Hello, and welcome to this practical lesson on friction.

My name's Mr. Norris.

This lesson is from the Forces topic.

This lesson, you're gonna have to use all of your scientific investigative skills to look at how we can best reduce friction.

We're gonna need to work really carefully to make sure that we get really accurate results that we can confidently draw a conclusion from.

The outcome of this lesson should be that by the end of the lesson, you'll have shown you can collect data to test the effectiveness of different lubricants, taking steps to ensure a fair test.

Here are some keywords we're going to be focusing on this lesson, lubricant, fair test, anomalous result, repeat measurement, and mean.

Each word will be explained as it comes up in the lesson.

This lesson has two sections.

In the first section, we will introduce the practical where we're gonna compare different lubricants, and then you'll do the practical.

And then in the second section, we will look at calculating mean average results from your results from the practical.

So let's get going with the first section.

A lubricant is a liquid that's added between two surfaces to reduce the friction.

Remember that friction forces happen because even surfaces that look really smooth actually have microscopic irregularities, so tiny bumps or spikes on the surfaces that catch on each other when the surfaces try and slide across each other, and that's where friction comes from.

So the lubricant reduces friction by keeping the surfaces slightly further apart.

So those tiny irregularities, the tiny bumps, catch on each other less when one object slides across another, or perhaps even not at all, reducing the friction significantly.

And oils and polishes are examples of lubricants.

So that's what we're gonna be testing today.

We're gonna be testing different lubricants to see which one reduces friction the most.

We're gonna be testing these three lubricants, cooking oil, washing-up liquid, and water.

Each lubricant's gonna be spread over the bench top.

Remember, in science, the tables we work at, we call them benches.

So here's an example of spreading some cooking oil across a bench.

This person's doing it with their hand.

Your teacher might show you an alternative way of spreading it across the bench evenly.

What you're then gonna do is flick a mass across that lubricated surface using an elastic band, a bit like this.

So stretch the elastic band, pull the mass back, and flick.

There we go.

So that's what we're gonna do.

The sliding masses slow down and stop because there's a friction force acting from the bench top.

So as that mass slides to the left, a friction force acts against that motion, slowing it down and eventually stopping it.

Let's do a quick check of your understanding of this practical so far.

You've got to choose from the words smaller and greater to complete the sentence.

So there are two gaps and two words to choose from for each gap.

So a better lubricant, a better lubricant means the mass will slide a, which word, distance because the friction force, when there's a better lubricant, will be, and then choose which word.

So five seconds to make sure you've made your choice for each gap.

Let's see how you got on.

A better lubricant means the mass will slide a greater distance because with a better lubricant, the friction force will be smaller.

Well done if you got that.

Now, in this experiment, we need to be sure that if the mass goes a greater distance, that's only because we used a better lubricant.

We can't let anything else change that makes the mass go further because then if the mass does go further, it could have been because of that other thing we let change instead of the fact we used a different lubricant.

And all we want to do in this experiment is find out which lubricant is best, so we can't let anything else affect the results.

Only which lubricant we used.

So that's why nothing else in the experiment could be allowed to change between tests, and that's the principle of ensuring a fair test.

We can't give the mass like an extra push or a little boost or a headstart in one of the tests 'cause then that wouldn't be fair on the other lubricants that didn't get the little boost or the little headstart.

We genuinely want to find out which lubricant is better, so that's the only thing we can let change between the tests, 'cause otherwise it wouldn't be fair on the other lubricants if the masses got a little boost or a bit of extra force in one of the tests.

So how are we gonna do that? What we should do is we should mark a start line on the desk with positions for the finger and thumb when we stretch the elastic band, and you should use a whiteboard pen that rubs out easily.

So here's an example of what I mean.

There's a start line with positions, little crosses for the finger and thumb, and then the mass can be flicked.

And each time the mass is pulled back, so the front edge of the mass is on the start line.

So watch them do that now.

Front edge of the mass on the start line, then it's flicked, okay? And the same every time.

So front edge of the mass on the start line, then it's flicked.

