Hello there.

I'm Mr. Forbes and welcome to this lesson from the Measuring and calculating motion unit.

In the lesson we're gonna look at how you can automatically measure the speed of an object using radar guns and light gates.

By the end of this lesson, you're going to be able to explain how speed cameras operate using radar guns and markings on roads.

You are also going to be able to carry out an experiment using a light gate to automatically measure the speed of a moving dynamic trolley.

There are just three key phrases that we need for this lesson.

The first of those is speed camera, and a speed camera is used to measure the speed of vehicles moving along roads, and it uses radar and photographs.

The second is a radar gun.

And a radar gun is a device that sends out radio waves which reflect off objects and you can use those reflections to measure the speed of things.

And the third is light gate.

And a light gate is used to measure the speed of things like a dynamic trolley as it passes through it.

And here are the explanations of those keywords again, and you can return to this slide at any point during the lesson.

The lessons in three parts and in the first part we're gonna be discussing how we can use waves to measure the speed of an object and that will lead us to the idea of radar guns being used in speed cameras.

And the second part we're gonna explain and plan a lesson to use a light gate to measure the speed of a dynamic trolley in an investigation.

And in a third part we're gonna analyse the results of that investigation.

So let's go on with measuring speed using waves.

You probably already know that speed cameras are used to monitor vehicles on roads to see if they're over the speed limit or not.

And then they can use that information to generate fines for people who are speeding.

So those devices detect whether the vehicle is travelling too fast for the speed limit on the road and also capture evidence of that.

And a speed camera sign looks a little bit like this.

It's meant to be a camera pointing towards the car, warning you that if you're going too fast a photograph's gonna be taken off you and you're gonna be fined.

And also there'll be road limits like this 30 miles an hour for example, to inform the driver of what the speed limit is.

Speed cameras operate using radar guns.

Radar guns send out radio waves towards a moving vehicle and detect the reflections of them.

And a part of a camera setup that looks something like this.

The speed camera's got parts to photograph the car, but also a radar gun to fire out those radio waves towards the car.

The camera sends out those radio waves and they're reflected off any moving vehicle that's in front of the radar gun.

And that wave is then reflected by the moving vehicle and its frequency is shifted, so its frequency increases or decreases slightly depending on how fast the object it's reflected off is moving.

So that shift can be used to measure the speed of the vehicle.

You don't have to know how to calculate that shift, but you should know that the faster the car is moving, the greater that shift is.

So the camera will be able to determine how fast the car is moving.

The speed camera is also designed to generate the evidence to centre of the person who's speeding to prove that they were going over the limit.

And it does that by taking two photographs, a very small period of time apart when it's detected something that's speeding.

Those photographs capture the registration plate of the car so the owner can be identified and also capture the position of the car at two very similar times, just moments apart.

And we can use that information to calculate how fast the car was moving.

So you can see here I've got a photograph of a speed camera on a road near where I live.

And those markings on the road are one metre apart exactly.

So if we take photographs a fraction of a second apart, we can tell how fast the car is moving using the speed equation.

And that evidence is what sent it to the driver to show that they've been speeding and to enforce the fine, let's have a look at an example of the sorts of calculations we can do to prove whether or not a car is speeding.

So we've got a pair of speed camera photographs taken 0.

5 seconds apart, so half a second apart.

And they show that the cars travelled a distance of 9 metres according to those lines that are marked on the road 1 metre apart.

The speed limit on that road is 30 miles an hour and I've converted that to metres per second for you, it's 13.

4 metres per second approximately.

And we're gonna work out whether the car is speeding or not.

So we can use the information we've gathered to use the speed equations.

So speed is distance divided by time and we know the car has travelled 9 metres in a time of 0.

5 seconds.

So we substitute those values in, and then we can calculate the speed from that.

And the speed of this car is 18 metres per second.

As I said, the speed limit on the road is 13.

4 metres per second.

So this car is significantly over the speed limit.

