Hello there, my name is Mr. Forbes, and welcome to this lesson from the "Hidden Forces" unit.

The lesson's all about stretching rubber.

So, we're going to be taking some rubber bands and we're gonna be putting forces on them and measuring how much they extend.

It's a similar experiment to one you might have carried out before, but we're gonna see if the results produce the same sort of patterns.

By the end of this lesson, you'll have carried out an investigation into what happens when you put different weights on a rubber band, and you'll have compared the behaviour of that rubber band to the behaviour of a spring when you put weights on that to see if it's the same.

And here are the key words that you'll need to understand to get the best from the lesson.

The first of them is elastic, and we're gonna investigate that type of behaviour, 'cause we're looking at rubber, and we're gonna determine whether it's elastic or not.

The second is extension, and that's the additional length when you put a force on something.

So, it's the increase in length that we'll measure.

The third is directly proportional, and you should already know that the extension of a spring is directly proportional to the force you put on it.

We're gonna see if that's true for elastic bands or rubber bands.

And finally, long molecular chains, and that's an idea we're gonna use to try and explain the behaviour of rubber bands which contain long molecules.

This lesson's in three parts, and in the first part, we're gonna have a lead up to an investigation into the stretching of a rubber band.

So, you're going to be measuring how that rubber band extends when you put different forces on it.

And it's very similar to an experiment you may have carried out with a spring.

In the second part, we're going to describe what your results show.

We're gonna see what the pattern in the stretching of a rubber band is and compare that to the pattern in the stretching of the spring.

And in the final section, we're gonna try and compare the stretching of the spring and rubber band and explain the differences between them in terms of the materials they're made from.

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

When you put forces on a rubber band, that rubber band will stretch, and it'll behave in a similar way to a spring in that it will get longer, it will extend when you put forces on it.

But there are some differences.

When you stretch a spring, it stays the same thickness but just gets longer.

But when you stretch a rubber band, the rubber band will actually become thinner as you stretch it, as it gets longer.

And the more force you put on it, the more it will stretch.

And so, it'll get even thinner when there's a larger force.

Both the spring and the elastic band, sorry, the rubber band will return to their original shape and size once you remove those forces.

And we call that type of behaviour elastic behaviour.

So, a rubber band is an elastic material.

Now, you can investigate the stretching of a rubber band in the same way as you may have investigated the stretching of a spring.

You get the rubber band suspended from something and then you can put some hanging masses on it, and those hanging masses will produce a constant downward force, and you can measure the extension.

So, if I place two newtons on the rubber band, it will get a bit longer and I can measure its extension.

And if I place three newtons on, it'll get longer again, and I can measure its new extension.

The extension is the distance it's stretched from its original length, so how much longer it's got.

And in this lesson, you're going to test whether the rubber band is gonna behave in the same way as a spring.

So, we're going to put weights on it and measure its extension.

And you're going to find out the answer to the question: Is that extension directly proportional to the force on it? So, when I double the force, does the extension double? And that's the behaviour of a spring.

We don't know yet if the rubber band is actually gonna follow that pattern.

So, let's check if you understand the purpose of the investigation and how you can make a prediction based on it.

I've got four pupils here, all of them trying to make a prediction.

I'd like you to select which two of the pupils have made predictions about the extension being directly proportional to the force.

So, I've got Laura saying, "The rubber band will extend by the same distance "for each newton." Got Izzy saying, "The rubber band will get longer "the more force you put on it." I've got Jun saying, "There is no extension "where there is no force pulling on it." And I've got Alex saying, "The band will get thinner the more force you put on it." And now you can select the two students who've made predictions that the extension's directly proportion to the force piece.

So, pause the video, select the two, and then restart.

Okay, welcome back.

And the two students that have made predictions about direct proportionality are Laura and Jun.

So, well done if you selected those two.

Right, so let's outline the procedure that you're going to follow to try and measure the extension of a rubber band.

And it's slightly different than measuring the extension from a spring because a spring is quite stiff and it's easy to find the bottom of it, but rubber band is a bit loose, and we need to have a technique to try and find out what its length is before it starts stretching.

So, you're gonna hang a rubber band from a clamp stand as shown in the picture here.

So, there's the rubber band and here's the clamp stand.

You're then gonna position a vertical ruler, and the vertical ruler is gonna be used to measure the length of the rubber band or its extension.

We need to place the ruler so that the zero is exactly in line with the bottom of the band, and we'll see how to do that in a minute.

