# Lesson video

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Hello there, I'm Mr. Forbes, and welcome to this lesson from the Forces Make Things Change Unit.

This lesson's all about stopping and we're gonna be examining the forces and the distances involved in stopping a car.

By the end of this lesson, you're going to be able to describe the factors that affect the stopping distance of a car.

You're going to be looking at thinking distance, the distance you travel while you're thinking about stopping, and then the braking distance, the distance you travel when you brake the car.

So we're gonna explain what affects each of those factors.

These are the key words you'll need to understand to get the motion from the lesson.

Firstly, the reaction time.

And for driving, that's the time taken between noticing some sort of hazard and taking an actual action that will deal with the hazard.

And we have thinking distance, that's the distance a vehicle will travel before the driver has reacted to a hazard.

During that time, a car won't slow.

Then we've got braking distance, and that's the actual distance a car will travel after you've pressed the brakes on it.

And finally, stopping distance, which is the sum of those thinking and braking distances.

You can return to the slide at any point during the lesson.

The lesson's in three parts.

And in the first, part we're going to be looking at reaction time and how that affects the thinking distance, the distance of vehicle travels before action is taken to start stopping it.

Then we're going to look at the braking distance, the distance that the car will travel after some sort of action's been taken, such as pressing the brakes.

And finally , we're going to look at some of the factors that affect the brakes and how effective they are and the energy transfers that happen after you press the brakes.

So when you're ready, we're gonna start by looking at reaction times.

Most people will learn to drive, and, as part of learning to drive, they'll learn how to spot potential hazards and how to deal with them, how to react to the different hazards.

Sometimes you can see those hazards from a long way away, such as in this photograph.

You can see hundreds of metres ahead and predict what's going on.

But other times, the situations are much more complicated and hazards can happen very close, very nearby to you, and you have to react as quickly as you can in both situations.

Any action that you're going to take to avoid hazard involves, first of all, noticing a dangerous situation.

So you see something happening, such as a child running out from behind a bus or another accident in front of you somewhere.

So you've noticed something's dangerous and you're gonna have to deal with it.

Then the next thing is thinking about how to react to it.

So you've got to mentally think, "There's a hazard ahead.

I need to stop the car.

I need to turn the car so I need to brake quickly." So you've got to notice something and think about it, and then you've got to start taking an action before the car can even start slowing.

So taking an action will involve, well, if you're driving a manual car, you've gotta move your foot from the accelerator and put the foot on the brake and push down.

So that takes a fraction of a second as well.

So each of those three stages are part of your reaction time during which the car isn't going to slow down because the brakes haven't even been pressed yet.

So we've got three different parts and that gives us a reaction time.

Let's see if you understand the idea of reaction time.

I've got a driver with a reaction time of 0.

8 seconds.

They're travelling at a steady speed of 30 miles an hour, and she spots a hazard.

What speed would she be travelling 0.

5 seconds after noticing the hazard? More than 30 miles an hour, 30 miles an hour, or much less than 30 miles an hour.

Pause the video, make a decision, and restart, please.

Welcome back.

Well, it'll still be 30 miles an hour.

At that time, 0.

5 seconds, that's less than the total reaction time, so the brakes won't be pressed yet.

So 30 miles per hour is the likely speed of them.

So the driver, seeing the hazard, but not pressed the brakes.

Wonder if you've got that.

As I mentioned earlier during the reaction time, the car is going to be travelling at a constant speed 'cause the brakes aren't acting.

It will travel a thinking distance.

So a distance before any action is being completed.

And we call that a thinking distance.

And that happens before the car even starts to slow.

We can calculate the distance by rearranging the speed equation.

So the distance you'll travel will be the speed you go in the time it takes to react.

So if I write that out in terms of driving a car, the thinking distance is the speed times the reaction time, speed limits in the UK in miles per hour.

What we're going to be doing, our calculations in metres and metres per second.

