Hello there.

My name is Mr. Forbes.

And welcome to this lesson from the "Forces Make Things Change" unit.

In this lesson, we're going to be talking about how to stop a vehicle safely, and that will involve discussing the forces, momentum, and the features of a car that allow it to stop safely.

By the end of this lesson, you're going to be able to use the force equals change in momentum divided by time equation to explain how we can control the size of the forces involved when an object is stopping.

We'll also look at how the features of a car are specifically designed to reduce those forces and prevent injuries.

These are the key words that will help you understand the lesson.

The first is stopping forces, and we're gonna use the term stopping force to mean any force used to bring an object to a stop.

Then we have stopping time, and that's the amount of time it takes for the object to stop.

Momentum, and that's the product of the mass and the velocity of a moving object, given by the equation there, p for momentum equals m for mass times v for velocity.

Then there's crumple zones, and they're parts of a car deliberately designed to crumple in collisions to reduce the size of forces.

And finally, airbags, and airbags are inflatable balloons basically in cars that will inflate very rapidly in a collision and prevent injuries to the passengers and driver.

You can return to this slide at any point in the lesson.

There are three parts to this lesson.

In the first part, we're going to look at how we can control the time it takes an object to stop.

And to do that, we have to alter the size of the stopping force.

And the second one, we're going to see how we can control stopping forces by increasing the length of time that something takes to stop.

And in the final part of the lesson, we'll be looking at a range of car safety features.

So let's start by looking at how we can control the stopping time.

When something like a car is moving along, you need a force to bring it to a stop.

You need that force to cause it to decelerate.

Under the normal circumstances, those forces are between the tyres of the car and the road surface.

And we can call those the braking forces acting on the car.

There are other circumstances where the car will come to a halt without braking.

It might, for example, collide with an object, and that collision would produce a large collision force and that would bring the car to a stop quite suddenly.

Using larger forces will decrease the time it takes for the car to stop, so it decreases the stopping time.

We can find the stopping time using an equation linking momentum, force, and time.

And that equation you may have seen before is this one.

The stopping time is the change in momentum divided by the stopping time.

And we're gonna rearrange that because we wanted to discuss stopping times, so that rearranges to stopping time is equal to change in momentum divided by stopping force as shown there.

For any given change in momentum, so if we've got an object moving at certain velocity and in certain mass, it's got a particular amount of momentum.

If we use gentle braking, then looking at the equation, you can see that the braking force is gonna be small with gentle braking.

And the result of that is, the time it takes for that object to stop is going to be large.

So when the braking force is small, the time to stop is large.

On the other hand, if you've got harder braking, if you're using a much larger force, then that's going to decrease the time it takes for the object to come to a stop.

Because in both circumstances, we had the same amount of momentum change.

So let's see how we can use that equation and expand on it to solve a problem.

I've got a car of mass 1,000 kilogramme moving at 12 metres per second.

The car uses a force of minus 800 Newtons to gradually slow to a stop.

So I've got the force negative there because the car's slowing down, the force is working against the direction of motion.

I'm gonna calculate the stopping time.

So to do that, I write out my equation linking stopping time, change in momentum and force, and I can expand on that 'cause I know the change in momentum is going to be the mass times the change in velocity, which are representing here by v minus u, that's end velocity minus starting velocity and then divided by force again there.

And if I substitute the values in from the question, I've got a mass of 1,000 kilogrammes, I've got a change in velocity from 12 metres per second down to zero, so my end velocity is zero, my initial velocity was 12, that's why I've got zero minus 12 metres per second there.

And then divide that by the force, that gives me a stopping time of 15 seconds.

Now, I'd like you to carry out a similar calculation.

I've got the same car, so it's got the same mass, it's travelling at the same velocity, 12 metres per second, but it's carrying out an emergency stop manoeuvre.

And in an emergency stop, we're gonna use the largest braking force the car can produce, and that's minus 4,600 Newtons.

And what I want you to do is to calculate the new stopping time.