That allows the elastic band to be stretched by the same distance, giving the same force from the elastic band every time, 'cause we can't accidentally push the mass with a greater force in one test 'cause that wouldn't be fair on the other lubricants that didn't get the extra force, okay? So we wouldn't know if the extra distance was 'cause we gave it a little bit of extra force or because the lubricant really was better, and our job here is to find out which lubricant really is better.

So we can't let anything else affect the results.

Let's do a quick check on that.

So we've just said that marking the bench top helps to control the distance the elastic band is pulled back, and it also helps to control the distance the elastic band is stretched across the start line.

Now, can you think of anything else, any other factors we should try to keep the same every time so they don't affect the results? So have a little think, pause the video if you need to, and see if you can come up with anything else we should keep the same every time so that the only thing that changes between the tests is the lubricant and everything else is the same.

What else should we keep the same? Five seconds.

Off we go.

Let's see if you came up with the same things that I thought of.

So did you think of the part of the bench top that's used as the track? 'Cause different bench tops around the room might have different surfaces cause different amounts of friction, and we can't let that change between the tests 'cause that could affect the results.

Did you think of the mass that's been used each time, and which side of the mass? The top or the bottom, in case the surfaces are different, so it would affect the friction.

That's always gotta be the same every time.

And what about the depth and evenness of the lubricant? We've gotta make sure it's well spread into a thin layer every time 'cause it wouldn't be fair if there wasn't just quite enough lubricant one test or there was too much lubricant the other test, affecting the results.

We need to make it a fair test between all the lubricants and test each lubricant in the same way every time.

Something else we've got to look out for when you're collecting data is you may get anomalous results.

These are results that don't fit with the other results, possibly because a mistake was made with that test.

So, here is one result where somebody flicked a mass, the lubricant used was cooking oil, and the mass travelled 51 centimetres before it stopped.

So is that an anomalous result that doesn't fit with the other results? Well, there aren't any other results, so we can't tell.

We don't know if that's an anomalous result or not yet.

We need repeat measurements to decide if a result is truly anomalous or not.

So here's the next result where the person repeated the same test with the same lubricant and the same mass and flicked it in the same way, and this time, it went 30 centimetres.

Oh, that's very different.

The 30 centimetres does not fit with the first result of 51 centimetres.

So maybe that 30 centimetre result is anomalous, except we don't know if it's the 30 centimetre result that's anomalous or the 51 centimetre result that's anomalous.

We need more than just two repeats to be able to tell, 'cause if you've got two results like these two and they're both different to each other, you don't know which one's correct.

Just 'cause you did the first one first doesn't mean that that's the correct one.

It might be that the 30 centimetres is the correct one and the 51 centimetres is anomalous.

So you must do a minimum of three repeats when doing repeat measurements.

So here's a third repeat.

Ah, right.

And now we know which of our first two results is probably the anomalous one.

It's the 30 centimetre result that doesn't fit with the other two.

So we need to do repeat measurements to decide if a result is anomalous where a mistake might have happened, and we don't really want to include that data because it's not valid because a mistake happened.

So anomalous results are assumed to be mistakes, so what do we do with them? We draw a single line through them so they can still be read.

And ideally, anomalous results should be retested if possible, but sometimes in experiments in a school lesson, there's not time to do that.

If you do get time to do that, fantastic.

That's what you could do, and you could record your retested result next to the line-crossed-out result.

You never fully cross out an anomalous result because it might turn out later that it's actually a sign that something more complicated was happening, and you still need to be able to see what results you've previously discarded were just in case they turn out to be useful in the future.

So one line through to show it's an anomalous result, but make sure it can still be read.

Quick check on anomalous results.

Which statements correctly describe anomalous results? A, anomalous results do not fit the pattern of other results.

B, anomalous results are always mistakes.

C, anomalous results should be rubbed out.

D, anomalous results should be retested if possible.

Which of those correctly describe anomalous results? Choose as many as you think.

Off you go.

Let's go through each one.

Statement A, anomalous results do not fit the pattern of other results.

That's the definition of an anomalous result.

That's correct.

B, anomalous results are always mistakes.

That's not correct.

Anomalous results are assumed to be mistakes, but they may turn out to be evidence of something unexpected or unusual later.

