Hello there, I'm Mr. Forbes, and welcome to this lesson from the Moving by Force unit, which is all about testing parachutes.

You're gonna carry out practical tasks where you build different parachutes and see which one of them falls the fastest or which falls the slowest.

By the end of this lesson, you are gonna have built a set of parachutes of different sizes, drop them, time how long it takes them to reach the floor, and use that information to calculate their speeds.

That will allow you to find out which of them travelled fastest and which travelled slowest.

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

These are the keywords you'll need to understand to get the most of the lesson.

First is drag or drag force, and that's the force that acts on an object as it's moving through something like air or water or any other fluid, that's a gas or a liquid.

Second is parachute, and that's the device we're gonna build, and that slows us down by increasing drag force.

The third is the force that actually pulls you down towards the ground, and that's gravitational force.

And the final one is resultant force, which is the result of the forces acting together, the overall effect of the forces.

And here's some definitions of those keywords for you to refer to.

If you forget what any of them mean, you can return to this slide at any point just to check out again.

The lesson's in just two parts.

And in the first part we're gonna talk about why objects fall downwards and the forces acting on them, and then why parachutes should have an effect.

Then you're gonna build some parachutes and test them out to see whether or not the large parachutes or the smaller parachutes will slow you down the most.

The second part is about processing the information you collected during the experiment and calculating some average speech from it.

Okay, let's start with a quick recap of the forces that act on you when you're falling through the earth.

When you jump out of a plane and open a parachute, there are two main forces that act on you.

The first is the gravitational force that pulls you down, and that's a constant force because you're attracted towards the earth.

Sometimes that force is called weight.

The second force that acts on you is a drag force, and that acts upwards acting against the motion.

And that drag force is because you are having to push the air particles out of the way to be able to fall through them.

When you're falling, that drag force increases as you get faster and faster, because you're pushing more and more air out the way.

But the gravitational force stays constant on you.

Eventually, you're gonna reach a steady fall speed, and you reach that steady fall speed because the two forces eventually become equal to each other and effectively cancel each other out.

So there's no acceleration, because there's no resultant force.

Now the whole point of a parachute is to slow you down.

If you jump out an aeroplane and fall for a while, you eventually reach a top speed of something like 200 kilometres per hour.

It's about 55 metres per second.

That's very fast.

When you open the parachute, you increase the drag force acting on you a lot, and that causes you to slow down tremendously.

Eventually you'll reach the speed of something like 25 kilometres an hour, much, much slower.

That's only seven metres per second.

You're still falling, but you're falling at much safer speed, so when you reach the ground, you're gonna be able to land safely.

In this lesson, we're gonna be testing different sizes of parachutes and the effect they have on falling speed, but we're not gonna be using cloth parachutes 'cause one of the problems with them is they need a few seconds to open up fully and have an effect.

Instead, we're gonna build parachutes out of pieces of paper or card.

So, paper parachutes.

We're gonna do that by folding the card in a particular way so that it falls fairly steadily downwards in a straight line.

To do that, I'm gonna show you this video of how to make them.

What we're going to do here is to fold a piece of paper into the shape of a tray to make a form of a model parachute.

First of all, we have to fold up each of the sides and crease it well.

It's important to make that crease go all the way along the paper, because that makes it easier to fold the corners in in a few moments time.

Once all four sides are done, we can then start squeezing in the sides and squashing them to form those little triangles and do the same thing for each of the corners.

The more you can crease it, the sharper the crease, the better it's going to end up.

And there we have a paper tray, which acts as a model parachute.

If you drop it from a height, it will fall nice and straight down to the ground.

Okay, you should have got a good idea of how to make the parachutes there, and we're gonna make some a little bit later on.

If you need to refer back to the video, just come back to this slide and see it again.

Right, so our paper parachutes should look something like this, a folded tray, and we're gonna drop them so that the flat side's down and those edges that have been folded upwards there.

And as you drop it, it's gonna fall fairly vertically.

There might be some wobbles side to side, but with a bit of practise you'll be able to drop it so it falls pretty much in a straight line.

The two forces acting on it, again, are the drag force and the gravitational force.

And because of the shape, those two forces very quickly come into balance and the parachute falls at a steady speed.

When you drop a parachute that's shaped like this, you should get an effect falls directly down to the ground, a bit like this.

So you saw there, there was some variation where it moves a little bit side to side, but as I've said, with practise, you'll be able to get it to fall fairly steadily very well.

So as I've said, we're testing whether or not the sizes of the parachutes have an effect on the falling speed.

So I've got a quick check about whether you can make a prediction.

A pupil drops two pieces of paper that have the same weight, one scrunched into a ball and the other folded into a parachute shape.

Which piece of paper will reach the floor quickest? Is it, a, they both reach it at the same time; b, the scrunched up ball paper; or c, the paper folded into a parachute.

And I want you to decide whether you're sure those are right or you're sure those are wrong or somewhere in between.

So pause the video, make a selection, and then restart once you're happy.

Welcome back, let's have a look (indistinct) those.

First of all, you should realise they're not gonna reach the ground at the same time.