So pause the video, use the same calculations as I've done before, and once you've got an answer, restart please.

Okay, welcome back.

Well, your calculation should look something like this.

I've skipped writing the delta p over f, that symbol, I'm just going straight to t equals m, v minus u divided by f.

Let's save a little step there.

I've substituted in the values, and they're similar to those as before, but this time I've got a stopping force of minus 4,600 Newtons, and you should have got an answer of 2.

6 seconds.

So the car comes to a stop much more quickly than it did before, and that's why it's an emergency stop.

Now, that stopping time we just worked out was just for a single car.

The stopping time also depends on the momentum of the vehicle.

You saw in the equation, we used change in momentum.

And so, again, momentum is the mass times the velocity.

And what that means is, if I've got a car with greater mass, it's gonna have greater momentum and it's going to do be much more difficult to stop.

Similarly, if I've got a car at a higher velocity, it's also got greater momentum and it's going to take a longer time to stop.

And that's particularly important for very large vehicles.

So if I've got something like this, a train, which we've got a mass hundreds of times greater than a car, and it can be travelling much faster than a car, it's gonna have a huge amount of momentum and it's going to take a long time to stop even if there's a large braking force.

So all the wheels on the train are gonna produce a braking force and that's going to be needed to bring it to a stop in a reasonable amount of time.

Let's say an example of the effect of mass.

I'm gonna do a calculation again on stopping time and then ask you to do one.

So I've got an empty bus, it's got a mass of 5,000 kilogrammes and it's moving at 6 metres per second.

And the bus can use a braking force of minus 4,000 Newtons to slow to the stop.

Calculate the stopping time.

So I use the same technique as it did before write out the equation, linking change in momentum, time, and force, expand on that a little to show the mass and the change in velocity, and then substitute the values in, 5,000 kilogrammes times, and again, it slowed down from 6 to 0, so the change in velocity is at minus 6 metres per second and then divided by 4,000 Newtons, the stopping force.

And that'll gives me a time of 7.

5 seconds for that bus to slow to a stop.

Now it's your turn, and I'd like you to work out the stopping time, but this time the bus has got some passengers on.

So, the same bus travelling at the same velocity and it's got the same size braking force, but now it's got passengers and its total mass has increased to 6,000 kilogrammes.

Calculate the new stopping time for the bus please.

Pause the video, work out that stopping time, and restart.

Welcome back.

Well, your answer should look something like this.

We've got exactly the same sort of calculation, but this time we've got a different mass, it's 6,000 kilogrammes instead of 5,000, and that gives a stopping time of 9.

0 seconds.

Well done if you've got that.

Okay, it's time for the first task of the lesson, and I'd like you to answer these two questions.

The first is about a lorry.

We've got a lorry fully loaded with cargo.

Why does that take a long time to come to a stop than an empty lorry travelling at the same speed with the same braking force? Then in the second question, I'd like you to calculate the stopping time for a motorcycle and they give the mass and change in velocity and force there.

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

And welcome back.

Well, the explanation about the lorry has to be to do with the change in momentum.

It's got a different mass.

So my answer would be something like this.

The momentum of the full lorry will be much greater than the momentum of the empty lorry.

And you can see I've put the equation there to show that I know the momentum depends on the mass.

Then I've explained why the stopping time would be greater by saying "The stopping time is given by t equals delta p over f, change of momentum by time, so the braking time has to be greater and the momentum is greater if I've got the same braking force.

Well done if you've got the answer.

And now to the calculation part, and again, I use the same calculations as I used earlier in the lesson.

I write out the equation linking the stopping time to the change in momentum and force.

I expand that equation slightly, so it's in terms of mass and change in velocity.

And then I substitute the values from the questions, and that gives me a stopping time of 3.

0 seconds.

Well done if you've got that one.

Now it's time to move on to the second part of the lesson.

And in this section, we're going to be looking at how you can control the size of stopping forces by altering the time of an impact.

If you've ever played a game of cricket, you'll know the cricket balls are quite hard and can travel at pretty high speeds.