It's time for the first check of the lesson, and what I'd like you to do is to determine whether a lorry is speeding.

So while lorry is travelling through remote way roadworks where the speed limit reduced to 50 miles an hour and that's 22.

4 metres per second.

And a pair of speed camera images showed that the lorry travelled 9 metres in not 0.

4 seconds.

I'd like you to show whether the lorry is speeding or not.

So pause the video, work out if it's speeding, and then restart please.

Welcome back.

Well, if we substitute those values into the equation like this, we find the speed of the lorry is 22.

5 metres per second.

So it's very slightly over the speed limit.

So technically it is speeding, it is breaking the law.

Well done if you've got that.

In laboratory experiments, we rarely use radar but we can use sound waves to measure the speed of moving objects.

And we do that by having a motion sensor.

A motion sensor is usually connected to a computer to record the values it measures.

So I've got a sound sensor here, positioned to measure the motion of a dynamic trolley.

And what it does is it sends out high frequency ultrasound waves so you can't hear them, but they are very similar to normal sound waves.

They're sent from the transmitter and they travel forward, hit an object and reflected off it and the echoes are detected.

And from those echoes we can find the speed of the object in two different waves, ways, sorry.

One of them is frequency shifts, just like the radar gun, the frequency of the sound wave will change slightly depending on how fast the object's moving.

Or we could just use timing information, how long it takes the sound wave to reach the object or reflect back to the transmitter and detector.

So there are two ways we can use sound waves to measure the speed of objects.

So let's have a look at that in a little bit more detail.

An ultrasonic sound sensor then is connected to a computer or data logger to measure speed and we can position it on a desk like this.

So the blue box there represents the ultrasound transmitter and it's connected to a laptop computer.

And it's gonna send sound pulse out towards a solid fixed object.

In this case, I've just got that sort of grey board at the end.

So the ultrasound transmitter sends out a sound pulse and that travels towards some object.

When it reaches the object, it's going to reflect off it.

So you're going to get reflected sound pulses travelling back towards the transmitter, which also acts as a detector, and it detects those sound waves.

And the time taken for the sound to travel to the object and back again is measured and recorded.

And as we know the speed of sound, the computer can then use that timing information to calculate the distance that that sound pulse has travelled to the object and back and it uses that to calculate the distance the object is away based upon the speed of time.

If the object was moving, it could also use the changes in the information to calculate speeds.

Let's see if you can do a calculation involving the speed of sound.

So I've got an ultrasound sensor and it's sending out sound pulses to a moving object.

The pulses take 0.

002 seconds to travel to the object and back again to the transmitter.

How far away is the object from the sensor? You'll need to know that the speed of sound in air is 330 metres per second.

So I'd like you to pause the video, try and work out the distance the object is away from the transmitter and then restart please.

Welcome back.

Well, hopefully you selected 33 centimetres.

Now I'll show you why that's the correct answer and not 66 centimetres which many people select.

So first of all, I'll write up the equation for speed.

Speed equals distance divided by time or v equals x divided by t.

If I rearrange that I get distance equals speed times time.

Substituting in the speed of sound, 330 metres per second and the time, 0.

002 seconds.

I get a distance of 66 centimetres.

But remember the sound wave has travelled to the object and back.

So it's travelled twice the distance to the object.

Therefore the distance just to the object is half of that, which is 33 centimetres.

Well done if you've got that.

Okay, it's time for the first task of the lesson.

Now what I'd like you to do is to answer two questions based upon speed and time measurements.

So pause the video, read through these two questions and answer each part of them and then restart please.

Welcome back.

Well, let's have a look at the answers to those.

Well, first of all, we were asked to work out the speed limit in metres per second.

So the road had a speed limit of 60 kilometres an hour.

I need to convert that to metres per second and I can do that using speed equals 60 kilometres divided by one hour.

That's 60,000 metres divided by, well, one hour in seconds is 3,600 seconds.

So the speed limit on the road is 16.