So we're going to have to do some (indistinct) conversion.

These are article speed limits.

Our town speed limits are 20 miles an hour around schools and things like that, and 30 miles an hour on most town roads.

If you're in the countryside, the national speed limit is 60 miles an hour.

And if you're on a motorway, the speed limit can only 70 miles an hour for cars, it's slower for some other vehicles.

One mile an hour is naught.

447 metres per second.

And we need to know what those speed limits are in metres per second.

So I'm gonna convert those for you.

We've got speed limits in metres per second and in town you can see I can calculate the speed limit.

It's 8.

94 metres per second.

I'm going to calculate the rest of the speed limits as well.

So we've got a town at 30 miles an hour, 13.

4, the countryside much faster, 28.

8 metres per second, and on a motorway, 31.

2 metres per second.

What I'd like you to do now is calculate the thinking distance for a car travelling at 26 metres per second if the reaction time for the driver was naught.

50 seconds.

So pause the video, work out the thinking distance and then restart please.

Welcome back.

Well the thinking distance there was 30 metres approximately.

We've got thinking distances, speed times, reaction time, that's 26 metres per second time, and naught.

5 seconds, that gives me 30 metres so that car won't have started slowing down for at least 30 metres if the driver reacts at the best possible time, their fastest reaction time.

Well done if you got that.

The thinking distance is directly proportional to the reaction time of a driver you saw in the previous calculation.

You multiply the reaction time by the speed.

So as you increase the reaction time, if you double it then you're going to double the thinking distance.

Doubling the speed doubles the thinking distance.

And I've got a table of thinking distance here for some different reaction times.

So I've got 10 miles an hour, 20, 30 up to 70 and I'm gonna show you the thinking distance for that reaction time of 9.

5 seconds.

If you react, and that's very fast reaction time, at 10 miles an hour, you are only going to travel 2.

2 metres before you start braking.

If you're going at 20 miles an hour, it's about twice as far, 4.

5 metres and 30 miles an hour, 6.

7 metres.

And so there are town road you can stop or start stopping I should say within 10 metres there.

But if you're going on a fast road, countryside or perhaps a motorway, we've got 14 metres before you even start stopping on a country road and 15 or over 15 metres if you're travelling on a motorway.

Now if you've got higher reaction time, which is much more likely, naught.

5 seconds is incredibly fast reactions for driving a car, then you're going to increase the thinking distance.

So let's have a look at those figures with a reaction time of naught.

8 seconds.

And as you can see, all of those increase substantially.

So a more realistic reaction time of naught.

8 seconds shows if you're travelling at 70 miles an hour, you're going to be travelling at 25 metres before you've actually started braking.

Typical reaction times are between 9.

5 seconds and 1.

5 seconds, depending on how hard you're concentrating.

So if you're fully concentrating on your driving, you'll be towards the lower end.

If you're not concentrating, you'll be at the higher end or even beyond that.

So training can help reduce those reaction times.

The more you drive and if you drive professionally, your reaction times will tend to be better, but there are a lot of things that will increase your reaction time.

If you are tired, you'll react much slower.

If you've been drinking alcohol and depressant, that will make your reaction times much higher.

And a range of drugs will also make your reaction times higher, medication for colds and coughs will have something that can actually make you feel tired and make your reaction times longer.

They're also greatly increased by what you are doing.

So if you are trying to operate a phone while you're driving, then your reaction times are going to be much higher because you'll be concentrating on the phone and not concentrating what's in front of you by the time you notice you'll have travelled a substantial distance.

You can measure reaction times in several different ways, but you are going to carry out a fairly simple experiment to measure reaction times.

We're going to be using a drop test.

And a drop test basically involves dropping an object and catching it as fast as you can.

We're going to have a ruler with some timing markings held by one person vertically.

And as you can see in the photograph, there's some small markings that are attached those to the ruler.

And there are times so when I drop the ruler and catch it, I can read off the time it took me to actually catch the ruler.

