Hello there, I'm Mr. Forbes and welcome to this last lesson from the Moving by Force Unit.

In this lesson, we're gonna bring together all the ideas we've seen in the lesson so far, and use 'em to explain how we can change the speed of a parachute as you're falling.

By the end of this lesson, you're going to be able to describe the forces that act on a falling object and how you can alter those forces to increase or decrease your falling speed.

So when you're ready, let's go.

These are the key words and phrases you need to understand to get the most from the lesson.

The first of 'em is gravitational force, and that's the downwards acting force that pulls you towards the centre of the earth.

The second is drag force.

That's the force that acts on you when you move through a fluid, which is a gas or a liquid.

The third is resultant force, that's the overall effect of a combination of forces acting on an object.

Next is parachute, which is something we used to slow down as we're falling.

And the final one is terminal velocity, which is what we'll discuss during the lesson, and that's the maximum falling speed that you achieved during the fall.

And here's a set of explanations for each of those keywords that you can refer back to during the lesson.

This lesson is in three parts, and in the first part we're gonna describe the forces that act on an object as you're falling.

So much of that will be a recap of what you've seen before in this unit.

And the second one, we are gonna discuss how the combinations of those forces lead to a maximum falling speed that we call a terminal velocity.

And in the final part we'll discuss how using a parachute can alter that terminal velocity, slowing you down.

So when you're ready, let's start with the first one, forces and falling.

So let's start by imagining the forces acting on a skydiver.

At first, they're in the plane, standing in the doorway ready to jump, and the forces on them must be balanced.

So we've got a gravitational force balanced by a normal reaction force.

The gravitational force will be pulling them down.

The normal reaction force from the floor of the plane will be pushing them up so they're not falling.

When they step out of the plane, there's no longer a normal reaction force from the plane.

And so there's only one force acting on them, and that's the gravitational force pulling them down.

As you can see in the picture here, they've just jumped out and it's that gravitational force pulling them towards the centre of the earth.

So they'll soon start to fall.

Once the skydive is in the air and falling through it, there are gonna be two forces that are acting on them.

The first of them is still going to be that gravitational force, and that's gonna act continuously through the fall.

And that's pulling them the earth.

The second force is going be the drag force, the force you get when you move through a fluid, that drag force is going to be pushing up, acting opposite to the direction of the gravitational force.

Okay, let's see if you remember those two forces.

I've got two forces acting on the skydiver here, force A and force B.

And all I'd like you to do is to name what those two forces are.

So pause the video, think of the names, and then restart, please.

Okay, hopefully you remembered these two.

Force A is the drag force or the air resistance, and that was the force acting upwards on the parachutist.

And force B is the gravitational force, and that's acting downwards, pulling them to earth.

Well done have if you've got both of those.

Now, as you're falling through the air, the size of the gravitational force pulling you downwards doesn't change.

If you're moving at low fall speed, that gravitational force will pull you downwards, making you get faster.

As you get faster, the size of that gravitational force doesn't change at all.

So the same size gravitational force at medium speed there.

And even when you're going very, very fast, the gravitational force is still the same size.

It's constant throughout the fall.

So gravitational force doesn't depend on the speed you're travelling at.

The size of a drag force is very different though.

As you're moving at low fall speed, the drag force is actually very small.

When you get a little bit faster, that drag force increases slightly as shown there.

When you get faster still, the drag force is even larger.

So the drag force does depend on the speed of the skydiver.

It increases dramatically as you get faster and faster during the fall until it reaches a maximum size and you reach a maximum speed.

Now, those two forces behave in the same way for all falling objects when they're falling through air or any other fluid.

The gravitational force is going to stay constant on them.

So we've got high and low fall speeds here, and the gravitational fall speed is the same, but the size of the drag force acting on the object is going to increase.

So at low fall speed, we've got that small drag force.

And at the high fall speed, we've got a much larger drag force there.

Okay, I've got a quick check for you now.

I've got three tennis balls falling through the air as shown in the diagrams here.

They're all identical.

But what I'd like to know is which of those tennis balls is moving the fastest or falls the fastest through the air? So I'd like you to pause the video, look carefully at the diagrams, make your decision and select it, and then restart.

Okay, welcome back.

Let's have a look at those three again.

You should have selected A.

And the reason you should select that one is because you can see that the drag force is the largest on that ball.

And if the drag force is the largest, it must be moving fastest.

The ball in the middle won't be, has got a smaller drag force.

So that's moving the slowest.

Well done if you selected the right one.

Okay, we've reached the end of the first part of the lesson.

Okay, we've reached the end of the first part of the lesson, which is all about the forces acting on an object moving through the air.

And I'd like you to try this task, please.