So in the photograph here, you can see there's a fast ball approaching the batter.

Now, that ball could easily cause injury if it hit an unprotected part of the body.

It would cause large forces, and it would hurt a lot.

So protective padding is used around the body to protect you from those forces.

So we have heavily padded gloves, so if the ball hits your fingers it won't break them, and padded legs, so we've got quite thick leg pads there to put the knees and lower leg, and even a hard padded helmet to protect from head injury.

So, that padding is to reduce the size of the force on the body when the ball hits you.

We can find the stopping force using the same equation as we saw before.

So stopping force is change in momentum divided by stopping time, or written in symbols, f is delta p, and we use delta p for change in momentum remember, divided by time.

And for any particular change in momentum, if we've got a hard surface that doesn't bend, then you're going to have a very short stopping time.

So the stopping time is gonna be very small, and that will mean that the force involved will be large.

On the other hand, if you've got a soft surface or padded surface, then when the object hits that surface, it's going to take a longer period of time to stop, and that longer period of time is going to give you a smaller stopping force needed to stop that object.

Let's have a look at an example of that sort of calculation using a cricket ball.

So we've got a cricket ball with a mass of 0.

16 kilogrammes, and is bowled at 25 metres per second.

The ball hits an unpadded leg and it stops in 0.

02, two hundredths of a second, so a very short stopping time there.

Calculate the force of the impact.

Well, I'll write out the equation as I've seen before.

F is delta p divided by t, or in terms of mass and change in velocity, it will look like that.

And then I substitute in the values.

So I've got 0.

16 kilogrammes, and there's my change in velocity again.

It's gone down from 25 metres a second to zero, so the change in velocity is zero minus 25 and divide that by the time of 0.

02, and that gives me a force of minus 200 Newtons, 'cause it's a stopping force.

Now, it's time for you to try an example, and I want you to use the same cricket ball travelling at the same velocity, but now the ball hits a padded leg and it stops in a time of 0.

10 seconds or a 10th of a second.

Calculate the new size of that impact for me, please.

So pause video, do the calculation, and restart.

Welcome back.

Well, your calculation should look something like this.

Again, I've missed off the first step so that we can just concentrate on mass and velocities.

I've substituted the values in there.

The change this time is that the time is 0.

10 second on the bottom of that equation there, and that gives me a force of minus 40 Newtons.

And as you can see, it's a much smaller force.

The padding has reduced the force of the impact quite dramatically.

For that sort of reason, gymnasts used padded floors to reduce any stopping force as they land.

So, if a gymnast was doing some gymnastics on a hard floor like this and they fell, then they'd hit that solid wooden floor, and the time of the impact would be very small, and that could cause some serious injury because there's very large forces involved.

So, if they're doing something where they may fall, like on a beam here, then they've got crash mats around them.

If you fall onto a crash mat, what that does is, the crash mat squashes beneath you as you fall and the time of the impact is much, much larger.

So the forces during the impact would be smaller as well.

So let's see an example to show the difference between landing on a hard and soft floor.

I've got a gymnast of 50 kilogrammes and they land on a floor travelling at 10 metres per second.

They bend their legs so the landing takes 0.

5 seconds.

So they're actually trying to reduce the impact by bending their legs.

Calculate the stopping force of that impact.

Well, I write out the equation as I've done before, and in terms of mass and changes of velocity there, and I substitute the values in, 50 kilogrammes minus 10 metres per second change in velocity, at 0.

5 second impact time, and that gives me a force of minus 1,000 Newtons.

So that's quite a large force to bring them to a stop in just half a second.

It's your turn to try the similar calculation, but this time the gymnast gonna land at the same velocity.

Well, there's a thicker crash mat used beneath them, and that increases the landing time to 0.

8 seconds.

Calculate the new stopping force, please.

So pause the video, look at that stopping force, and restart.

Welcome back.

Well, the calculation should look something like this.

And as you can see, all we've done is change the time and the calculation is now 0.

80 seconds, and that gives us a smaller stopping force minus 625 Newtons, still quite a large force but reduced from what it was earlier.