So, we need that zero to be exactly aligned there, otherwise we're not measuring the extension properly.

So, let's see if you understand how to set the equipment up.

I've got a picture here of part of the equipment set up, and I'd like to say what's wrong with that arrangement of the ruler and the elastic band.

Is it because the rule is not vertical, the bottom of the elastic is not in line with the zero on the ruler, or the rubber band is already stretched? So, pause the video, make your selection, and restart.

Welcome back, hopefully, you noticed that the elastic band is not aligned with the ruler properly in that picture.

So, we need to sort that problem out.

To align the bottom of the rubber band with the ruler, you ought to use a technique like this.

Basically you need to put another ruler into the rubber band as it's shown in the picture here, and need to pull it down very gently just so you get its natural length so it's not all scrunched up.

So, pull down gently and then use that ruler, make sure it's horizontal, and align it with the other ruler at the zero point.

So, you'll need to adjust that vertical ruler until it's exactly in line with the bottom of the elastic band once you put down that tiny force on it.

It's very important that you don't overstretch the rubber band here.

So, you don't wanna put a large force on it, just very gently so that it's not all tangled up.

And that will give you the zero point for the rubber band.

Okay, what you're going to do next is measure the extension for different forces.

So, you're going to hang weights from the bottom of the rubber band with the weight holder and record the extension in centimetres for each weight you put on it.

So, you can see I've got a table here, and I'm gonna put a force of nought to 10 newtons on it, and I'm gonna measure the extension for each of those.

So, to make sure you know how to calculate extension, I'd like you to look at this diagram, and I'd like you to work out what's the extension of that rubber band.

Remember that that rubber band was aligned with the zero on the ruler before we put your weight on it.

So, pause the video, choose your answer, and then restart.

Welcome back, and hopefully you saw that the end of the rubber band is at 5.

5 centimetres.

So, that's the extension.

It's 5.

5 centimetres longer than it was without the weight on it.

Well done if you chose that.

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

So, the instructions are fairly straightforward.

Gonna set up the equipment, zeroing the end of that rubber band.

So, it's in line with the top of the vertical ruler.

You're gonna hang a mass hanger onto the end of the rubber band and measure the extension and then you're gonna add masses one at a time, measuring the extension of the rubber band, going up to a total of about 10 newtons, depending on how strong your elastic band is.

To help you understand that procedure, I've got a short video that shows you how to do it.

So, let's watch that now.

(ambient background noise) (ambient background noise) (weighing clinking) (weighing clinking) (weighing clinking) (weighing clinking) (weighing clinking) (weighing clinking) (weighing clinking) (weighing clinking) Okay, you should be ready to carry out the experiment.

What I'd like you to do is to pause the video, get your readings, and then restart, and we'll look at what you've got.

Okay, welcome back.

Hopefully the experiment went well for you.

I've got my results here.

You can see I've filled them in in the table, and they're increasing as they go on, but only to analyse those further to see exactly what's happening to the elastic band.

Your values will be a bit different than mine 'cause your elastic band won't have been exactly the same, but you should get a similar sort of pattern.

So, well done if you've got those.

Right, now it's time to move on to the second part of the lesson.

We've collected our data from the experiment, and what we're going to do is use that data to describe how a rubber band stretches, and we'll see if it stretches in the same way as a spring.

So, I've got my data organised in the neat table here table.

I've got lots of different forces and extensions, and what I need to do is to plot a graph to see if there's a pattern in the relationship between force and extension.

So, I'm gonna need a sheet of graph paper, a little bit like this, and I'm gonna plot force along the bottom in that direction, the X direction, and I'm going to plot extension up in the vertical direction or the Y direction.

And I'm gonna need to choose some suitable scales for that data, in order that my points will fit on the graph paper neatly.

So, let's see if you can choose some suitable scales.

I'd like you to complete these statements, suggesting what values for the scales and the visions should be used based on that results table I've collected there.

So, the maximum for the force scales should be what Newtons with divisions of how many Newtons.

And the maximum for the extension scale should be how many centimetres with divisions of one centimetre and subdivisions of what? So fill in those blank for me, please.

Pause the video, fill them in, and restart.

Okay, welcome back.

There were a variety of options you could have chosen there.

I've chosen a maximum force scale of 12 newtons 'cause that's a maximum force we've got there.

You might have chosen to go up to 15 perhaps.

The divisions of one Newton would be typical because I'm using exact values.

So, one Newton, two Newtons and so on.

The maximum for the extension scale should be, well, I've chosen 18 centimetres.