Imagine a skydiver standing on the edge of a plane doorway and stepping off.

Their movement is shown in the four figures below.

And what I'd like you to do is to draw and label the forces acting on them at each of those four stages.

So the first stage as they're about to jump off the edge of the plane out the doorway.

The second stage, they instant they push themselves off, so they're no longer on the plane.

The third one, as they're falling slowly through the air.

And finally, as they're falling quickly through the air.

So pause the video, add the force arrows and labels to those diagrams please, and then restart.

Okay, welcome back.

Let's go through each of those four stages in turn.

So in the first stage as they're about to jump off the edge, well, there's two forces acting on 'em here.

There's the gravitational force pulling them downwards, which you've drawn with the green arrow.

And because they're standing on the surface, there's the normal reaction force acting on them.

And I've drawn those arrows the same size because they're both the same size force, one's upwards and one's downwards, and they cancel out and there's no resultant force.

And that's why the skydiver isn't falling yet.

In the second diagram, they've just pushed themselves out of the plane.

We've still got the gravitational force pulling them downwards, but because they're just got outta the plane and not started moving downwards yet, there's no other force, there's no drag force acting on them.

In the third diagram, the gravitational force is still there and it's still the same size throughout.

And now there's a small drag force directed upwards.

And finally, when they're falling much more quickly, we've still got the gravitational force and the drag force.

But this time the drag force is much larger.

Well done if your diagrams look anything like that.

Okay, we're ready to move on to the second part of the lesson now, which is all about terminal velocity.

Terminal velocity is the maximum speed you reach when you're falling.

And to understand why there is a terminal velocity, we'll have to think about Newton's 1st law of motion again.

So let's do that.

Newton's 1st law of motion tells us what happens when there are forces acting on an object, and the different things that happen if there's a resultant force or no resultant force.

So thinking about no resultant force first.

If the object's not moving, then it will remain not moving.

It will stay stationary.

And you can see that in any object resting on a desk where the force are balanced on it, and it started off stationary, it doesn't start moving on its own.

You have to put a force on it to start it moving.

If the object was moving anyway, then it'll continue to move at the same speed and in a straight line.

So a moving object that's got no resultant force stays going at that constant speed in a straight line.

If there is a resultant force on the object, then there is a change in speed.

It will accelerate and it'll accelerate in the direction of the resultant force.

So it could speed up or slow down.

So resultant forces will cause objects to speed up or slow down.

So I'd like you to try and use Newton's 1st law of motion to decide what's gonna happen to the skydiver in each of those four stages we saw earlier.

First stage, just as they're about to jump out of the plane, then the instant they push themselves out, then as they're falling slowly and finally as they're falling quickly.

And you can see I've drawn the sizes of the forces acting on them in each case.

So we'd like you to look at those carefully and try and think about what would happen.

So pause the video, make a decision for each of the stages, and then restart.

Okay, let's have a look at each of those four diagrams. In the first one, with the normal reaction force on the gravitational force, both those forces are equal in size.

So there's no resultant force and the skydiver wasn't moving.

So according to Newton's 1st law of motion, no resultant force on them.

They are stationary, so they're going to remain stationary.

And the second diagram, it's the instant they push themselves outta the plane.

So they're stationary, but there's only one force acting on them.

So there is a resultant force acting on them.

So they're going to start speeding up.

They're gonna start moving downwards.

In the third picture, they're falling through the air and there's two forces acting on them, gravitational and drag force.

But the drag force is smaller than the gravitational force.

So there is a resultant force still acting on them, acting downwards.

So that resultant force is gonna cause them to speed up.

They're going to fall downwards faster and faster.

In the final diagram, the drag force and the gravitational force are the same size.

So those two forces are cancelling each other out, and there's no resultant force on them.

That means they're going to move at a steady speed again.

It's a moving object with no resultant force, so you get a steady speed.

Well done forgetting those.

So as you've seen the gravitational force acting stays the same, but the drag force increases as you get faster.

And that changes how the skydiver is moving through the air.

So in the first stage, when they've just stepped out the plane, there's a large resultant force acting downwards.

They're gonna speed up downwards.

The second stage, there's a smaller downwards resultant force because that drag force is cutting out some of the gravitational force pulling downwards.

In the third stage, again, a small resultant force again.

And finally you get to a stage where there's no resultant force acting on the skydiver at all.

And at that point, if there's no resultant force, there's nothing to change the speed of the skydiver.

The size of the resultant force is decreasing.

Eventually it becomes zero.

So linking that back to Newton's 1st law of motion, when there's no resultant force, there's no more acceleration.

And by that I mean there's no change in speed anymore.

So in this situation here we've got the skydiver with a drag force and gravitational force equal to each other.

And so they've got a steady falling speed.