So the crash mat has reduced the stopping force.

Okay, it's time for a couple of questions based upon those ideas.

So, I'd like you to answer these two.

So we've got two identical eggs, they're dropped from the same height.

Egg X is dropped onto a wooden floor, and egg Y onto a crash mat.

I want you to explain why one egg breaks and the other doesn't.

And then the second one, I'd like you to calculate a stopping force for me.

So I've got a baseball player catching a 0.

15 kilogramme baseball travelling at 45 metres a second, so fairly faster, and the ball stops in 0.

07 seconds.

I'd like you to calculate the stopping force, and then I'd like you to suggest two ways of reducing the size of that force as the player catches the ball, and explain how they'd work.

So pause the video, answer those questions, and restart please.

Welcome back.

And here's the answer that I would've given to the first one.

Both eggs are gonna be reaching the ground at the same velocity or same speed, and they've got the same momentum in that case 'cause they're the same size egg.

Egg X will be stopped by the hard floor in a short period of time because the floor's not flexible.

And that means there's going to be a large floor, sorry, a large force.

So a large force because there's a short stopping time.

Egg Y, well, that hits a soft, more flexible surface.

That surface bends when it's hit, and that means the impact is gonna take place over a longer period of time.

And so we can have a smaller stopping force to change the momentum of the egg, so hopefully the egg doesn't break.

Well done if you've got something like that.

And for question two, these are my calculations.

So to calculate the stopping force, we're using the same equations as we've used several times already by putting the values in from those 0.

15 kilogrammes, that change in velocity of 45 metres a second, and the stopping time on the bottom there of 0.

07 seconds, and that gives me a stopping force of minus 96 Newtons.

Two possible ways of stopping, sorry, reducing those forces would be wearing padded gloves.

So a bit like the cricket pads and the gloves you saw in the cricket match.

That padding will increase the catch time, so that will reduce the forces.

And the second idea is, if you move your hands away from the ball, then you're gonna catch it over a longer period of time as well.

So making your hands move along in the same direction as the ball will reduce the stopping force as well.

Well done if you've got those two.

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

And in it we're going to look at some car safety features used to reduce the size of forces on passengers and the driver in case of emergency stopping all collisions.

The number of cars on UK roads has increased dramatically over the last 60 years.

I've got a table to show you though some of those numbers.

We've got back in 1966, 7 million cars on the road, which is quite a lot.

But since then, it's increased to current values of over 33 million.

So that's 33 million cars on the road each day potentially.

The number of accidents because of those cars has also increased because it's more people driving, more likelihood of collision when you've got more cars on the roads.

However, the number of passenger fatalities and serious injuries has actually decreased over that same period.

I'll show you the part of the table of that.

Back in 1966, there were 8,000 people a year dying in cars, so that was just drivers and passengers, and that reduced in the 1980s, and then significantly reduced further around the 2000s on the down to about 1,460 now.

So, we've got a reduction in fatalities and serious injuries as well.

And the reason for that is not because there's, you know, fewer cars or anything, it's because the cars themselves are much better at protecting the passengers.

So even though there are more collisions, it's less likely you're gonna be seriously hurt in a car crash.

Okay, to check if you understood what I said there, I'd like to say if this is true or false and justify your answer please at the same time.

So, evidence shows that modern cars are much safer to travel than old cars.

Is that true or false? And then justify your answer by selecting one of those two options.

Pause the video, make those two selections, and restart, please.

Welcome back.

Well, that's true.

Evidence does show that modern cars are much safer than old cars.

The number of passenger deaths has fallen even though there are many more cars on the road.

So well done if you selected those two.

One of the main reasons that fatalities are lower is because cars are specifically designed now with rigid and collapsible regions.

So parts of the car are deliberately designed so they scrunch up or collapsed during any collision.

And other parts are designed to be really very solid and not change shape at all.

So, for example, for this car, the front part would be specifically designed to crumple if there was an accident.