So they're not always mistakes, but we do assume they're mistakes for now.

Statement C, anomalous results should be rubbed out.

Nope, they must still be readable.

And statement D, anomalous results should be retested if possible, that's correct.

Well done if you identified A and D as correct.

So, you're now ready, I think, to do the practical.

So step one in the practical will be to mark a start line and positions for the finger and thumb, place a metre rule alongside, and practise flicking the mass in the same way every time using your marked start line and the marked position for the finger and thumb.

They're the steps you need to take to ensure a fair test.

Step two is then evenly spread a small amount of lubricant onto the test area.

Step three is then flick the mass, measure and record the distance it travels against that metre rule.

And step four, repeat the process to take three repeat readings for each lubricant, and check for anomalous results as you go.

Now, if you're just watching this video and you don't have access to the equipment, then there is a video of example results being collected which you can watch now.

<v Instructor>In this investigation,</v> we're going to be testing three different lubricants to see how well they can reduce friction.

Now, to do that, we're going to fire a small mass, which you can see here, and we're going to fire it across the surface of the table with an elastic band.

And we're going to try that with three different lubricants to find out which one enables the mass to move the furthest.

Now, the problem is, when we're testing it out, we can pull it back by all different amounts.

So we can make it go that far, or just that far, or even this far, just by pulling back the elastic band by different amounts.

And it's also important how stretched the elastic band is and how far apart our fingers are.

That can make a difference to how far it goes as well.

So what we've got to do is to keep a number of things exactly the same each time we take a measurement.

So to make sure we keep those things the same, let's start by drawing a start line with a ruler and mark on two points roughly where we're going to put our thumb and forefinger.

And if you take the mass and put our finger, our first finger and our thumb on those crosses, pull the mass back so that it's in line with the start line, and leave go, it fires that distance.

Let's try that again.

Exactly the same, it's gone a different distance.

And if we keep repeating that, we can get an eye for the fact that it keeps going about the same distance each time.

So if we keep those points marked nice and clearly throughout, let's just make them a little bit bolder, then we can use those as a guide each time we release the mass.

And so to take the first reading, we're going to put a little bit of oil onto the table, some cooking oil, and spread that really thinly over the whole surface so we've got a nice, thin, even coating of oil over the whole of the table.

And then when we're ready, we're going to move the ruler and place it alongside so we can measure the distance that the small mass travels, then release the disc and measure how far it goes.

Now, this first one, as we can see, has gone 51 centimetres in front of the start line.

Let's try a second measurement.

It's a bit slippy.

Oops.

And that one, I was in a bit of a rush there, but that went 30 centimetres, which is a very different reading.

So let's try that a third time to see which of those two is the most accurate.

And for our final reading, and as you can see, it's gone 55 centimetres, which is close to the first measurement that we took, so we can assume the first and the third ones are closest to the correct measurement.

And for this second set of readings, we're going to use some washing-up liquid.

Again, just a small amount spread very thinly over the whole of this tabletop surface.

We've already cleaned the oil off, so that's going straight onto a nice clean surface.

Then when we're ready, we're going to fire the mass across that surface and again measure how far it goes.

So it's a bit slippy, I think.

So we'll give it a go, and there.

It's gone 46 centimetres.

Let's try a second measurement.

And that time, it's gone 37 centimetres.

So one third and final go again.

And that final one has gone exactly 37 centimetres again.

So those are our three measurements for the surface being, the lubricant being the washing-up liquid.

And the final lubricant that we're going to test is just water.

Again, we've cleaned the table, got all of the washing-up liquid off it, and now we're just gonna spread a thin layer of water over the surface, nice and thin, just the same way we've done for the other lubricants.

And then we're going to test our mass by firing it with the elastic band for a final set of three results.

So here's the first one.

I forgot to put the ruler on, so let's pop the ruler on, and that is about 51 centimetres.

Try the second one.

And that one's gone about 31 centimetres.

So one final attempt, and that one is about 33, 34, so 33 1/2 centimetres.

And that's our final set of results for this investigation.

<v ->For feedback on that task,</v> here is an example set of results.

You might have got results similar to this.

These are the example results from the video recording of the experiment being done.

