconds using stopwatches and also clocks with a second
hand. We’re also going to learn how to
calculate durations in seconds and make simple conversions between minutes and
seconds.
What is a second? It’s a unit of measurement that we
can use to measure time, short periods of time in fact. Did you know you’ve been watching
this video for about 40 seconds already? Have you any idea how long a second
takes? What does it feel like? It’s really not that long at
all. It’s about the length of time it
might take you to click your fingers or how long it takes you to blink. Our first experience of seconds is
probably when we come across an analog clock or watch that has a second hand on
it. On this clock, it’s the thin red
hand we can see here.
And whilst we sometimes might see
the minute hand move, probably never see the hour hand move, it’s just so slow, the
second hand is the hand of the clock that we can see moving really clearly. And if you listen carefully, you
might even hear a ticking sound as this second hand moves. Each tick means that one more
second has gone by. Tick, tick, tick, tick. Now the rhythm of that animation
isn’t quite the same as seconds going by, but it should give us a good idea as to
what the second hand does. It’s a steady movement at about
this sort of speed: one, two, three, four.
If we zoom in on the clock face, we
can often see small marks all the way around. And we’ve met these before, haven’t
we? We can think of them as marking out
the number of minutes in an hour. There are 60 of them, and that’s
one of the ways that we know that there are 60 minutes in an hour. But if we watch our second hand
carefully, we’ll see that it stops at each one of these marks. And it travels once all the way
around the clock face in a minute. So just like 60 minutes are the
same as one hour, 60 seconds are the same as one minute. And we can use this fact to help us
solve problems. Here’s a good example of one where
this fact is going to come in very useful.
48 seconds have gone by. How many seconds are left until a
minute passes?
In this picture, we can see one of
the pieces of equipment we can use to measure periods of time, a stopwatch. Now there are two main kinds of
stopwatches. There are those that are digital
and those that have a hand that goes all the way around the clock face, like this
one. I suppose we could call it an
analog stopwatch. This particular stopwatch has got a
red hand. And we can see that it’s traveled
almost all the way around the clock face. Can you see where it started? Well, the blue area shows where
it’s traveled. So we can see it began by pointing
directly upwards to the number 60. Then someone must have pressed the
start button to start timing. The hand then moved around the
clock face. And now it’s at this point here, 48
seconds later.
Obviously, we can see in the first
sentence that 48 seconds have gone by. But we could also tell this by
looking at the clock face. Do you notice anything about the
numbers on this clock face? Instead of being numbered from one
to 12 as they are on an analog clock, our stopwatch is numbered in multiples of
five, all the way up to 60. This is because this stopwatch is
used for measuring very short periods of time, in seconds. So even if we didn’t have this
first sentence to help us, we could still read our stopwatch and see that 48 seconds
have gone by. Five, 10, 15, 20, 25, 30, 35, 40,
45. Now we’ve made all the jumps of
five seconds that we can. If we made another, we’d have gone
too far.
But we have got some more notches
to count. So let’s carry on counting from 45
in ones. 46, 47, 48. So we can see from our second hand
that 48 seconds have passed. But we are asked how many seconds
are left until a minute passes. To solve this problem, we need to
remember a fact about seconds and minutes. One minute is equal to 60
seconds. That’s why our stopwatch goes all
the way up to 60. You know, we could even alter the
wording of our question here. Because both measurements are in
seconds, it’s much easier to work out the answer. If one minute is worth 60 seconds
and 48 of those 60 seconds have gone by, the answer to our problem is going to be
the difference between 60 and 48.
We could find the answer by using
subtraction, maybe starting at 60 and counting backwards until we get to 48. Or let’s use our stopwatch to help
us. Let’s start at 48 seconds and see
how many jumps we need to make to get to 60. And you know, we could do it in two
jumps. First, we could start on 48 and
jump to a nice round number. 48 plus two takes us to 50. And from here, it’s only 10 more
seconds until we get to 60. We’ve counted on two seconds and
then another 10. That’s 12 altogether. We know that 60 seconds are the
same as a minute. And so if 48 seconds have gone by,
we can work out how many seconds are left until a minute passes by counting on from
48 to 60. There are 12 seconds left.
We often use seconds to measure
very short activities. For example, both the men and the
women’s 100 meters world record times are about the length of time it would take
from the hand of our stopwatch to go from here to here. That’s not long at all, is it? To measure short activities like
this, we need a start time. And with a stopwatch like this,
it’s usually when the second hand is pointing straight upwards. But just as important is reading
the end time. And if we have both the start and
the end times, we can work out the duration. The duration is just how long
something lasts. In this example, we can see we
started at zero and 10 seconds have gone by. And the length of time it takes for
world record holders to run 100 meters is about 10 seconds.
Do you think you could answer a
question where we need to do this, not run 100 meters, but calculate duration? Let’s have a go.
Seconds are used to measure a short
period of time. Complete the following: In the
given activity, jumping jacks take what seconds.
This question is all about how long
it takes to do some jumping jacks. These are a great form of exercise,
but you probably wouldn’t want to be doing them for too long. Perhaps that’s why, in this
question, the activity’s been measured in seconds. As we’re reminded right at the
start here, seconds are used to measure short periods of time. And we need to find out how long
these jumping jacks have taken. To help us complete this sentence
and find how long these jumping jacks take, we’re given three stopwatches. Or really, as we’re going to find
out, it’s the same stopwatch but shown three times.
