Hello, and welcome to this lesson, which is about the stretching a spring practical.

It's in the physics unit Energy of Moving Objects.

And my name is Mr. Fairhurst.

At the end of this lesson, you should be able to measure the extension of a spring and describe the properties of the spring as it increases in length.

We're going to come across these keywords during the lesson.

If at any point you see these keywords and you want to check what the meanings are, you can always pause the video and come back to this slide and have a look.

Okay, so let's make a start.

The first part of the lesson, we're going to investigate how the spring stretches.

So, let's start by looking at a spring.

We know that when the spring is stretched, we're transferring energy into the elastic store.

We start off with the chemicals in our body.

When they react, they allow our muscles to work, and we can use that to pull the spring at both ends to stretch it so the energy in the chemical store is transferred into the elastic store as we stretch the spring.

And as you can see, I've got equal amounts of energy in the elastic store at the end as we had in the chemical store at the start, and that's assuming that no energy has been dissipated into the surroundings.

Now, when we release that stretched spring, the energy is then transferred from the elastic store and it makes a spring move.

So, it's transferred into the kinetic store and once again, an equal amount of energy is transferred from the elastic store and into the kinetic store.

We're assuming again that no energy is dissipated, whereas in the real world some of it will be and we'll always have a little bit less energy in the kinetic store than we started off with in the elastic store.

And there'll be a little bit of energy dissipated into the thermal store and the surroundings will be just a little bit warmer.

Okay, have a look at this question.

Pause the video and start again once you've selected your answer.

Okay, so how did you get on? How much energy is transferred to the elastic store if 12 joules of work is done stretching a spring by 2 centimetres.

Well, we don't have to do a calculation here.

We've been told the energy that was done stretching the spring, work done is the energy that we've given to the spring.

We've assumed that no energy is dissipated, so we'll have exactly the same amount of energy at the end as we had at the start.

So, the correct answer is B, and well done if you've got that.

As we stretch a spring, we transfer energy to it and we also extend its length.

And this idea of extension is quite a difficult idea to get your head round at first.

In this example, we've got three weights added to the spring which have extended it from its original length.

And we've got that marked on the diagram.

Now, when we had just one weight add it on, the extension was a lot smaller, but it was still from the length of the spring with the weight on back to the original length of the spring.

So, we're always measuring with the extension, the increased length of the spring from its starting length when it was not stretched.

And the larger the stretching force we add on to that, the greater the extension is going to be.

Now, we can control the size of the extending force by hanging weights on the spring and using the weight as the force.

Now, each 100 gramme mass that we're adding pulls down with a force that's equal to its weight, and we can calculate the weight of each of those masses by using the equation for weight.

Weight equals the mass in kilogrammes times of gravitational field strength.

And what we're going to use are masses that are each got a mass of 0.

1 kilogramme so we can calculate the weight of each of those.

So, 0.

1 kilogramme times 10 newtons per kilogramme.

The gravitational field strength, if you remember, is the number of Newtons of force that the gravitational field of the Earth pulls down on each kilogramme, and that gives us a weight or a force of 1.

0 newtons for each of those 100 gramme masses.

Have a look at this question and work out the force pulling on the spring of 100 gramme mass hanger that has got four 100 gramme masses slotted onto it.

Pause the video whilst you do so and start again once you're ready.

Okay, how did you get on? Hopefully you said 5.

0 newtons.

Well done if you got that right.

We have the mass hangar which is 100 grammes plus four extra 100 gramme masses.

It can be quite challenging to measure the extension of the spring accurately to the nearest millimetre.

And if you look at this picture, it's quite hard to actually judge where to measure from, and that's part of the issue.

We're going to measure the length from the top of the loop, and that is because that is at the very bottom of the part of the spring that is stretched, the loop shouldn't be stretched as we're pulling the spring longer and that should stay quite regular.

So, we can mark that point there and measure to the top of the loop.

Now, we can use a set square to make the measurement a little bit more accurate.

And if we place a set square on the top of the loop in this diagram, we've got quite a stretched spring.

We can see here going down from the top, the markers are 36, 37, 38 centimetres.

2, so 38.

2 centimetres from the set square.

Now, it's important to look at the set square with your eyes level to the measurement.

If you're looking from too higher position, you'll be looking over the top of the set square and you'll see some of the measurements below 38.

2 centimetres and you'll get a reading that's too big.

And if your eye line is below the level of the set square, then you'll be looking upwards and more of the scale will be obscured by the set square and you get a reading that's too small.

And that sort of error when we're looking at the measurement from the wrong angle is called a parallax error.

