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Hello and welcome to this lesson on measuring forces.

This is from the forces topic.

My name is Mr. Norris.

In this lesson we're gonna be developing our practical skills and learning how to measure forces using a newton metre.

So the outcome of the lesson is that by the end of the lesson, hopefully you should be able to use newton metres to measure forces accurately.

Here are some keywords we're gonna be talking about in this lesson.

newton, newton metre, scale, accurate and zero error.

Now each word will be introduced as it comes up in the lesson at the appropriate point.

But on the next slide there's an example of each keyword used in a sentence.

You could pause the video on the next slide and have a read through to get prepared as possible for the lesson.

So pause the video now if you'd like to have a read through of the lesson keywords.

This lesson is divided into two sections.

The first section introduces or recaps using a newton metre, and then there is a first practical element where you can go and practise that skill of using a newton metre.

The second section is about adjusting and selecting a newton metre.

So looking in a bit more detail about how to use newton metres really well.

So let's get started with the first section.

So different forces have different sizes.

You only need a small force to lift an apple.

Apple's quite low mass, you don't need much force to lift it, whereas pushing a car would require a much larger force.

So we can use a longer arrow to represent that larger force.

So how do we measure the size of a force? Well, we use units called newtons, so that's a lowercase n when you write the word newton and a capital N for the symbol for newtons.

So the unit is named after Sir Isaac Newton.

So it's a capital N for Newton when it's his name.

And when it's the unit, it's a lowercase n.

A force of about one newton is about roughly speaking, the force needed to lift an apple.

So every apple's gonna be slightly different, but roughly speaking, a force of about one newton is about the force needed to lift an apple, something that's about a kilogramme, like a bag of flour or a bag of sugar that's a kilogramme, that would need 10 newtons of force to lift it.

So that gives you a sense of roughly how big one newton and 10 newtons of force are.

So this is a newton metre.

I'm sure you'll have used these before.

This is what we use to measure the size of a force.

So let's just go through the different parts.

At the top is an adjustment screw, more on that later, that's used to adjust the newton metre.

Within the newton metre there's the plastic casing, which just about shows up on the photo.

And then within that plastic casing is a spring that gets pulled down.

The marker is that kind of yellow disc that gets pulled down when the whole newton metre gets pulled down and you can see the marker gets pulled down in line with a point on the scale.

And that's how you take the reading by reading the scale, where is the marker that yellow disc against the scale.

And finally the last part is the hook, which is where you attach the object, which is gonna exert the force on the newton metre, the force that you're trying to measure.

So how do you take a measurement? Let's just look at that in detail.

So here is a newton metre that's been set up on a clamps stand.

And you can see some masses have been hung on the hook.

So that's step one, attach the object that will exert the force to the hook and the object will then stretch the spring and pull down the marker to the level on the scale, which indicates the size of the force that it's pulling with.

So then you read the position of the marker against the scale and we're pointing right at the position of that yellow disc against the scale.

So let's have a look at a couple of newton metres now.

So let's look at the newton metre on the left of the screen.

The the red one that's on the left of the screen, that newton metre, the marker is directly in line with the six line on the scale.

So that newton metre is reading six newtons, 6.

0 newtons.

That's fairly easy to read because the marker's exactly on one of the marked lines.

And the same for the other newton metre.

On the right of this slide, the green newton metre that's on the right of the slide, that marker is exactly in line with one of those thick lines, which in this case represents 0.

5 newtons, half a newton because that line is exactly halfway in between the zero line and the one line.

So it must be halfway between zero and one, so it's 0.

5 newtons.

But of course you need to be able to read the newton metre whatever bit of the scale the market is at.

So let's just make sure we can go through that.

So to read a scale accurately, you've got to first work out what the intervals are.

The intervals are the jumps that the scale goes up in.

So on that first scale, it goes up in steps of intervals of 0.

5 newtons.

So each line is 0.

5 newtons 0.

51, 1.

52, 2.

53 and 3.

5 and the last line that's marked on the diagram is four.

But what about the next scale along? Well that's going up in steps or intervals of two newtons.

So each line, so it goes 2, 4, 6, 8, 10, 12, 14, and the last line is marked as 16 going up in intervals of two newtons.

So to read a scale accurately, firstly make sure you know what steps, what intervals is going up in.

And if needed you can do this calculation to make sure you've got that right.

Look at the first number that's marked on the scale after zero and you divide that by the number of intervals, the number of steps to get to that number from zero.