The rulers drop through someone's open hand.

So you'll see the ruler starts to move and you've got to basically try and catch it so the other person, so one person drops the ruler and the other person attempts to catch it between their fingers.

And then once you've caught it, the times, the time it took you to catch it can be read directly off the paper strip that you've attached to the ruler.

And the times you're gonna collect with this are typically much, much smaller than the reaction times when you're driving a car because you're gonna be concentrating on just one thing.

Whereas in a car you're concentrating on hundreds of things at once.

So I'd like you to attempt to make a reading for a reaction time test.

I've got three photographs of me catching a ruler here.

I'd like to know when was my reaction time at the best.

So what's the smallest reaction time shown in this set of photographs? Pause the video, work it out and restart please.

Welcome back.

Well, you should have selected a the reaction time that was naught.

1 seconds if you read carefully off the strip of paper.

And the other reaction times was slightly higher, naught but.

15 seconds and naught but.

12 seconds.

And as I said earlier, these reaction times are much faster reaction times than you'd get when you were driving.

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

I'd like you to follow the instructions here to find your reaction time.

I'll show you a brief video of me dropping the rule and catching it to show the idea.

But basically you work in a pair.

Your partner should hold the ruler and drop it without telling you when they're gonna drop it.

You've got to catch it between your fingers as quickly as possible and read your reaction time off the strip.

You'll need to repeat that five times to get some sort of mean reaction time.

So here's a video of me doing one of those tests.

Stick the reaction time strip to the ruler.

<v ->One person holds the ruler vertically.

</v> You hold your hand open so that the start line is in line with the top of your hand.

The ruler is dropped without warning and catch it as quickly as you can.

Your reaction time is shown by the position you hold the ruler.

Repeat the experiment at least three times to find a mean value for your reaction time.

Some example results are shown here.

Hopefully that gave you the idea.

And for part two, I want you to use the reaction time that you actually have and then use that to estimate what thinking distance you'd have when you are driving at 30 miles an hour, 30.

4 metres per second and at 70 miles an hour, which is 31 metres per second here.

So pause the video, try that experiment and calculation and restart please.

<v ->Welcome back.

</v> Well here's my mean reaction time.

I had drop results there, naught.

2, naught.

15, naught.

10, naught.

15 and naught.

10 seconds.

That gives me a mean reaction time of naught.

14 seconds.

And I use that to calculate my estimated thinking distance at 30 miles an hour, 1.

9 metres and at 70 miles an hour, 4.

4 metres.

Well done if you've got reaction times like that or faster.

Okay, now it's time to move on to the second aspect of stopping.

And that's braking.

When you press the brakes, your car going to travel for a certain distance before it comes to a complete stop.

We call that the braking distance.

So let's have a look at that.

When you press the brakes on the car, the car's not gonna stop instantly, it's gonna gradually slow down.

There's gonna be some sort of brake force acting on it to slow it.

So here's a car, I start braking and I apply a braking force on it, but that's not gonna cause it to instantly stop.

The car's gonna generally slow down, that force is gonna act on it, it's going to decelerate the car and it's going to slow down and come to a stop.

So eventually the car stops off here and you can see I've got a distance that the car travelled during that braking.

And we call that the braking distance.

The braking distance is going to depend on the size of the deceleration of the car.

And we've got the equation of acceleration or in this case deceleration that's going to be equal to force divided by the mass.

So the greater the force of braking, the greater that deceleration is going to be.

And more quickly the car will come to a stop.

So a greater force gives a shorter braking distance.

And if you look at the equation again, you can see mass is also involved.

So we've got the mass here and the greater the mass, the smaller the deceleration is going to be.

So a vehicle that's very large like a lorry is going to have a much longer braking distance than a smaller vehicle.

So there's those two factors that are involved in the deceleration, the force and the mass.

Let's see if you understand those factors here.