The whole point of a parachute is to change the speed that you're falling at, and if it didn't have any effect, we wouldn't use them.

The scrunched up ball of paper should reach the ground the quickest, because it's got the smallest drag force acting on it, so it reaches the fastest speed as it falls so it takes the shortest time to reach the floor, and the parachute should reach the floor in the slowest time because it's got a larger surface area, it's got a bigger drag force, and therefore it'll go a smaller speed and take longer to reach the ground.

Well done if you've got those three.

So in this experiment you're gonna test different size parachutes, and what you're gonna do is make those parachutes from identical sheets of paper, but fold them into different sizes.

And the reason we want identical sheets of paper is because we want this to be a fair test.

We want all the parachutes to weigh the same amount, to have the same gravitational force pulling them downwards.

So, it's only the drag effect that we're changing.

Okay, so you're gonna take three sheets of identical paper and fold them slightly differently so you get different size tray shapes, basically.

Before carrying out the experiment, we should make some kind of prediction about what's going to happen based upon our knowledge of drag and gravitational forces.

Andeep's made a prediction here.

"The greater the surface area of the parachute "the greater the drag." Well, that's not quite a prediction.

That's saying his knowledge of drag forces.

So you need to use that to then make a prediction about fall speeds, which he does.

He says, "That this means a larger parachute "will fall at a lower speed "and take longer to reach the ground." So now we've got a prediction about what he thinks gonna happen.

You can make a different prediction, if you like, before starting the experiment.

Right, we'll need a method to be able to test that prediction.

So Andeep's gonna test his prediction using this method.

He's gonna measure out three heights, a drop height where he's gonna release the parachute from, and then a bit lower down a start line.

And that start line's lower down so that the parachute can accelerate to its top speed so that it's falling at a constant speed.

And then the end point of his experiment's gonna be the floor.

So when the parachute reaches the floor, then he's gonna stop his timer.

So he's gonna drop the parachute from the drop height so it reaches a steady speed.

It's speeding up during that part of the journey.

Then, he's gonna time how long it takes to pass from the start line to the floor at that steady speed.

He needs some example distances, so it suggested 0.

5 metres for it to speed up in, and then a metre at least so that he's got enough time to start and stop his stopwatch.

So those distances should work quite well.

This very short video clip will show you a parachute falling to the floor so you get an idea of what the experiment's all about.

Basically just dropping the parachute, letting it pass the start line, start the clock at the same time as that, and then stopping the clock when it reaches the floor.

And let's watch that clip.

I've slowed the video down so you can see these times more clearly.

Right, so we saw the parachute for during the clip, and you can calculate the fall time from the readings from the clock.

The time at the start line was 15.

71 seconds.

So it didn't need to start the stop clock as it passed the line.

I already had it running.

The time when it reached the floor was 16.

71 seconds.

And so that actually took exactly one second to fall from the start line to the floor.

So a nice simple value there.

Your values will probably be somewhat different than that.

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

I'll go through the instructions with you before you start.

So, first of all, you need to make those paper parachutes with different surface areas, three different sizes, large, medium, and small.

Remember there's a video clip of that earlier in the lesson.

So you can go back, if you haven't made the parachutes already.

Then I want you to drop the one with the largest surface area, the largest parachute, from the drop height, and time how long it takes for it to fall from the start line to the floor and record it in a table that looks like that one.

Now you can see in that table that we've got repeat readings for the time.

So I want you to repeat the drop test with the larger parachute two more times and record those fall times for me, please.

And then repeat the experiment with the medium and then the small paper parachute.

So, in the end you've done nine different tests with those three different parachutes.

We'll calculate the mean times and things like that a bit later on.

So if you understood those instructions, you can now carry out the experiment safely, please.

Okay, you should have carried out the experiment now, and you should have results that look a little bit like this.

Now remember, your parachutes won't be exactly the same size as mine, and you might have dropped your parachutes through a different height.

So your data won't match these numbers, but they should show the same sort of pattern.

You should get fairly consistent results for each of the parachutes.

So you can see my large parachute there, 1.

20, 1.

34, and 1.

24 seconds.

It's not too much variation between those.

The medium parachute, they're all around one second mark.

And my small parachute was about 0.

8 seconds for those three readings.

So you're looking for consistent readings like that for each parachute, but different for each sized parachute.

Now, as you've collected all your data, we need to process that a little bit to find mean times that it took the parachute to fall and then calculate these speeds from those mean times.

So we're gonna do the second part of the lesson now, finding fall speed from timing results.

The first stage of processing the results is to find the mean fall time.

We're gonna do that for each parachute separately.

We don't just take all nine results and find the mean of them.

We find the mean for the large parachute, then the mean for the medium, and then the mean fall time for the small parachute.

And we do that like this.

We add up the three different time measurements, and then we divide it by three.

So, for the large parachute here, the mean time is those three values, the three times added together, 1.

20 plus 1.

34 plus 1.

24, and then divided by three.

Gotta be careful when using the calculator here and make sure that you do the addition first and then divide that total by three.