So on that first scale diagram that's on the slide now, the first number marked after zero is one and there are two steps to get there.

So what's each step where you have to divide that one newton step into the two little steps it takes to get there.

So of course the scale goes up i0.

5 newtons, which is hopefully you could see that anyway, but it's helpful to talk through a really clear obvious example.

What about the other scale on this slide? Well, the first number marks after zero is four newtons and there are two steps to get there.

So four newtons divided into two.

So that scale goes up in two newton intervals.

So that's how to work out what the steps are.

If you're not sure, look for the first number marked after zero and then divide by the number of intervals it took to get there.

Okay, we're gonna do a little check that you can do this now.

So on this newton metre, look at the picture please.

How many intervals are there between zero and one newton? That's the first question.

Just how many intervals are there? And then secondly, how much force does each interval measure? So can you have a go at those first? I'll give you five seconds to start with for those two.

Let's check you are on the right track.

So how many intervals between zero and one newton? Well, there's five intervals there.

There's five gaps between the zero and the one line.

So therefore how much force does each interval measure? Question two, well there's one newton divided into five gaps.

So one newton divided into five gaps gives you 0.

2 newtons for each interval.

So this scale goes up i0.

2 newton steps.

The interval is 0.

2 newtons.

So now we know that we can measure wherever the scale's pointing to 'cause we know that the interval is 0.

2 newtons.

So now have a go at part three of this check.

Okay, what does each newton metre read? It might be worth pausing the video on this one.

To check you can read each newton metre, get a reading for each of those three newton metres.

Off you go.

Now pause the video if you need to.

Right? Let's check your readings.

The first newton metre looks like it's exactly on the one newton line.

The second newton metre picture looks like it's exactly on the sixth newton line.

So the both of those should been really easy to do.

And the third newton metre looks like the marker is on the second line after the eight newton line.

So the scale goes up i0.

2 newton intervals.

So it's the second line.

So the first line would be 8.

2, that must be 8.

4 newtons.

So very well done if you got all three, but don't worry if you didn't 'cause we got another chance to practise now.

So exactly the same questions, but now for this newton metre.

So on firstly on this newton metre, how many intervals are there between zero and one newton? And secondly, how much force does each interval measure? So have a go at those two.

They should be fairly quick.

I'll give you five seconds.

If you're not ready, pause the video if you need to.

So for question one, there are 10 intervals between zero and one newton.

Therefore how much force does each interval measure? Well, one newton is broken into 10 intervals.

So we've gotta divide with the one newton by into the 10 intervals, therefore each interval is 0.

1 newtons.

So well done if you've got that, that scale goes up i0.

1 newtons.

So now we know that you could have a go at part three.

What does the newton metre read in each of those pictures? Pause the video if you need to and make sure you've got a reading for each picture.

Off you go.

Pause the video if you need more time.

So the first one that is north 0.

5 newton's, half a newton.

The second one is exactly on the three lines.

So again that should have been really easy.

That's 3.

0 newtons.

And the third picture, it's on the second line after one newton and this scale goes up in naught 0.

1 newtons.

So that is just 1.

2 newtons.

We'll do one final task with one last picture.

What does this newton metre read? So I've given you, you just need to choose A, B, C, D or E.

What does that newton metre read? So look carefully at the scale, work out the interval.

Once you've worked out the interval, then try reading the scale.

Pause video if you need to.

The correct answer is D.

This newton metre reads 6.

4 newtons.

That's because between four and six, that's a chunk of two newtons but the gap between four and six is divided into five intervals.

So the two newtons between four and six two newtons is divided into five intervals, two divided by five is 0.

4, therefore each interval represents 0.

4 newtons.

So that marker is at the first interval after six newtons.

So it must be 6.

4 newtons.

Very well done if you got that one.

That was a tricky scale to work out.

Okay, time now to do a task where you get to actually use newton metres.

Now if you are watching this and you don't have access to equipment, there's actually nothing that you can really do on this task 'cause you've already practised in the previous checks everything that you could do from photographs that are just done again in this task but with a real newton metre.

So I'll go through the task and if you've got access to the equipment then this is what you should do.

Okay so collect a newton metre, a small spring and a 300 gramme mass.

And secondly, work out what are the intervals on the scale of your newton metre just like we just did from the photographs.

And then thirdly, measure the forces required to lift a pencil case, pull a pencil case along a bench, extend a spring 10 centimetres and the force needed to lift 300 grammes and record those in a table.