Now, you probably might have spotted, as the video was being done, there were two clear anomalies in that set of data which have been crossed out with a single line, there and there, because they are anomalous results that do not fit with the other results for that lubricant.

Another thing just to note is that the third result for water in the video, it was mentioned as it was thought to be 33.

5 centimetres, but really, all results should be recorded to the same number of decimal places.

And all the other results were given to the nearest centimetre, so we should probably give that result to the nearest centimetre as well.

That's why it's been recorded as 34 centimetres.

So, well done for your effort on that practical.

Let's look at what we should do next with the data we've collected.

So here's the example data from the experiment.

And we've not filled in the final column yet to calculate the mean.

A mean or average result should be calculated.

And in science, we should round a mean to the same number of decimal places as the measurements 'cause we can only make measurements to the nearest centimetre.

Therefore, we should only give our result to the nearest centimetre.

Otherwise, we'd be claiming we could measure to a better level than we really could if we give our mean to more decimal places than we can actually make the measurements to.

So our means in this case should all be rounded to the nearest centimetre so the means are the same as the measurements that we made.

So how do you find the mean? This will be something you'll have done before.

Find the mean of this set of numbers, 25, 22, 24, and 27.

You have to add them all up.

And there are four numbers in that set, so you divide by four, and that gives you 24.

5.

However, all of those numbers were given to the nearest whole number, so in science, we should also round our mean to the same number of decimal places as the measurements, which in this case is no decimal places.

So the mean in this case, we should give it as 25 because we round it to the same number of decimal places as the measurements.

So now your turn.

Find the mean of this set of numbers.

So you'll need to get out your calculator and find the mean of that set of numbers.

Follow exactly the same process.

Off you go.

Pause the video if you need to.

Okay, you should have found the mean of that second set of numbers.

Pause the video if you've not done it yet.

So how do you do it? You add up all of those numbers and divide by the number of numbers that there are, which in this case is five.

So that gives you an answer of 30.

2.

However, in science, a mean should be rounded to the same number of decimal places as the measurements, which in this case is no decimal places, so to the nearest whole number.

The mean should be given as 30 in this case.

Very well done if you did all of those steps correctly.

And let's just think about anomalies again.

In science experiments, anomalous results, to start with, we just assume they're mistakes until we do further investigations, possibly in the future.

So because they're assumed to be mistakes, we don't include them in the mean.

They're not allowed to contribute to the final results 'cause anomalous results are results where we think a mistake happened, even if we're not sure what the mistake was.

So for the set of results from the example results, for cooking oil, we think the result for 30 was an anomalous result, so we're not gonna include it in the mean.

So we would calculate the mean result just by including our two good results, and we divide by two because there's only two numbers that we're counting towards the mean.

So that will come to 53.

And it comes out to a whole number, so we don't need to round it because it's already to the correct number of decimal places, which is no decimal places in this case, so to the nearest centimetre.

So in it goes for the mean for cooking oil, not including the anomalous result.

So I'm gonna get you to have a go at this in just a moment.

So if you were asked to find the mean of this set of measurements in centimetres, 25, 22, 24, and 37, I mean, the 37 should be jumping out at you as that is very different to the others.

It doesn't fit with the others.

It should be classed as an anomalous result.

So when you're finding the mean of that set of measurements, do not include the anomalous result.

Only include the non-anomalous results, the results which are valid.

So 25, 22, 24 all divided by three gives you 23.

7.

But then what's the final thing we need to do? We need to make sure it's rounded to the same number of decimal places as the measurements, which in this case is no decimal places, or to the nearest whole centimetre, which in this case would be 24.

So that's what we should give the mean as for that set of four measurements.

One was anomalous.

Your turn now.

Please find the mean of this set of repeat measurements where they're all in centimetres.

Pause the video if you need to.

Off you go.

Right, I'm gonna go through the answer to that now, so pause the video if you're still working on it.

So looking at those results, 31, 29, 32, 20, and 30, well, they're all about 30 apart from 20.

20 looks anomalous to me, so I'm not gonna include it in the mean.

It's very different to the others, which are all around much closer to 30.

So I'm gonna find the mean of four numbers.