7 metres per second.

Then we can work out whether or not the motorcycle is speeding like this.

We can gauge speed is the distance it travels divided at a time.

It travelled eight metres in north 0.

4 seconds, and that's 20 metres per second.

So this motorcycle is above the speed limit.

It is speeding.

For the second question, we were asked to find out how deep a lake was using the speed of sound in water and a time measurement.

And again, we can use the speed equation for that rearranging it.

So we get the distance is the velocity times the time.

The velocity is 1,500 metres per second and the time is 0.

003 seconds.

Then lake is, was a distance travel to 45 metres.

But remember, that's the distance to the bottom of the lake and back to the sensor.

So the depth of the lake is half of that value.

So we get depth of 22.

5 metres.

Well if you've got those answers.

Now it's time for the second part of the lesson.

And in it we're going to be planning and carrying out an experiment using light gates to measure the speed of an object.

Light gates are used in a range of motion experiments to measure the speed of moving objects and they look like this.

Basically they are sort of hoop shape and objects pass between the two ends of that hoop.

The light gate has a transmitter on one side of the gap, let's say this one, and a receiver on the other side of the gap, over here.

And a beam passes between those, an invisible beam of infrared radiation and objects like this slide between the gap in the transmitter and receiver interrupting that beam and data loggers can measure that interruption time and use it to calculate speeds.

We use light gates in motion experiments because they can measure times very accurately with a very high precision.

The gate produce a small infrared beam between transmitter and receiver like this.

So we have a transmitter and a receiver and the transmitter sends out the beam and the receiver's detecting it.

If an object passes between the transmitter and the receiver, something like a ball falling through the gap that breaks the beam, a beam can't reach a receiver and the light gate automatically turn on an electronic timer.

So as soon as that beam is broken, the timer turns on.

But then as the object moves past the beam and it's not interrupting it anymore, the timer automatically stops.

So what we've got is a very precise, very accurate time for the object to pass between the transmitter and the receiver.

And that time is recorded and we can use that to work out its speed of the object.

So let's see how we can use that idea to measure the speed of a trolley moving down a slope.

So I've got a view from above here and I've got a light gate that's positioned on both sides of a ramp.

And as you can see there, there's an infrared beam which I've drawn in red between the transmitter and the receiver.

If I've got a trolley with a piece of card mounted on the top, when the trolley reaches the light gate, that piece of card will interrupt that beam and it'll cause the timer to start.

Then a short time later the trolley will have moved through the light gate completely and be on the other side and the beam is no longer being broken.

So the time is gonna automatically stop as soon as that piece of card has passed through the light gate on the top of the trolley though.

So what we're going to get is a timing measurement for how long it took that piece of card to completely pass through the light gate.

So we have a distance and we have a time.

Okay, let's see if you can calculate the speed of a trolley as it passes through a light gate based upon some measurements.

So I've got a trolley here and it's gonna pass through the light gate, which is mounted on the stand there.

And the light gate record a time of 0.

40 seconds for that trolley to pass between.

And as you can see on top of the trolley is a length of card that's not 0.

2 metres long.

What I'd like you to do is use that time information and that length of information to calculate the speed of the trolley.

So pause the video, point that out and restart please.

Welcome back.

Well, hopefully you selected 0.

5 metres per second though.

And the way we work that out is, well we've got the distance of knot 0.

2 metres, that's the card length and a time of not 0.

40 seconds.

We have the speed equation, speed equals distance divided by time or v divided by t, substitute that length and that time in there and we get a velocity of 9.

5 metres per second.

Well done if you've got that.

Now let's get on to planning our experiment and we're gonna be using a dynamic trolley.

And as you know, if we place it on a ramp.

the ramp's gonna put a force on it and it's going to speed the trolley up.

So I've got a trolley here and I've mounted a piece of card on the top of it for using in my timing experiments.

And as it's on the ramp, there's going to be a forward force acting on it due to the ramp that's mount there.