So, if you hit something that was in front of you, and that front section of the car would actually crumple and distort.

Similarly, the back, if some car ran into the back of your car, that part of the car is also designed to specifically crumple up.

The central section of the car is designed to be very strong and rigid.

So the place where you'd be sitting is designed not to distort at all.

There are very strong steel sections there to make sure it doesn't distort.

And so that section stays safe for the passengers, protects them.

Those sections of the cars that are designed to scrunch up in collisions are called crumple zones, and they're designed to deliberately crunch and collapse as the car comes to a stop.

And they can look quite damaged when they work.

So we've got one here, here's a car that hit a tree, and you can see the front section of the car looks pretty devastated though.

But look carefully at the central section of the car where the passengers would be, and you see there's no damage at all to that section.

Even the windscreen hasn't broken.

So that crumple zone has, I would say, absorbed some of the impact.

It's protected that central section of the car by collapsing.

Similarly, this car, we've got the rear section hit by another car, and it's crumpled.

But you can see there's only a slight distortion in the door of that rear section of the car.

The passenger in the backseat would be protected by that back part of the car crumpling up.

So crumple zones are designed to crumple, and then you've got the solid sections designed to hold their shape in any collision.

Crumple zones work by reducing the size of the stopping force or the impact force during a collision.

And they do that based upon the relationship we've seen before.

The stopping force is a change in momentum divided by the time of the impact, we got that equation there.

And we saw that if you can increase the time of a collision, the duration of any collision, then the stopping force would decrease.

If we had a very rigid car or a solidly constructed car with no flexible sections at all, what would happen is that car would come to a stop in a very short period of time, hundredths of a second, and that would produce very, very large impact forces.

Those impact forces would be transferred to the passengers causing severe injuries.

But the crumple zone is designed to be crushed during a collision, and as it crushes and collapses, that makes the collision last over a longer period of time.

So with a much longer period of time of collision, maybe 10th of a second instead of hundredth of a second, you get much smaller impact forces, and that means that the passengers don't feel as large an impact force.

Let's see an example of the effect of a crumple zone.

So I've got a test car, it's got a mass of 1,200 hundred kilogrammes and it's crashing into a wall at 10 metres per second.

And this car doesn't have a crumple zone, and it stopped in a very short time of 0.

04 seconds, four hundredths of a second.

And I'm gonna calculate the impact force.

So as before, I write out the equation to find the force based upon that change in momentum and time, and expand on that to change it into looking at changes of velocity and mass, which is what have been provided.

I substitute the values in there, and as you can see, I get a very large impact force there, minus 300 kilonewtons, so 300,000 Newtons.

And that's a very large force which would damage and crush the car completely.

What I'd like you to do is to work out the size of the impact forces if the stopping time could be increased to a third of a second or 0.

30 seconds.

So, it's exactly the same car, but this time we're gonna use a crumple zone at front.

So I'll pause the video, work out the size of that impact force, and then restart please.

Okay, welcome back.

Well, again, write out the initial equation, substitute in the values, and you can see all the values are the same except for the time here of 0.

30 seconds, and that gives collision force or an impact force of minus 40 kilonewtons.

So it's significantly reduced from there early on.

It's still quite a large force, it still caused a lot of damage to the car, but it's much, much smaller than before.

Well done if you've got that.

A second safety features come into all cars, are seatbelts.

Seatbelts are required by law for the drivers and all passengers, and that was introduced a few decades ago and it's saved thousands of lives since the compulsory use of seatbelts in cars.

So you can see here, I've got a child in a child seat.

Again, extra padding there for protection where they've got a seatbelt across them.

And what seatbelts do is when the car comes to a sudden stop, the seatbelt applies stopping forces, slowing the passenger down gradually with the car so you don't move forwards and hit the back of the seat, so that child is in the seat and the seatbelt will hold them in place.

It's designed to stretch slightly over time, so that it lowers the impact force.

So, again, we're extending the time of the forces acting and that reduces the size needed by the force.