An empty van is a mass of 2,000 kilogrammes and a loaded van as a mass of 4,000 kilogrammes.

They both produce the same braking force.

Which of these statements is correct? So read through the statements, make your decision and restart please.

Welcome back.

You should have selected this one.

The loaded van has a longer braking distance.

Okay, it's more massive.

It's going to be harder for that vehicle to stop.

So well done if you selected that.

As you saw, the braking distance depends upon the size of the braking force and the size of that braking force depends very much on the grip between the tyre and the road surface.

So if you've got tyres that are worn and these tyres are in fairly good conditions, but if you've got worn tyres, you're going to get a lower grip.

So a lower force, and that's going to increase the braking distance.

If you're driving and wet, so you've got a wet road, that again is going to decrease the grip between the tyres and the road.

Lowering the braking force and giving you a larger braking distance.

It can be up to double in on wet roads compared to dry roads.

And worse still, if the roads are icy, then the grip is lowered by a large factor.

So you're getting very small braking forces and your braking distance is gonna be much, much larger.

Typically more than 10 times as large when you're on an ice road.

The braking distance also depends very clearly on the speed the car's travelling at.

If you're travelling at a very high speed, then you're going to find you've got a very large braking distance, doubling the speed of the car, increases the braking distance by a factor of four.

So I'll look at some example values here again, I've got my typical speeds and I'm gonna look at braking distances for good conditions.

So remember if the conditions are worse, these could be increased by up to a factor of 10.

So at 10 miles an hour I can stop a car within 1.

5 metres.

So the braking force is quite large.

I can easily stop a car when I'm going slowly.

If I double up speed, as you can see, my braking distance goes up by factor four.

It's six metres though.

So after I press the brakes, it's gonna take six metres for the car to stop, increasing that to 30 miles an hour.

typical town roads, 14 metres if I'm travelling on a fast road, so a countryside or a dual carrier, something like that, 55 metres is the minimum distance taken for the car to stop after you've pressed your brakes.

And on a motorway, obviously it's gonna be even larger.

It's 75 metres.

So it's 75 metres is the minimum distance you could stop a car on a motorway in good conditions.

Let's see if you understand the braking distance here.

A lorry travelling at 20 miles an hour has a braking distance of 12 metres.

What's it braking distance going to be if it's travelling up 40 miles an hour? So pause the video, make selection and restart please.

Welcome back.

You should have selected option D, 48 metres.

The braking distance is going to be about four times as large because the speed is twice as fast.

So that's 48 metres.

Hold on if you've got that.

Now as we understand thinking distance and braking distance, we can find the stopping distance of the car and the stopping distance of the car is the minimum distance that the car is going to come to a complete stop in after noticing a hazard.

So we've gotta have the thinking distance while you react and the braking distance while the car slows down.

So for a car travelling at 30 miles an hour with a reaction time of about one second, the thinking distance is about 13 metres and the braking distance is similar at 14 metres.

So if we wanted to know the stopping distance for that car for that driver at that speed, we can add the stopping distance is the thinking distance plus the braking distance.

Add those two values together and that car could come to a stop within 27 metres.

So if the hazard is closer than 27 metres, the car's gonna still be moving when it reaches it.

Okay, let's see if you can do the same sort of calculation.

I've gotta drive it with a reaction time with naught.

8 seconds, driving a lorry at 25 metres per second.

At that speed, that braking distance is 18 metres 'cause it's a very heavy lorry.

I'd like you to calculate the stopping distance for the lorry, please.

Pause the video, make your selection and restart.

Welcome back.

Well that lorry will stop in 100 metres.

The thinking distance is the speed, time to time, naught by eight seconds and 25 metres per second.

That gives a thinking distance of 20 metres.

So before the lorry starts slowing, then the lorry is gonna take 80 metres to come to a stop.

So we get the stopping distance by adding those together 20 metres plus 80 metres, that gives a hundred metres.