You may have chosen 20 centimetres 'cause it's above the maximum value of my extension there.

And subdivisions are one millimetre because I've got readings that go to about 0.

7 of a millimetre.

So, I need those subdivisions to plot accurately.

Well done if you've got answers similar to that.

Okay, I've got my data, and I'm gonna plot that on a graph now, and you're going to do this in a little while.

So, if I plot my first point from my data, get a point like this, then my second, and third, and that's straight-ish, but it looks like it's curving a little bit, perhaps, for that third one.

Then if I start to plot the rest of the points, I'll notice it's very, very different.

I get an increase in the stretchiness here.

The extension is going up by bigger values each time, and as I continue on, that starts to decrease again and the line flattens.

So I've not got a typical straight line shape that I might have been expecting.

If I try to draw straight lines, none of those pass through the points very well at all.

So this graph isn't a straight line graph, so we should get rid of those.

What I should do with this is you can see it's a smooth S shape, so I should try and draw a curve, a single smooth curve through those points and that shows me the shape of the extension for a rubber band.

Okay, I've got three identical sets of data here and I'd like you to do is decide which of those lines I should have drawn for that set of data, line A, B, or C.

I'd like you to pause the video, make a selection, and restart.

Welcome back.

Hopefully, you chose line C.

That's a single smooth curve passing through the points.

Line A is a straight line and those points definitely don't follow straight line.

And line B is a sort of straight lines joining the dots together individually, and we should never plot those sorts of graphs in physics.

We should look for a straight line or a smooth curve, not dot to dots.

So well done if you chose C again.

Okay, it's time for you to plot a graph based on your experimental results.

So I'd like you to use your data from your experiment to produce an extension against force graph for your rubber band.

If you didn't get any useful results, you can use the data I've got in the table there instead.

Once you've plotted that graph, I'd like you to try and come to a conclusion that describes the relationship between the extension and the force and give a reason for your conclusion please.

So pause the video, plot your graph, reach your conclusion, and then restart when you're ready.

Okay, welcome back.

And here's the sort of thing you should have come up with.

I've got a graph of extension against force there, and you can see it's that typical S shape curve.

Your conclusion should be that the extension is not directly proportional to the force because the line is not a straight one.

It does go through the origin, 0,0, which a line of proportionality would do, but for this one, it's definitely not a straight line.

So it's definitely not a directly proportional relationship.

Well done if you've got that answer.

Okay, now we've plotted our graph.

We've noticed that the extension is not in direct proportion to the force on the rubber band.

We're going to try and explain that stretching pattern in the third part of the lesson.

When we measure the extension of a spring, we can see that it's directly proportional to the force acting on it.

So we get a straight line graph like this that passes through the origin.

But the extension of a rubber band is different.

The rubber band gives us a pattern similar to this.

So that sort of S shape.

It's very, very different.

The extension's not proportional to the force acting on the rubber band.

We can see that the elastic band's behaving differently depending on the force on it.

In the first section of the graph here, when we've got a small force less than three newtons, the rubber band doesn't stretch easily, its extension isn't going up very much at all.

So it's quite stiff in that part when we've got small forces.

But then once we've started stretching it a bit, the rubber band seems to get a bit looser, more stretchy, because the rubber band here is stretching quite a lot for each newton we put on it, it's much more stretchy.

But then once we get to large forces, the rubber band becomes less stretchy again.

You can see the gradient of the graph here is shallower.

The extension's not going up much for each newton we put on it.

So we've got sort of three phases to the stretching of the rubber band.

It's not stretchy at first, it's quite stretchy in the middle and it's not stretchy again towards the end.

Now we've only tested one rubber band and we've got a pattern like this, an S shape.

But if we test a range to rubber bands, we find they all fall the same sort pattern in the same way that springs all fall the same sort pattern.

But for rubber bands, they always produce this S shaped curve where they're not stretchy for small forces, quite stretchy for medium sized forces, and then not stretchy again for large forces.

I test another one and I get the same sort of shape.

So no matter what I do, I find that all rubber bands are hard to stretch at first, then they become easier to stretch for medium forces, and then hard to stretch at the end.

So we get those S-shaped graphs.

Okay, so now as you know the difference between rubber bands and springs and how they stretch allow you to identify some like to look carefully at this graph and decide which of those lines represent the stretching of rubber bands.

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

Okay, welcome back.

We're starting with the top one, that is a rubber band.