Now, when we're taking our measurements, it's going to be easiest if we line up the measurement ruler so that the 0.

0 centimetre mark, the very top of the ruler, is at the level of the top of the loop so that all of the measurements we're taking are going to be measuring the extension, the increase in length of the spring from its original length.

So, we can double check that with our set square, and then we can take our measurements of extension as we go through the investigation.

Have a look at this, what do you think is the best definition of the extension of a spring? Pause a video whilst you make your selection and start again once you're ready.

Okay, so what do you think is the best definition of the extension of the spring? The correct answer is the increase in length of the spring from its original length.

So, well done if you've got that right.

I think the most common answer that people give that's not right is that it's simply the increase in length of the spring.

It's the increasing length of the spring from when? The last measurement.

The last measurement could have been different for different people so we can't really use that.

So, we need to use the length of the spring from its original length.

So, here we've got all of the equipment set up in the lab ready to take our measurements.

And what we did, we very carefully set the measuring ruler so it's exactly vertical using a plumb bob.

That's a piece of string with a heavy weight at the bottom we just hung next to the metre ruler to make sure it was lined up exactly vertical.

You'll notice in this picture that the clamp stand itself is not quite vertical compared to the ruler, whereas the table leg is.

So, what we can't do is just pick something in the lab and line the rule up against that and assume it's going to be vertical, it may not be.

And an alternative to using a plumb bob would be to use a set square on the tabletop so that the ruler is at right angles to the tabletop.

In other words, exactly vertical.

We've also made the ruler hang over the edge of the table, and that's because we've got quite a long extension on the spring and with the weights hanging below the spring, that's most likely to take it over the edge of the table in order to measure the longest extension.

So, we need to prepare for that before we begin taking any other measurements.

And the risk of doing that is that the whole clamp stand will topple over and fall off the table.

So, rather than let that happen or risk that happening, we've clamped it to the table with our clamp.

And we've also put some padding on the floor, we've just used a cardboard box, so if the spring does break and the masses fall onto the floor, they're not going to be damaged.

The bottom weight, the mass hanger itself, is quite delicate when the heavy weights land on it if it goes straight onto the floor, so we need to avoid damaging that and also to protect our toes.

Have a look at this question, see what you think and choose all the ones that are relevant.

Pause the video whilst you do so and start again once you've made all of your choices.

Okay, so which of these are safety precautions that should be taken when investigating the extension of a spring? Well, we need to clamp the stand to the table so it doesn't topple over.

We need to put padding under the hanging masses primarily to protect the masses and to stop them getting damaged if they fall, but also to protect our feet.

And we need to wear safety glasses.

You could argue that you should tie back long hair because it might get caught in the spring if your hair is particularly likely for that to happen to then by all means do tie it back.

We wear safety glasses in this case because with the tension in the spring, if it does snap it's likely to ping and we've got a sharp end possibly with if it breaks and therefore it could catch you in your eye, so we wear safety glasses just to protect against that eventuality.

So, that's the one I suppose you may have missed off, but very well done if you've got the top two, and very well done indeed if you've got all three.

What I'd now like you to do is to have a go at that investigation.

I'd like to measure the extension of the spring for different forces and to describe what happens to the spring as it's stretched.

Now, to do that I've got a set of instructions here for you to follow.

Set up the apparatus as you've already seen, and then carefully hang a 1 newton weight to the spring.

Lower it carefully with your hands so it doesn't bounce.

If you don't do that, it might drop off or it might damage the spring and cause the experiment to go wrong.

I'd like you to measure the extension of the spring with that weight added and then remove the mass completely, observe the spring, and if it's changed then just jot down how it's different from what it was at the start.

If it looks exactly the same, you don't need to write anything down.

And then, I'd like you to repeat that.

First of all, add the 2 newton weight, measure its extension, and then take it off the spring, observe the spring, and then add the 3 newton weight and so on until you've got a weight of 12 newtons.

You can record them in a table, something like this, with the description, as I just mentioned, of any changes to the spring.

You will need to fill that part in if there are any changes.

And once you've got all of your results, plot a nice clear graph on a piece of graph paper to show the extension of the spring against the force that you've used to stretch it.

So, pause the video whilst you do all of that.

Once you've a good set of results, start the video again and we'll carry on.

Okay, so how did you get on? Here's my results.

Yours should look something like these, but they clearly won't be exactly the same because you may have used a different spring and so on.

Now, for my spring, it only started to look different when it got to about 10 newtons.

At first, it looked very similar to the original spring, but the turns of the coil were slightly pulled apart.