That steady falling speed is called the terminal velocity.

Sometimes it's called the terminal speed.

Now let's have a look at how streamlining will affect how fast an object can fall through the air.

So imagine two objects that have got the same weight, but one of them is more streamlined than the other.

So I've got them here.

And the teardrop shaped one is a bit more streamlined than the little block, but they both have the same weight.

And that falls acts downwards.

That one's streamlined, and that one's not streamlined.

Both of 'em are falling at the same speed.

So we'll draw the drag force on for both of those objects.

Now the streamlined one has a small drag on it, 'cause that's what streamlining is about.

It's about reducing drag.

With a non-streamlined one has a larger drag force on it.

If you look carefully, the drag force on the non-streamlined object is equal to its weight.

So it's reached its terminal velocity.

Well, the drag force on the streamlined object is smaller than the weight.

And so that's still going to be speeding up downwards.

It's not reached its terminal velocity yet.

So that one hasn't reached its terminal velocity.

That one has reached its terminal velocity.

So you can see streamlined objects will fall faster through the air.

They'll reach a higher terminal velocity than objects that aren't streamlined for the same weight.

We can do a similar thing to compare objects that have got different weights and find out whether a heavier object will have a different terminal velocity than a light object as long as they're in the same size and shape.

So I've drawn some of those here, the same size and shape.

One of 'em is polystyrene, so that's going to have a very small weight and one's a block of metal, which is going to have a much larger weight.

So they've got different weights that have drawn on there.

As they're travelling at the same speed, they'll have the same drag force on them because they're the same shape, neither of them is more streamlined than the other.

So the polystyrene, when you look at it, you can see that the drag force and the weight are the same.

So that has reached its terminal velocity and isn't gonna get any faster.

The metal block though, the drag force on that is much smaller than the weight.

And so that hasn't reached its terminal velocity.

It's going to continue to get faster and faster.

So eventually it will reach its terminal velocity, but it will be travelling much faster than the falling polystyrene.

Okay, let's see if you can link terminal velocity to the forces acting on falling objects.

I've got a tennis ball and a cricket ball here, and both of them are exactly the same size and same shape, but the cricket ball is much heavier than tennis ball.

Now imagine I dropped these from a very great height, both at the same time.

I'd like you to draw the vertical forces acting on both the balls when the tennis ball has reached its terminal velocity.

So pause the video, sketch diagrams of that, and then restart when you're ready.

Okay, welcome back.

Let's have a look at the forces acting on these balls.

First of all, they've got different gravitational force acting on each.

The cricket ball is much heavier, so there's a larger gravitational force on it than on the tennis ball.

But because they're both moving at the same speed through the air, they've both got the same size drag force because they're both equally streamlined.

The cricket ball hasn't yet reached its terminal velocity, even though the tennis ball has.

Well done, if you've got those.

Okay, now it's time for a task and I'd like you to apply your knowledge to skydiving.

So I've got pictures of some skydivers here, and skydivers can change their terminal velocity in the air.

They can speed up to catch up with other people or slow down to meet them.

I'd like you to write down an explanation of how they could do that, please.

So pause the video, have a think, and then restart when you've answered it.

Okay, let's have look at some answers that you could have used here.

So to speed yourself up, you need to tilt yourself forward.

So you're falling headfirst or backwards, so you're falling feet first.

What that will do is make you more streamlined.

That will reduce the drag force acting on your particular speed.

And so you'll speed up until the drag force equals your weight again.

So just by tilting your body, you can go faster.

You can do the same sort of thing to slow yourself down.

If you tilt yourself so that you're falling belly first, then you are going to fall a bit slower because the drag force is going to increase on you.

So you'll slow down for a bit until again, your weight and the gravitational force are equal.

So changing your angle of fall will change your terminal velocity, and that's how skydives can speed up and slow down to catch up with each other.

Well done if you've got those.

Okay, now it's time to move on to the last section of the lesson, and we're going to look at what happens when you open a parachute and how that affects the speed of your fall.

So let's do that.

As we saw earlier, a skydiver can speed themselves up or slow themselves down by altering their position and adjusting the drag forces that are on them.

So when they're falling headfirst, they can go at speeds of 200 kilometres an hour, pretty fast, but when they lie flat on their bellies, they can slow down and reach speeds of just 200 kilometres an hour, still fast, but a bit slower.

And that allows 'em to catch up with each other and form patterns and rings like that.

Increasing the drag force on them reduces their terminal velocity so they have some control of their speed in the air.

Now a parachute is a large sheet of cloth, and when you open it up, it opens into a canopy like you see in the diagram here.

It's a very large surface area, much greater than the surface area you can produce on your body.

So it's not streamlined at all.

It's the opposite.