And it's also quite wide, so seatbelts are deliberately designed to be wide to spread that force over an area across your chest, and that will reduce the pressure acting on you, causing less damage to you if you come to a sudden stop.

Modern cars also have airbags.

An airbag are basically inflatable balloons that are set off during a collision.

So if there is a collision, the airbags inflate in a fraction of a second.

There's actually a small explosion that produces gas, that fills, basically, a balloon very, very quickly, in, you know, hundredth of a second.

And then if there is a collision and you move forward, then your head won't bang into the steering wheel.

It will hit this inflated balloon, and that will have a cushioning effect.

It will mean that your head comes to a stop in a longer period of time, and significantly reduce the forces acting on it.

And because your whole face is gonna hit that cushion, it's going to spread the force across your whole face causing less damage as well.

Front passengers are also protected by airbags, so they'll have it in the dashboard so they don't fly forward in the dashboard.

And in fact, most cars now have airbags in the side as well.

And those airbags will protect you from any side collision.

So if you move to the side suddenly, you don't bang your head on the window or something like that.

You can see here a deflated airbag.

So what's happened in this collision is both of those airbags have rapidly inflated.

The passenger and the driver have been protected from hitting the solid surfaces in front of them for a brief few seconds, and then the airbags have deflated quite quickly afterwards to allow the passengers to escape from the car.

Okay, you've seen three safety features.

So what I'd like you to do is to match those safety features to the descriptions.

So pause the video, read through the information there, match the three safety features to the three descriptions, and then restart please.

Welcome back.

Well, let's have a look, airbag first.

So the airbag inflates in the form of soft cushion to prevent passengers hitting hard surfaces.

Well done if you've got that one.

A crumple zone is designed to increase the duration of an impact when crushed in a collision, so the front and rear crumple zones there.

And finally, the seatbelt slows the passenger down more gradually and holds them in place during an impact.

Well done if you've got all three.

And now it's time for the final task of the lessons.

So what I'd like you to do is to have a look at these two questions.

First of all, I've got an image of a cycling helmet there with a hard outer shell and soft internal padding.

And I'd like you to explain how those features help protect the cyclist in an accident.

And then I've got some cars in safety tests.

Two cars, 800 kilogrammes crashed into a solid wall at a velocity of 13 metres per second.

Car X has no crumple zone, and it comes to a stop in 0.

05 seconds.

And car Y comes to a stop in 0.

20 seconds because it does have a crumple zone.

I'd like you to calculate the impact forces for those two collisions please.

Pause the video, write out your answers, and restart please Welcome back.

Well, these are my explanations of why the helmet might work.

We've got the hard outer layer to protect the cyclist from sharp objects.

It's always a useful feature.

And it also spreads the force over the whole area of the helmet, and that will reduce the pressure caused in any collision.

The soft padding will increase the duration of any impact.

So as your head comes to a stop, it'll push into the padding, and it'll slow down a little bit more gradually than it would if it hit a hard surface, and so that reduces the forces acting on your head.

Well done if you've got those.

And here are the calculations.

First of all, I'll write out the equation.

And then for car X, I'll fill in the values into that equation, and that gives me a stopping force of minus 208 kilonewtons.

And for car Y, the one with the crumple zone, you'll see that the force has been reduced to minus 52 kilonewtons.

Well done if you got those two.

And now we've reached the end of the lesson.

And here's a summary of all the information.

So, the stopping force is equal to the change in momentum divided by the stopping time.

And as you can see, it can be written in symbols there.

The stopping time for a moving object can be reduced by using larger stopping forces.

So, for example, we can use more powerful brakes to make something come to a stop in a short period of time.

The stopping force can be reduced by making the duration of any impact or any stop increase.

And an example of that is wearing padding.

So padding will protect you from hard impacts.

We also saw some cars safety features, and they included crumple zones designed to impact, sorry, designed to increase the impact duration and lower the forces, seatbelts to slow the passengers down, and airbags to prevent collisions with the hard parts of the car.

Well done for reaching the end of the lesson.

I'll see you in the next one.