You've got a characteristic S shape there: Not stretchy at first, stretchy, then not stretchy.

B is a spring.

So the next rubber band is C, that's another S shape.

D is also a rubber band, and E is a spring.

It's a straight line that's going through the origin.

So well done if you identified those three.

As we've already found out, spring stretch in proportion to the force, they're directly proportional to the force acting on them.

So a spring is a load of turns of coil, a metal coil, all close together.

When I put a force on it, you can see it gets longer and a larger force again, it gets a bit longer still.

What's happening there is the turns of the coil are moving apart.

So it's allowed to stretch in a simple way because those coils can separate simply.

There's nothing complicated about their arrangement.

So the turns of that coil separate uniformly due to the force, and that gives me an extension that's directly proportionate to the force on the spring, giving me those straight line graphs we've seen before.

The structure of rubber is much more complicated than the structure of a spring.

Rubber bands contain lots of long molecules, long chain molecules that are all coiled up with each other.

So you can see my little figure here, I've tried to identify some of the separate molecules.

Obviously, this is a much simplified version, and they're all tangled together a little bit.

When I put a small force on them, they start to uncoil.

But because they're tangled, it's quite difficult for them to uncoil.

And so it doesn't stretch much for that small force.

But once they've started to uncoil a bit, they start to be become more stretchy because they're less tangled together.

So the long chain molecules are lined up, and the larger forces can then cause it to stretch much, much more.

So the rubber band has become much stretchier for medium-sized forces.

Eventually though I've stretched those long chains as long as they'll stretch and it becomes much more difficult to stretch 'em out even further, it becomes very hard, because they're already stretched close to their limit.

So they'll become very stiff again and more stretch as much.

Now that's quite a complicated thing to picture.

So let's watch a quick video that tries to model that stretching behaviour with some wool.

The wool represents the long chain molecules in the rubber.

At first, they're all coiled together.

When a force is applied, the long molecules begin to uncoil.

The tangling makes this very difficult, and so the elastic doesn't stretch much.

Once the molecules are untangled, it becomes much easier for them to be stretched out.

The rubber stretches much more easily.

The long molecules stretch until they approach their maximum length and then they become much harder to stretch again.

The rubber band is hard to extend.

So hopefully that video helped you understand what's going on inside the long chain molecules of the rubber band.

Okay, to check if you understood that, I'd like you to decide whether this is true or false and give some reasons for your answer.

So a rubber band stretches differently to a spring because it's made of a softer material.

Is that true or false? Positive the video, make your decision, and then restart.

Okay, that was false, but I'll need you to justify that answer.

So I'd like you to choose from one of these two.

Is it because the rubber band has long molecular chains which behaved differently to metal coils, or is it because the rubber band is made from a harder material than a spring's so it's harder to stretch.

Again, pause the video, make a selection, and restart.

Okay, hopefully you chose answer A, and that's the rubber band has long molecular chains, so it's very different behaviour than metal coils.

Well done if you've got that.

And now it's time for the final task of the lesson, and this asks you to explain the shape of the graph.

So I've got a graph here showing the extension of a rubber band and different forces are acting on it, and I'd like you to explain the behaviour of the three parts of that graph: Part A for when there's small forces, Part B, medium sized forces, and Part C for when there's large forces acting on the band.

So I'd like you to write out a description of what's going on in each of those parts, please.

Pause the video, write your descriptions, and then restart to see the answers.

Welcome back.

Hopefully, your answers are a bit similar to these.

So for part A, during part A, it's not stretchy much.

You can see there these very small extensions for those small forces and that's because the long chain molecules are all coiled together inside that rubber band.

In part B, we've got a greater stretching.

You can see the extensions going up much more for each newton of force, and that's because those molecule chains are able to straighten out during that section, and so the rubber band can become longer.

And finally in part B, it's not stretching much.

Again, you can see the extension's quite small for each newton of force, and that's because the molecules have straightened out as much as they can and they can't stretch anymore.

If you've got answers similar to that, well done.

Now we've reached a summary of the lesson, so let's go through what you should have learned during it.

The extension of a rubber band is not proportional to the force on them.

But rubber is elastic.

So you can see a rubber band is hard to stretch at first and easy to stretch from medium sized forces, then harder to stretch, and that's very different from the stretching of a spring.

The behaviour of a rubber band can be explained by its internal structure inside.

It's got long molecular chains that are all tangled together and they're easy to stretch in some parts and more difficult to stretch in others.

So hopefully you've got a picture of that in your head.

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

Goodbye.