Well done if you've got that.

You should have noticed that the speed of the car affects the thinking distance because you're gonna travel further as you think.

And it also affects the braking distance.

So the speed of the car is one of the most critical things about coming to a stop.

If you're going fast, it's gonna take a much larger distance to stop.

Doubling the speed doubles the thinking distance and increases the braking distance by a factor of four.

If you're travelling at 30 miles an hour, it's going to give a stopping distance more than twice as far as 20 miles an hour.

And that's why in town roads you're typically going to have a stopping distance of 25 metres if you're travelling at 30 miles an hour.

But schools have speed limits of 20 miles an hour and that reduces the overall stopping distance to a typical value of about 12 metres.

So you're going to be able to stop before you hit a hazard on that slower road much easier.

Okay, now it's time for the next task for the lesson.

And what I'd like you to do is to calculate some stopping distances.

For question one, I've got a car moving at 30 metres per second with a braking distance of 75 metres.

And I'd like you to work out the stopping distances for those three different scenarios.

And I've got two cars.

And what I'd like you to do is to use the data to try and see how quickly would the driver need to react to stop before hitting the hazard that's 20 metres ahead of them.

So pause the video, work out your answers to those and restart please.

Welcome back.

Well here's the calculation for those in scenario A.

I've got a total stopping distance of 90 metres.

In scenario B, I've got a total stopping distance for 120 metres.

And for C, you have to look back at those calculations and look at what would happen if the braking distance was were three times greater and that gives total stopping distances of 240 metres and 270 metres.

Well done if you've got those.

And the second question asked you to work out some reaction times, minimum reaction times.

And as you can see there working through the process, I can find the thinking distances and from the thinking distances and the speed, I can calculate the reaction times and in reaction times for car X, you can see naught.

46 seconds, a very short reaction time needed there to come to a stop.

So that's gonna be very difficult.

And then in car Y going slower, they've got a much larger reaction time and they've got a full 2.

3 seconds and they'll still be able to stop in time before hitting the hazard.

Well done if you've got those.

And now we've reached the final part of the lesson and we're going to look at the energy transfers that happen when you press brakes.

The kinetic energy that a car has when it's moving is called kinetic energy.

We say it's got a kinetic energy store and we've got a formula equation for that.

Kinetic energy is half times mass times speed squared, or sometimes we'll say velocity squared.

So a car with greater mass has more kinetic energy as you can see from the equation.

If I increase that mass and keep the speed the same, the kinetic energy is going to rise.

Similarly, a car with a greater speed or velocity has more kinetic energy as well, 'cause if they increase that speed there, the kinetic energy is going be rise.

During braking, all of that energy from a moving car, and it's quite a lot of energy 'cause the mass and speed's both high, that energy must be conserved and it must be transferred into the car's brakes.

So when you press the brakes, a force is going to apply to the road and to the brakes and that's going to transfer energy into thermal stores of the car's brakes.

Let's see if you understand the idea of kinetic energy and the transfer of it.

A motorcycle speeds up from five metres per second to 10 metres per second.

What's going to happen to the kinetic energy of the motorcycle? So pause the video, make selection from the list there and restart.

Welcome back.

Well, the kinetic energy should be four times as large.

I've doubled the speed and according to the equation here, if I double that speed there, that's going to be something squared.

So doubling the speed makes that kinetic energy increase by a factor of four.

So for example, if I move from five squared 25, that would be to 10 squared or hundred.

That's an increase of a factor of four.

Well done if you've got that.

As I mentioned earlier, as a car slows down, the energy in its kinetic store needs to be transferred somewhere and that's transferred into thermal stores because a force, a braking force is applied and that causes the temperature of the brakes to rise.

So we can see some of the places that energy ends up.

We've got forces acting between the tyres and the road surface.

And that's gonna cause an increase in the temperature of those two surfaces as they rub together.

And that's what's causing these skid lines on the road surface.

