Lesson Video: Time in Seconds | Nagwa Lesson Video: Time in Seconds | Nagwa

# Lesson Video: Time in Seconds Mathematics

In this video, we will learn how to measure time in seconds using stopwatches and clocks with a second hand, calculate durations in seconds, and make simple conversions between minutes and seconds.

16:18

### 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.

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