Hello and welcome to NinetyEast. In this video we'll be looking at speed; which is something you're probably quite familiar with intuitively.
You'll know that the car will travel usually a lot faster than a bike will.
But you'll know that most journeys will be varying in their speed - because we have things like traffic lights, or maybe we stop to pick someone up.
Sometimes it's useful to know the average speed with which a car or person has travelled during a journey. To calculate that we need to know
a couple of different things: the distance that was travelled during the journey, and
the time it took to travel that distance.
If we've got that information we can plug it into this formula right here.
You'll see that we have a couple of units in here already: distance travelled in metres, and time taken in seconds.
That's because in science we tend to report average speed in metres per second.
But in everyday life we tend to report it in kilometres per hour, or miles per hour - and all of those are correct,
so long as an exam question or an assignment question hasn't specifically asked for one over the other.
Let's look at a couple of calculations in metres per second.
This person has run eight kilometres in 53 minutes.
We want to calculate the average speed in metres per second. We've got the formula here. If you'd like to pause and calculate the ave. speed
yourself, please do so and then you can join us again to see if you've got the same answer.
Okay let's get started. So we want the distance travelled in metres;
this means that we need to multiply eight kilometres by 1000 metres/km. Similarly we want the time
taken in seconds. So we need to multiply 53 by 60 seconds per minute.
It's REALLY useful to keep all the units in the calculation because it's like a second check to make sure you've got the numbers correct.
Because we've got 'km' both in the numerator and denominator, we can cross those off. And because we've got minutes in the same
numerator and denominator we can cross them off as well.
This leaves us with metres in the numerator and seconds in the denominator.
So we multiply 8 x 1000 m to get the distance travelled (which is 8000 m).
And we multiply 53 by 60 seconds to get the time it took to travel those 8000 m.
Grab your calculator and divide 8000 by 3180 seconds,
and you'll get the average speed of this person's run and that's 2.5 metres per second.
Let's compare this to a car, which we think intuitively will be faster. But we need to calculate it just to be sure.
So we've got a car that's travelled a distance of 30 km in 25 minutes.
It does (just from intuition) sound faster than 8 km in 53 minutes, but let's work out how much faster.
So the average speed is going to be the distance travelled in metres (so 30 km x 1000 m / km),
divided by the time it took (so 25 minutes x 60 seconds/minute).
Again, we just cross out the common units in both the numerator and denominator.
That's km here, and minutes in the denominator.
OK so let's work out the average speed : thirty multiplied by a thousand (30,000 m), divided by twenty-five by sixty (1,500 seconds).
So let's do that in our calculator.
You should get 20 metres per second.
So we were correct: the car was travelling faster than the runner
and it's a little under ten times faster;
probably about eight times faster.
How do I know that? Well I just divide 20 by 2.5, and you'll get eight.
So just to review, the speed of an object is how fast it's travelling at any one point in time.
Because objects will change their speed over time, we tend to calculate the average speed of a journey.
And that's calculated using this equation here (distance / time taken).
Hopefully you've found this video useful. Don't forget to check out the associated worksheet linked in the description box below.
The worksheet's got four different examples to work through with answers in m/s, km/hr and mi/hr.
If you have any questions please don't hesitate to send us them either directly through YouTube, or via email to NinetyEastTV@gmail.com. Thanks!