And if you compare it to the original spring, you can see that a little bit more clearly.

For 11 newtons of force pulling at the spring, it looked very much the same but just a little bit longer.

And then for 12 newtons, it was pulled quite a lot out of shape.

Each turn was pulled much further apart and the loop at the top got stretched out of shape.

And you can see the sharp end of the loop, that's the bit that can spring off and catch when your eye if you're not wearing your safety glasses.

And here's my graph of those results.

I've given it a nice title to say what it's showing.

I've labelled each of the axes nice and clearly and included the units of measurement that I used in the investigation.

So, hopefully your graph should look something very similar to that.

We're now going to go into part two of the lesson and we're going to have a think about those results and to describe how the spring was stretched.

So, we're not going to do any numerical calculation in this part, we're simply going to describe the spring and match the descriptions up against that graph.

So, there's the graph, it shows the extension of the spring against a stretching force and we can break this graph down into two parts.

If we look at these measurements between 0 and 9 newtons, it's very much a straight line.

In fact, we should really have drawn that with a ruler.

And then, the top part of the graph, it starts curving up, and that part, something different is happening.

And if we go back to our description of what was happening, we can start thinking about what might have been happening to the spring in each of these sections of the graph.

So, in this part where it's a straight line, all of our observations showed that the spring when the weight was removed, returned straight back to its original length.

The spring was what we called elastic, it's behaving elastic, it just sprung straight back to its original length each time.

And the stretch when we were stretching and pulling it out of shape, we call that elastic deformation.

Deformation means stretched out of shape and the elastic means will ping back to the same shape it used to be once we take the forces off.

Now, when we got to the curved section for 10, 11, and 12 newtons of force, we noticed that the spring being started to be stretched out of shape.

And we call that stretch inelastic deformation.

Inelastic is the opposite of elastic.

If you think about seeing things, we use the word visible to see things and invisible for things that we can't see.

So, in, the two letters at the start of the word mean it's the opposite, so inelastic means not elastic.

So, it's deformation, a stretch that is not elastic because the spring will not ping back to its original shape when we take the forces off.

It's been permanently stretched out of shape.

It's past what we call the limit of proportionality.

So, above a certain size force, we start stretching the spring out of shape with this inelastic deformation.

Pause the video whilst you have a go at this question and then start it again once you've made your choice.

Okay, so what is the stretch of a spring called when it returns to its original shape? Well, the correct answer is elastic deformation.

That word elastic means to ping back to its original length and shape when we take off the forces that are stretching it out of shape in the first place.

Inelastic deformation is when it's pulled out of shape permanently.

And the limit of proportionality is the point after which it starts being stretched out of shape permanently.

So, well done if you got elastic deformation correct.

What I'd like you to do now is this task, I'd like you to add some labels to the graph that you've already drawn for task A and also to write a description of what each of those labels tells you about what's happening to the spring.

And those are the labels that I've put on the screen that I'd like you to add to your graph.

So, just pause the video whilst you do that and start it again once you've got all your labels and you describe what each one tells you about the spring.

Okay, so how did you get on? These are my answers.

I've drawn in the elastic deformation up to about 9 newtons.

And I've said below the limit of proportionality, a stretched spring is not stretched out of shape.

It stretch is called elastic deformation.

I could perhaps, in fact, it's not permanently stretched out of shape because as we're stretching the spring, we're actually pulling it out of shape at that point.

But when we take the forces off, it will go back to its original position, its original length.

And then, for the inelastic deformation, which is the curved part of the graph above 9 newtons, I said above the limit of proportionality, a stretched spring is stretched permanently out of shape, I should have put.

Its stretch is called inelastic deformation.

It's the opposite of elastic deformation if you like.

And at 9 newtons, I've marked on the limit of proportionality.

And I've sort of said what that means already in my other two statements, so I'm not going to add anything extra to that.

If you've got something similar to that, and in particular if you've used the word permanently deformed or permanently stretched out of shape, very well done indeed.

So, very well done for reaching the end of the lesson.

This is a short summary slide that just describes the key points from the lesson.

And again, it's very similar to the answers to task B.

Below the limit of proportionality, a stretched spring is not stretched permanently out of shape.

Its stretch is called elastic deformation.

Above the limit of proportionality, a stretched spring is stretched permanently out of shape.

And its stretch is called inelastic deformation.

The extension of the spring is the increase in length from its original length.

And inelastic deformation causes a permanent extension.

So, again, very well done if you've reached end of the lesson.

I do hope to see you next time.

Goodbye.