You should also repeat the measurements to check for mistakes and that's a really important idea in science.

We always repeat measurements that we make to check for mistakes and mistakes still get recorded and don't get crossed out completely.

We still need to be able to be able to read mistakes because it might turn out later that they weren't mistakes at all.

They were just an unusual reading for a reason we didn't understand at the time.

So repeat readings to check for mistakes and record all the readings that you make and then if there's time you can measure the forces required to lift or pull other objects of your choice as well.

So if you've got access to a newton metre on that equipment, then you can do that now.

And if you don't then don't worry because you've just done everything you can from photographs already in this lesson and there'll be other opportunities later in the lesson to do more practise readings from newton metres too.

So if you can have a go at that task now.

You should pause the video and come back when you've done that task.

Right I'm gonna give you some feedback if you've just done that task.

Here's an example answer for question two 'cause question one was just collecting the equipment or part one was just collecting the equipment.

So there's an example newton metre, the intervals on that newton metre would be 0.

1 newtons because there the scale goes up in 0.

1 newtons 'cause there's 10 intervals between zero and one.

So that one newton divided into the 10 intervals means each interval is 0.

1 newtons.

So you should have written perhaps a sentence like that if you were able to do the task intervals by newton at represent 0.

1 newton's or whatever they represented on your newton metre that you used.

Here's some example results for parts three and four of the task.

Part three was the first measurement and part four was the repeat measurements.

So the forced to lift a pencil case, well that's gonna depend on the mass of the pencil case that you actually lifted.

So everybody would get different results for that.

However, you should check that your repeat measurements are very close to your first measurements because that would suggest if they were far apart then maybe one of those measure measurements was a mistake.

So hopefully they're close together and you can see in the example results they are quite close together, which is reassuring.

And then the force to pull a pencil case across a bench, again that will depend on the mass and the material of the pencil case.

However, for the same pencil case being pulled across the same tabletop or the same bench top, it should really require the same amount of force.

So in these example results it looks like one of those results might be a mistake and that's what's important to do repeat measurements to check for mistakes.

You really need yo do a third repeat measurement to find out if it's more like 0.

9 newtons or more like 1.

4 newtons for whoever collected this data, the force to extend a spring 10 centimetres.

Now if everybody in your classroom used identical or fairly identical springs, everybody should get fairly identical results for for that one.

So that would be a good one to check with the people around you about does everybody get similar measurements? And your repeat measurements should be similar to your first measurements for the force required to extend a spring 10 centimetres if everybody used the same style of spring and the same for the force to lift 300 grammes.

Now on earth the force needed to lift 300 grammes slowly.

Should be about three newtons but somewhere between 2.

9 and three newtons.

So again that would be a good one to check with all the people around you to check that everybody got about three newtons for that force.

Well done for your effort on doing that task.

Hopefully it gave you some really valuable practise on using newton metres.

Okay, so it's now time for the second part of the lesson on adjusting and selecting a newton metre.

So when there is no force applied, newton metres should read zero and this newton metre does that.

There's nothing on the hook and you can see it's reading exactly zero.

The market is exactly in line with the zero line.

So that's all good and there's just a zoomed in image there.

However, this newton metre should read zero.

There's nothing on the hook but it doesn't, it reads 0.

2 newtons.

When there's nothing on the hook it should read zero, but it reads an oh 0.

2 newtons.

That means all readings are gonna be 0.

2 newtons too big because if it reads 0.

2 when it should read zero, then when it should read 0.

2, it's gonna read 0.

4 and when it should read 0.

4 it's gonna read 0.

6.

And when it reads when it should read 0.

6 it's gonna read 0.

8 because it's every measurement starts off 0.

2 newton's too big.

So every measurement becomes 0.

2 newton's too big.

What about this newton metre here? Now this new tome does the opposite.

It's gonna make readings about 0.

2 newton's too small because the marker is starting at a position about where minus 0.

2 newtons would be.

The marker's starting off too high.

So you'd have to pull down the hook with a force of 0.

2 newtons just to make it read zero.

So if you pull it down so it reads 0.

2 newtons, you've actually used a force of 0.

4 newtons to do that.

So every reading it's gonna be 0.

2 newtons too small, you'll have used 0.

2 newton's more force than the newton metre actually reads.

So this problem is called a zero error when the newton metre should read zero, but does it? So let's do a quick check of that.

No force is applied to this newton metre.

The newton metre shows an error.

Could you have a go at answering each question please? Question one, what is the name for this kind of error? Should be obvious we've just said.