There's also going to be a very small frictional force acting backwards though, but the resultant force of those two, the difference between the forward force and friction, that's going to cause the trolley to accelerate as it rolls down.

And we measure the speed of that trolley near the end of the ramp with a light gate.

What I'm gonna ask you to do is to use light gate to investigate whether the speed of a trolley rolling down the ramp is affected by its overall mass.

So we're gonna have an arrangement like this where I've got a trolley placed upon a ramp and got a car mounted on the top of it and there's a forward force acting on it and we are going to position some extra masses on the trolley and that's going to increase the overall mass of it.

Increasing the mass of the trolley is going to increase the forward force acting on it.

We're going to see whether that actually affects how fast the trolley is travelling by the time it reaches the end of the ramp.

Now to make the test fair, we're going to make sure that only the mass of the trolley is gonna be changed during the experiment.

So we're going to have a fixed height of the ramp.

So we're not gonna adjust how slow it is or how til it is.

So we need that constant height there.

We're also going to need a constant distance for the trolley to roll down before it reaches the light gate.

So we need that to be a fixed distance as well.

Now I'd like you to identify the dependent, independent and control variables for this experiment.

So match each variable to the type of variable for the experiment by writing the letters.

Use D for dependent, I for Independent and C for control.

And do that for each of those four variables I got there.

So pause the video, decide which type of variables they are and then restart, please.

Welcome back.

Well first of all, the mass of the trolley is the independent variable.

The independent variable is the variable we change deliberately during the experiment.

Then we have the distance between the start and end lines, and that's a control variable.

It's something that's got to be kept constant.

The gradient of the slope is also a control variable.

We need to keep that constant and the dependent variable.

the thing we measure, is going to be the speed of the trolley.

Well done if you've got those four.

Okay, now it's time for you to actually carry out the experiment.

I'd like you to follow the instructions that are shown here, setting up the equipment and all the other stages to see if there is a relationship between the mass of the dynamics trolley and the speed at the end of the ramp.

You'll need to record all of your information in a results table.

So I'll show you that, but you can return to these instructions if you need to as well.

So, here is the results table I'd like you to record all your values in and we'll be processing those results after the experiment so you don't need to calculate the mean time on speed at this stage.

So pause the video, carry out the instructions and complete the table and then restart please.

Welcome back.

Well your results should look something like these.

I've recorded the length of the card and I've recorded three sets of times for the different masses of the trolleys.

So well done if you've got something like this.

And now it's time for the final part of the lesson.

And what we're gonna do is take our results and see if there is actually a relationship between the mass of the trolley and its speed at the bottom of the ramp.

First thing we need to do with our results is to find the mean values for the time.

So we're gonna calculate those.

So we're gonna take our data table here where we've got three times for each run of the trolley and we're gonna calculate any means.

Any anomalous results in there should be crossed out and ignored for the calculations.

But looking at this data table, I can't see any anomalous results.

So we're gonna use all of them.

So we're gonna start with the zero grammes added to the trolley and calculate the mean time.

And to do that, we add up the three times and divide by three because there are three values.

So when I do that, I get a mean time of not 0.

23 seconds.

And I can repeat that process for the second row where I've added 100 grammes and that gives me a mean time of not 0.

23 seconds as well.

And finally, for the 200 gramme mass, I can find a mean add three together, divide by 3, it's not 0.

21 seconds.

Okay, I'd like to see if you can calculate a mean now.

So I've got a different set of data here.

And what I'd like you to do is to calculate the mean time using that set of data.

So pause the video, calculate the mean time, and then restart please.

Welcome back.

Well the mean time there was not 0.

21 seconds.

We had those three values together and then divide by 3.

Well done if you got that.

The next stage in processing our results is to calculate the speeds using the mean time and the length of the card.

And the length of the card in my example is 10 centimetres here or 0.

1 metres.

So to calculate the speed we can go through each row of the table one at a time.