The first stopwatch is labeled
start time. This shows us what the stopwatch
looked like when the activity began. And if we want to work out how long
something’s taken, we need to know when it started. Our second picture shows us what
the stopwatch look like at the end of the activity. Knowing the end time is just as
important as knowing the start time with a question like this. Our final picture really just puts
these two facts together. The part shaded blue shows us where
the hand on the stopwatch has moved. We can see both the start time and
the end time. And it’s this third picture that’s
probably going to be most useful to us. It shows us exactly how long the
activity’s taken or, if we want to use a mathematical word, its duration.
We can see that the hand on the
stopwatch started when it was pointing directly upwards at the number 60. There are 60 seconds in one minute,
and that’s all the way around the clock face. So we really can think of this
clock face as going from zero all the way around to 60 seconds. And when the second hand points to
this number at the very start, it’s not pointing to 60, but really to zero. This is really useful to us. Because we’re starting at zero, we
just need to look at the end time, and this will tell us how long the activity’s
taken. If we look at the end time, we can
see that the second hand has moved just past the number 35. There are five seconds between each
number on the clock face.
But these are all labeled, so we
don’t need to start at zero and count in fives. We can just go straight away to 35
and then count on. Looks like we’re going to have to
count on another two more seconds. So we’ll say 35 and then 36,
37. In order to find out how long
something lasts or its duration, we need a start time and an end time. We’ve read both the start and the
end times on our stopwatch. And we’ve used these to find out
the duration of the jumping jacks activity. In the given activity, jumping
jacks take 37 seconds.
So far in this video, we’ve talked
about how we can measure time in seconds using the second hand on a clock face and
also using an analog stopwatch like this. But you know, there’s one more
piece of equipment we could look at. And that’s a digital stopwatch,
like this. Just like the analog stopwatch that
we’ve looked at, we’d press a button at the top here to start timing. Then we’d expect the numbers on the
screen to start changing as the number of seconds goes by, one, two, three, and so
on. And then when we want to finish
timing, we’d press the same button again to stop the timer so we can read the time
from the screen.
The reading on this particular
digital stopwatch is a little bit like the reading we might get on a digital
clock. We’ve got two digits and then two
dots and then two more digits. If you remember from our work with
digital clocks, the numbers on the right of the two dots are usually the number of
minutes that have gone by in an hour. But on this particular stopwatch,
the numbers on the right of these two dots are the number of seconds that have gone
by. So as soon as we press the start
button, we’ll start to see these numbers change: one, two, three, and so on. And this number will steadily
increase until we get to 59. But because we know that there are
60 seconds in a minute, it’ll show us that one minute has gone by on the left of
these two dots. And, of course, the number of
seconds will keep on taking away until we stop the clock. Let’s test what we’re like at
reading these digital stopwatches and try a question where we need to do this.
A tutor gives his two students a
quiz. The two stopwatches show the time
taken by Liam and Jacob to solve the quiz. What is the time taken by each of
them, respectively, in seconds? Who finishes the quiz first?
This question describes two
students, Liam and Jacob, who’ve been given a quiz to do by their tutor. It seems like it was a little bit
of a competition because we’re shown two stopwatches which show the time taken by
each of these two students to finish the quiz. The first part of our problem asks
us to find the time that it takes both Liam and Jacob to solve the quiz in
seconds. This word “respectively” here isn’t
as complicated as it looks. It just means in the order that
they’ve been mentioned. So when we work out both times, we
need to say Liam’s time first and then Jacob’s. Now, when both students started the
quiz, the stopwatch would’ve looked a little bit like this.
As soon as the tutor started
timing, the digits to the right of the two dots would’ve slowly increased, one, two,
and so on. These digits show the number of
seconds that have gone by. And we know with stopwatches like
this, the two digits to the left of the two dots show the number of minutes that
have gone by. So we can see straightaway how long
each student took. Liam’s stopwatch shows one minute,
10 seconds. And Jacob’s shows one minute, 25
seconds. But there’s a slight problem here
because we’re told to give the times in seconds. In other words, we’re gonna have to
convert between a time given in minutes and seconds to just seconds.
And to help us, we’re going to have
to remember a fact about minutes and seconds. This bar model shows Liam’s time of
one minute, 10 seconds. Do you remember how many seconds
there are in a minute? One minute is the same as 60
seconds. And 60 seconds plus another 10
seconds equals 70 seconds altogether. And we can use a similar bar model
to help convert Jacob’s time into seconds. He took one minute, 25 seconds,
which is the same as 60 seconds plus 25 seconds, which is 85 seconds in total. And as we’ve said, we need to give
these times in the order that they’re mentioned, so Liam’s time and then
Jacob’s.
In the final part of the problem,
we’re asked, “Who finishes the quiz first?” And because we’ve converted both
times into a number of seconds, we can just compare the numbers. 70 is less than 85. And when we’re talking about time,
the quicker we do something, the less time it takes or the less number of seconds it
takes. Liam’s taken less time, and so he
finishes the quiz first.
This question tested our ability to
read digital stopwatches but also how to convert between a time in minutes and
seconds to a time just in seconds. We knew that there were 60 seconds
in a minute, and this helped us. The time taken by each student to
complete the quiz, in seconds, in the order that they’re mentioned, is 70 seconds
and 85 seconds. And so we know the student who
finishes the quiz first is Liam.
So what have we learned in this
video? We’ve learned how the measure time
in seconds using stopwatches and clocks with a second hand. We’ve also learned how to calculate
durations in seconds and convert between minutes and seconds.