Number two, will measurements made be too big or too small? Think about what the newton metre should read that newton meter's got nothing on it.

And question three by how much will results be inaccurate? Okay, pause the video now to give yourself enough time to answer all three questions quickly.

Right you should have had to go at answering all three.

So question one, this is called a zero error 'cause the millimetre should read zero because no force is applied but it doesn't, it reads 0.

3 newtons when it should read zero.

So will the measurements made be too big or too small? Well right now the measurement is too big because it should read zero, but it's reading 0.

3 so all the measurements are gonna be too big by 0.

3 newtons.

So that's the answers to question two and question three.

Very well done if you've got those.

So what can we do about zero error? Well the first thing to do is check for it.

So when you first pick up a newton metre, hold it up with no mass on the hook.

Make sure it's being held vertically, not at an angle and check that it reads zero when it should read zero when there's nothing on the hook.

But if it doesn't, if there is a zero error and the newton metre doesn't read zero when there's nothing on the hook, then you can correct for it using the adjustment screw.

You might need to experiment with the screw, twist it first one way.

You might even need to do quite a few turns before you notice the marker moving back to the correct zero position.

And if you twist the screw the wrong way to start with, then you'll actually make the zero error bigger.

So you might have to experiment with which is the correct way to turn the adjustment screw to adjust the new two metre back to zero when it should read zero.

The following video will show a demonstration of this <v Narrator>On this newton metre</v> the scale does not line up with the zero mark and this is called a zero error.

If we take measurements with the newton metre like this, then every measurement we take will be a little bit too big because it's starting with the measurements a little bit above zero.

To adjust for this, we need to turn this adjustment screw at the top and move up the scale until it reaches zero.

And now with no zero error, all of the measurements you're going to take are going to be accurate, <v ->Right? We're onto the last part of the lesson now,</v> which is about selecting a newton metre.

So of course different mutters could be used to measure different sized forces.

So a property of each newton metre that's important to know about is the range of each newton metre.

So this is a zero to five newton newton metre 'cause it can measure from zero newtons to five newtons.

So the range of each newton metre goes up to the greatest force it can measure.

In this case five newtons.

Whereas this newton metre goes from zero to 10 newtons.

So the range is is a zero to 10 newton newton metre and this one goes from zero to 20 newtons.

So it's a zero zero to 20 newton newton metre.

So have a look at this.

The same force North 0.

5 newtons is being exerted on all three of these newton metres.

So on this first newton metre, the scale goes up in north 0.

4 newton steps.

That's the interval and that's what a force of half a newton looks like on that newton metre.

This newton metre has a different scale, different interval.

The scale goes up in 0.

2 newton steps and that's what a force of 0.

5 newtons looks like on that newton metre.

Actually it looks a little bit inaccurate.

I wonder if the person checked for a zero error properly when they were taking the photo.

And this newton metre has a different scale interval as well.

This scale goes up i0.

1 newton steps.

That's what 0.

5 newton's looks like on that newton metre.

Now I think it should be fairly clear which newton metre is probably gonna give the most accurate reading of the 0.

5 newton force.

It's the last one.

Okay? And that's because smaller forces can be measured much more accurately when the scale 'cause of in smaller amounts because you can see smaller differences more clearly if the scale goes up in smaller amounts, basically if the interval is smaller.

So that's a useful principle.

Smaller forces can be measured more accurately when the scale goes up in smaller intervals.

So the best newton metre to use is the one whose scale goes up in the smallest intervals.

But the range is still large enough that it can measure the force that you're trying to measure.

So this new tome has a range up to five newtons and the intervals are 0.

1 newton.

This next new tome in some ways is better 'cause it can measure a bigger range of forces up to 10 newtons.

But the trade off is the intervals are a bit bigger.

It can only measure forces to the nearest 0.

2 newtons.

Whereas this newton metre can measure up to 20 newtons.

The range is bigger but the intervals are even bigger again, it can only measure to the nearest 0.

4 newtons.

So the best newton metre to use is the one who scale goes up in the smallest intervals but the range is still large enough.

So outta the three, that first one's gonna be best for forces up to five newtons because it's got the smallest intervals and it can measure forces up to five newtons.

What about the next newton metre? Well actually that's the one that's gonna be best for forces between five and 10 newtons because anything below five newtons you should use the first newton metre, the green one on the left of the screen.

But anything between five and 10, you should use the yellow one 'cause that's got the next best interval for measuring forces above.