So the first row where there's zero grammes added, I calculate the speed like this.

Speed equals distance divided by time.

The distance is the length of the card that's not 0.

10 metres.

And the time there is not 0.

23 seconds.

And that gives us not 0.

43 metres per second.

And a fill that in the table.

For the second row of the table, all you can see, the mean time is exactly the same, so the speed's going to be exactly the same.

So I'm just gonna write that directly in.

And for the third row of the table, I've got a slightly different mean time.

So we'll do the calculation again, and again a mean time here for not 0.

48 metres per second, and I fill that in the table as well.

So now I'd like you to calculate the speed for the following set of results.

And we've still got a 10 centimetre length card, 0.

1 metre, and you can see the mean times in the table there.

So pause the video, work up the speed of that trolley and then restart please.

Welcome back.

Well, your solution should look like this.

We've got speed equals distance divided by time and we've got 0.

1 metres over 0.

20 seconds.

And that gives a speed of 0.

50 metres per second.

Well done if you've got that.

Once we've calculated on the average speed, we can try to look if there's a pattern in the relationship between the speed and the mass added and the trolley.

So I've got a complete data table here with all the speeds for the masses added to the trolley, and I can look at that and try and reach a conclusion.

Now, sometimes we need to process the data by plotting graphs to see if there's a relationship, but this is a little bit more simple because if you look at the speeds, they're all basically the same.

The speed isn't really affected by the mass of a trolley.

So I don't really need a graph to reach your conclusion.

I can just state that the speed remains constant or roughly constant when the mass changes.

So now I can state my conclusion about the relationship between the mass of the trolley and the speed at the bottom of the ramp.

And based on a set of results like this, my conclusion is simply the speed of the trolley at the end of the ramp does not depend upon the mass of the trolley.

And now it's time for the final task with the lesson.

And the task is to plan and experiment.

So a pupil wants to use a light gate to measure the speed of a small ball after it's fallen through different heights.

So they're gonna be dropping that ball from different heights between 0.

25 metres and 0.

5 metres.

And I'd like you to write out a plan to describe how they can carry out that experiment.

As a third test, your plan should include a diagram, a method, a blank results table, and a description about how to find out if there is a relationship between the fall height and the speed.

So pause the video, write out your plan and restart, please.

Welcome back.

And your example diagram should look something like the diagram we've drawn here.

We've got a ball and we're gonna be dropping it from measured height so it falls through the light gate.

So the light gate's got to be directly below it.

And I've marked out a start and end line.

So we can measure distances with a metre rule there and everything's held together on a stand.

The instruction should look something like that.

You'll need to be measuring the diameter of the ball because you need to know how large it is as it passes through the light gate.

So you can get a distance and you'll need to establish what positions you wanna drop the ball from.

And each of the stages is listed there in numerical order.

So well done if your plan looks something like that.

And the results table you need should look something like this.

We've got different drop heights.

In the first column we're doing repeat measurements.

So I've got three times for each drop height, I've got space to calculate the mean time and I've got space to record the speed once I've calculated that.

And your plan to find a pattern should involve plotting a graph, comparing the speed to the drop height and the speeds, the dependent variable and the drop height of the independent variable.

Well done if you've got that.

And now we've reached the final part of the lesson where I give you a quick summary.

Our radar gun uses changes in wavelength in reflected radio waves to measure speed.

So that radar gun can measure the speed of moving vehicles as they pass nearby.

Speed cameras then use the information to trigger a camera which records two images a short period of time apart and that provides evidence of speed based upon marking on the road.

Light gates use the interruption of infrared beams to turn a timer on and off.

So in an object passing between it, it turns the beam off and then when the object's passed, the beam's back on again and that triggers a timer.

And that will give us precise timing measurements.

When we've got an object of no length passing through the beam of the light gate, we've got the length and the time measurements and both of those are then used to calculate the speed.

Well done from reaching the end of the lesson.

I'll see you in the next one.