So we've got a temperature increase as those two surfaces rub together.

But most of the energy ends up in the brake discs because as you can see there, we've got a brake disc and we've got strong forces that act in them.

And that creates a very large temperature rise as the brake discs are clamped and slowing the wheel down and they get very, very hot.

In fact, brake discs can get so hot that they become fairly ineffective.

They overheat if you use the brakes often.

So if you are driving and you are accelerating and slowing and accelerating and slowing a lot, so a lot of brakings going on from high speed, the brake system's gonna get very hot and that brake system can actually fail if it overheats.

So hot brakes aren't as effective and can actually fail.

So imagine you are driving down a very steep road using the brakes a lot, you can actually have brain systems fail.

And obviously that's very, very dangerous.

In fact, so dangerous, that's something like a heavy lorry, if its braking systems fails goes down, then on those roads you often have escape route towards a lorry which can't brake, can turn off the main part of the road into a gravel track and that will slow it down, hopefully safely without a serious accident.

Now in normal cars, all of that energy from the kinetic store is transferred to thermal stores and that's generally wasted.

You're not gonna be able to use that energy anymore.

It's just dissipated into the surroundings.

So some modern cars don't wanna waste that energy and they've got regenerative brakes.

When the car brakes, they actually transfer some of that energy instead of just thermal stores, they have generators and it transfers the energy into a battery.

So the rotating wheel wheel turns the generator and produces an electric current.

And that current causes a chemical store in a battery to increase.

Most modern electric cars have regenerative brakes because it makes them much more efficient.

So instead of wasting the energy to the surroundings, we are able to store that energy in the batteries of the car and then reuse it when the car drives off again.

Okay, let's see if you understand that.

How do regenerative brakes save money for drivers of electric cars? So have a look at the three options there.

Welcome back.

Well, the answer to that one is they transfer the energy during braking in a way that allows it to be reused.

So instead of just causing heating, it's stored in the batteries and reused.

And that means that you don't have to charge the batteries from other devices.

And so you are basically getting that excess energy for free.

So that's going to reduce your costs.

Well done if you've got that.

Now it's time for the final task of the lesson.

And I've got three questions for you here.

I'd like you to read through each of those questions and write down your answers please.

So pause the video, work out your answers and restart.

Welcome back.

Well, let's have a look at the first one.

We've got a a fast car race.

Sam notices the brakes sometimes glow red hot.

Explain why that happens.

Well, that's happening because the cars are slowing down from high speeds.

There's a huge amount of kinetic energy in their stores and that's transferred to the braking systems and that can actually increase the temperature of them until they are so hot, they're glowing red.

So they're red hot.

Well done if you've got that.

And the second two questions.

Explain why it's more likely the brakes of a heavy lorry overheat than the brakes of a family car.

Well, the lorry's got more mass, so there's more energy, more kinetic energy in its stores, and that's transferred to the brakes.

That's gonna cause more heating, making them more likely to fail.

And suggest why regenerative brakes are more common in electric cars or less in petrol driven cars.

Well, electric cars already have an extensive battery system where you can store the energy.

So they've got systems that will allow the energy to be stored more efficiently, so it's because they've got more batteries.

Well done if you thought of that idea.

And we've reached the end of the lesson.

So here's a summary of everything.

The stopping distance of a vehicle is the sum of the thinking distance and the braking distance.

The thinking distance is proportional to the speed and the reaction time of the driver.

So if you're going fast, then you're gonna have greater thinking distance.

And if you're going fast, greater thinking distance.

Reaction time is affected by a wide range of things like tiredness, alcohol, and other drugs and attention levels.

The braking distance is very greatly affected by the speed.

Doubling the speed increases the braking distance about four times.

And finally, braking distance is also affected by road conditions.

So if there's rain and snow or poorly maintained tyres or something like that, you're going to have a much larger braking distance.

Well done for reaching the end of the lesson, I'll see you in the next one.