So add up the four valid measurements and divide by four, 'cause there's four of them.

That gives 30.

5.

And I'm gonna round it to the nearest whole centimetre so it matches the measurements, so I'm gonna give the mean as 31 centimetres for those measurements.

Very well done if you did all of those steps.

Okay, time for the final task of this lesson.

I would like you for the first thing to calculate the other two means for the sample results from the video.

So we've already got the mean for cooking oil where we didn't include 30 because it was an anomalous result, but I would like you to calculate the mean for these results for washing-up liquid, and the mean for these results for water, following exactly the same process I just showed you.

And then look at what Jacob is saying about these results, "It's hard to decide if the 46 centimetre result for washing-up liquid is anomalous or not." So for now, it's not being crossed out.

So in part one of the task, assume it's fine, but Jacob's right.

Actually, if you look at it, it is possibly verging on anomalous, but it's hard to decide.

So for part 2a of this task, I would like you to calculate the mean for washing-up liquid again, but this time treating the 46 centimetre result as anomalous.

We treated it as not anomalous before in part 1, and then in 2a, you are gonna treat it as anomalous and recalculate the mean.

And then in part 2b, suggest what Jacob could do to help decide whether the 46 centimetre result is anomalous or not.

So get your pen, get your calculator, you should have it handy from the checks that we just did, and pause the video now and have a go at all of the parts of that task.

Off you go.

I'm gonna give feedback on the task now.

Make sure you've done all parts.

So calculate the means for the other two lubricants.

For washing-up liquid, you've got results of 46, 37, and 37.

So add them all up, divide by three, that gives 40 exactly, and you didn't need to round it because it was already to the nearest whole centimetre.

So in it goes.

The mean washing-up liquid is 40.

And then the mean of water, the mean result for water, well, 51 has been identified as an anomaly because it doesn't fit with the other two results.

So we're only calculating the mean based on the two valid results, which is 31 and 34.

So that gives a mean of 32.

5, and then round it to the nearest whole centimetre so it matches the results, gives a mean of 33 for water for those sample results.

So well done if your means exactly match those.

And now part 2a of the task, calculating the mean for washing-up liquid again, but this time treating the 46 centimetres result as anomalous because it's difficult to decide.

It is a bit different to the other two results.

So I've treated it as anomalous.

Now, you should be able to see, actually just by looking at those two results, that if result two is 37 and result three is 37, then the mean of those two will just be the same number, 37.

So that was actually really easy to do.

But if you did do the calculation, the two results are 37 and 37, which you have to add up and then divide by two, gives you 37.

And it's already a whole number, so you don't need to round it.

So that was that.

Now actually, the mean before was 40, and that was not treating 46 as anomalous, and the mean now is 37 if the result for washing-up liquid is treated as anomalous.

So the mean is either 37 or 40, which actually aren't too different.

So that is what you'd expect, actually, if a result is perhaps borderline anomalous but it's not really clearly anomalous that it actually hasn't had a huge effect on the mean.

It's only changed it from 40 down to 37.

So that's why in borderline cases, you can kind of make a judgement call either way and it shouldn't affect your mean result too much.

And for part 2b, what could Jacob do to help decide whether that result really is anomalous or not? Well, here's what I think he should do.

Ideally, I think it would be great if he could repeat the measurements again to see if the measurements cluster around 37 centimetres, because if you did lots more repeat measurements and almost all of them were close, really close to 37 and not close to 46, that would mean the 46 probably is an anomaly.

Whereas if Jacob did lots more repeat measurements and he got readings all across that range between 37 and 46, then that would suggest that the 46 is not an anomaly.

So repeat readings can help you decide whether a result is a mistake and an anomalous result or not.

So very well done for completing this lesson about the practical to investigate friction.

Here is a summary.

Lubricants are liquids added between two surfaces to reduce the friction between them.

In this lesson, we tested three different lubricants.

In a fair test, only the variable being tested is changed, so any differences in results can only be due to that variable.

And all other factors must be kept the same for all measurements.

Repeat measurements help check for anomalous results, results that do not fit with other results so are assumed to be mistakes.

And a mean combines the repeat measurements into a single final result.