If you forget to do that, you'll get the wrong value.

So once I've done that, calculated it, I get a value of 1.

26 for my mean time there.

I'll put it in the table.

For the second parachute, I do the same process, add three values and divide by three.

That gives me a mean time of 1.

05 seconds, and I put that in the table there.

And for the final one, the same process again, add the three values, divide by three.

That gives me a mean time, of 0.

80 seconds.

And I complete my table by putting the value in there.

Right, let's check if you can calculate the mean times for a set of results.

I don't want to use your results yet.

I want you to use the examples given in this table.

And I'd like you to calculate the mean fall time for the large parachute only.

Use the data in the table shown there, please.

So pause the video, calculate the mean time for the large parachute, and then restart when you're ready.

All right, you should have got the answer to that by adding these three values together and then dividing by three.

And that gives a value of 1.

36 seconds.

So if you've got 1.

36 seconds and wrote it there, well done.

You've got it right.

Okay, we've not actually found the speeds yet.

So that's the next stage of the process.

We're gonna calculate the steady fall speeds for the three different parachutes.

And to do that we're gonna use the speed equation.

So we've got a set of example data here that I'm gonna lead you through.

So first of all, I need to know how far the parachutes fell.

My parachutes used a distance of one metre exactly.

So I'm gonna use that in my calculations.

I've got a set of mean times there.

So I need now to use those distances of one metre and mean times to find speed.

And to do that we're gonna use the speed equation, which is speed equals distance divided by time.

So starting with the large parachute, I identified the distance and the time from the data in the table.

And those two values are there.

It's one metre or 1.

26 seconds.

So I'll write that out as part of the speed equation.

Speed equals one divided by 1.

26, and I calculate that and it gives me a speed of 0.

79 metres per second.

And I can write that into my results table.

I'll repeat the same process with the medium parachute.

I look for the data in the table.

It's one metre again, and it's 1.

05 seconds, the mean time that it took to fall.

Calculate the speed and that's 0.

95 metres per second.

And the third example for the small parachute, identify the data and write the equation again with that data in, speed equals one divided by 0.

80.

Do the calculation, and that's 1.

25 metres per second.

So I've completed all my calculations, and I've got all my three fall speeds there.

Right, there's a quick check here to see if you can calculate fall speeds.

So I'd like you to calculate the fall speed for the large parachute that's falling 1.

0 metres using the data in the table.

And I only want you to calculate for the large parachute, please.

Ignore the medium and small parachutes for now.

The fall distance was 1.

0 metres, so I'd like you to identify the right time to use and calculate the fall speed for that parachute.

Pause the video, do the calculation, and restart when you're ready.

Okay, this is how you should have done it.

First of all, you identify the data.

So, the fall speed is gonna be the distance divided by the mean time.

The fall distance one was 1.

0 metres, the mean time 1.

41 seconds.

So we put those two values into the equation, and then we do the calculation on our calculator, and that gives us a fall speed of 0.

71 metres per second, which we put in the table.

Well done if you've got that answer.

Okay, now it's time for you to calculate the fall speeds for your parachutes.

If you don't have any suitable data, I've provided some for you here.

It's important to notice that my parachutes have only fallen 0.

5 metres for this data.

So that's the distance you use in the calculations.

And you can see you've got to calculate the mean times and the mean fall speeds from that.

So, what I'd like you to do, calculate the mean times for each parachute using your results if you've got them or my results if you don't.

And then calculate the fall speeds for the parachutes using your results, or my results if you need to.

Okay? So pause the video, do all your calculations, and then restart, and we'll have a look at some answers.

Welcome back.

Let's process my results to see the fall speeds from my parachutes.

So first of all, calculate in the mean times.

To get the mean times, add the three values and then divide by three for each parachute separately.

So, I got a mean time of 0.

92 seconds for the first.

And then I do the process for the other two as well, 0.

84 and 0.

63.

And then I calculate the fall speeds, making sure I use the correct fall distance and the correct time.

So, calculating the fall speeds, fall speed is distance divided by mean time.

Made the distance 0.

5 metres and the time 0.

92 seconds, and you get a fall speed of 0.

54 metres per second.

And again, I repeat those calculations using the data for the medium and for the small parachute and I get a completed set of results.

So, well done if you've got anything like that.

Okay, we've reached the end of the lesson now.

So, just a quick summary of the information you should have learned.

When an object falls, it'll speed up until it reaches a steady speed.

And the reason for that is because, although the gravitational force acting on the object falling is constant, the drag force will increase as you get faster and faster.

Eventually you reach a steady speed when that drag force and the gravitational force are the same size and there's no resultant force acting on you anymore, and therefore, there's no acceleration.

So, you're not gonna speed up or you're not gonna slow down.

And the results of your experiments should have shown that the larger the surface area of the parachute the bigger it is, the greater the drag force it provides, and so you get a lower fall speed.

It'll take longer for the large parachute to reach the ground than it did for a shorter parachute.

As I said, we've reached the end of the lesson now, so I hope you enjoyed the practical work, and I'll see you in the next one.