It's designed to be as un-streamlined as possible.

That means it's gonna produce a very, very large drag force when you're moving quickly through the air.

So you get a massive upwards force, much greater than your weight, and that's gonna start slowing you down.

So increasing the drag force is going to reduce your terminal velocity a huge amount.

So let's have a look at what that parachute does to the falling speed.

So first of all, here's our skydiver.

They've not opened the parachute yet and they've reached their terminal velocity.

So we've got a gravitational force downwards and a drag force upwards, and they're equal.

So we've got balanced forces.

They must be falling at a steady speed, and that was the terminal velocity.

Now, when they open the parachute, you've all of a sudden got a very large upwards force.

The drag force is much, much larger than the weight, and what that's going to do is producing an unbalanced force.

And that upwards force is gonna cause a decrease in the speed.

So the moment you open a parachute, your speed starts decreasing, and you can see that effect in videos of paragliding.

When somebody opens the parachute, they appear to shoot upwards.

What's really happening there is the person who opens the parachute slows down, or the person who's got the camera still falling at a steady speed as it just looks like somebody's shooting upwards, and in fact, they're just slowing down.

Okay, so obviously the skydiver is slowing down, but they're not gonna stop in mid air.

So let's have a look at what happens next.

Here's the parachutist, just open the parachute.

There's a large upwards force.

Unbalanced forces means that they're gonna slow down and they're gonna slow down quite dramatically.

But as they slow down, you should know that the drag force is going to decrease.

So after a couple of seconds, the drag forces decreased.

That's shown here.

And so even though they're slowing down, they're not slowing down by as much now.

And as they slow even further, that drag force is eventually going to be the same size as the force pulling them down.

And so we've got balance forces again.

And so the parachutist has reached a steady speed, and this steady speed is a lot slower than it was earlier.

Okay, let's check if you understood that.

What's happening to this parachutist? Have a look at the force diagram and let that help you decide.

Are they A speeding up, B, slowing down, or C, falling at a steady speed.

So pause the video, make your selection, and then restart.

Okay, you should have selected this slowing down.

You can see that the drag force here is greater than the gravitational force pulling them downwards, and that means that they're gonna slow down because there's an unbalanced force on them.

The resultant force is acting upwards, and as they're falling, that's going to slow them.

Well done if you selected that.

So what's happening to this parachutist? I'd like you to look at the diagram and make your decision again.

Are they A speeding up, B, slowing down, or C, falling at a steady speed? Look carefully at the diagram, pause the video, and restart when you're ready.

Okay, in this case, they're falling at a steady speed.

There's equal forces, so there's no resultant force.

And if there's no resultant force, they're not speeding up, they're not slowing down, they must be going at a steady speed.

So well done if you selected that.

Now the whole point of a parachute is it's going to reduce that terminal velocity.

You're still going to be falling, but you're going to be falling at a much slower, steady speed.

So when going head first down, without the parachute open, you're falling at 250 kilometres an hour.

But by opening the parachute, you're going to gradually slow down until you reach a new steady falling speed, a new terminal velocity that's only 25 kilometres an hour.

It's still pretty fast, so you've gotta be careful when you land, but it's much, much slower than it was earlier.

Okay, it's time for the final task now and it's this one.

Jo jumps out of a plane and reaches a terminal velocity.

And what I'd like you to do is to complete the fourth diagrams showing those four different stages, please.

Just before she opens the parachute, just after she opens the parachute, just before she reaches the ground, and just after reaching the ground.

So pause the video, complete those force diagrams, and then restart, please.

Okay, here's the solution to those.

In the first diagram, we've already got the gravitational force drawn.

You should have added a drag force at the same size because she's moving at a steady speed there.

In the second scenario, she opens the parachute, there's a much larger drag force than her weight.

So a much bigger upwards arrow there for the drag.

Just before reaching the ground, she must have reached a new terminal velocity.

So the two forces are equal again.

And finally on the ground, she's still got the weight pulling her downwards, but there's no drag now, it's a normal reaction force.

She's standing on the ground.

Well done if you've got those four.

Okay, we're at the end of the lesson now.

So here's a quick summary of what you've seen.

The gravitational force acting on a falling object is constant.

So that force is always the same size.

But the drag force increases with speed.

So as you go faster, there's a greater drag force.

When those two vertical forces are equal, you don't have a resultant force.

And so the object has reached a steady speed, and we call that the terminal velocity.

You also saw that streamlined and heavy objects have a greater terminal velocity than things that aren't.

A parachute is designed to increase the drag force when it's opened, and that will slow a skydiver to a safe landing speed, a new, much lower terminal velocity.

And that's the end of the last lesson in this unit.

I hope to see you in the next one.

Bye-Bye.