It's got the best interval for measuring forces that are above five newtons that the first newton metre can't measure 'cause the range isn't big enough.

And then that third newton metre on the right of the screen, the red one, this is gonna be best for forces between 10 and 20 newtons.

Because if a force is smaller then you should use the other two, one of the other two newton metres that have the smaller interval.

So let's do a check about which newton metre is best to measure a particular force.

Choose the most suitable newton metre.

Newton metre A, newton metre B or newton metre C, to measure each force of those forces.

Forces one to six on the left of the screen there.

So remember you are looking for the smallest interval possible, but the newton metre has still got to be able to measure the force.

So have a go now pause the video if you need to and press play again when you've chosen A, B, or C for each force.

One to six, off you go.

Okay, so I'm gonna go through the answers now.

So the strategy for doing this is we should try and use new to metre A 'cause it's got the smallest interval, unless the force is too big for A to measure, then we still switch to B because B's got the next biggest interval.

And if the force is too big for A or B to measure, then we should switch to C as a resort 'cause that's got the biggest interval.

So we want to only use C if we have to to measure a large force.

So let's go through them in turn.

Force one is 6.

2 newtons A can't measure it but B can.

So let's go with B force two, neither A or B can measure it.

The force is too big for A or B to measure.

So we have to go for newton metre C, force three and force four are both below five newtons.

So three and four can both be measured by newton metre A because they're below five newtons and we want to use A if we can it's got the smallest interval.

Force number five 11 newtons can't be measured by A or B, we'll have to use C to measure it 'cause the range of B is 10 newtons and this is an 11 newton force.

We have to use C.

And then force number six is below five newtons.

So we can use a again with how's the smallest interval.

So that was how to do that task.

Well done.

So we're now ready to do a second practical task in this lesson.

So again, if you're in a classroom, then you will have been provided with a selection of newton metres and objects, which you can measure.

And you are going to measure the force exerted by each object one hung on a newton metre.

But some of these forces will be very small and some of these forces will be bigger than the scale on some of the newton metres.

So you will need to choose the most appropriate newton metre for each object by considering the scale on the newton metre metres.

And remember to check for a zero error every time you pick up a newton metre and you're about to use it.

Then check for a zero error by checking it read zero with no false applied and then adjusting it if necessary.

If you are watching this without access to the equipment, then there will be some example photos of newton metres on the screen.

So you can still practise the skill of reading a newton metre.

They'll come up on the next slide.

So if you've got access to the equipment, you should go and do that task now, recording your measurements for different forces and which newton metre you use to measure each force.

Okay, well done for your effort with that task.

If you are watching this at home, then here are some photos of example newton metres that you could pause the video now and attempt to read what each newton metre, what force each newton metre is measuring.

So you've had to go at practising the skill of reading a newton metre before I go through the answers, pause the video now to do that if you'd like that extra practise at reading newton metres.

Okay, I'm gonna go through the answers now to this task, to the example newton metres, what they read.

So the first three pictures all show the same newton metre, the yellow newton metre in the first three pictures.

The interval on the scale goes up in 0.

2 newtons.

That's useful to know when making the readings.

First picture shows six newtons exactly, it's exactly on the sixth line.

The second picture shows 5.

4 newtons because it the, the market is on the second line below five newtons.

So it's 5.

2 and then 5.

4 newtons, the third picture, it's on the third line below three newtons.

So it's 3.

6 newtons.

So 3.

2, 3.

4, 3.

6 newtons.

There's an argument about whether it's really just above that line, possibly it's 3.

5 newtons.

It's definitely a bigger force than 3.

4 newtons.

The markers definitely beyond the 3.

4 newton line.

It's a bit tricky to tell from the photograph whether it really is on the 3.

6 newton line or whether you could say it's halfway between 3.

4 and 3.

6.

So calling it 3.

5, I've gone for 3.

6 is what it looks like, I think mostly to me.

But if you said 3.

5, I think that's okay.

The fourth picture is the green newton metre.

This has a different scale interval.

The interval goes up in steps of 0.

1 newtons.

So that to me looks like 3.

8 newtons 'cause it's on the eighth line after three.

So 3.

8 newtons and then the final one, this scale goes up in steps of 0.

4 newtons.

So it's beyond 12.

It's not the first line or the second line.

It looks like the third line after 12 newtons going up in steps of 0.

4.

So 12.

4, 12.

8 must be 13.

2 newtons if it's on the third line after 12.

And the interval is north 0.

4 newtons.

So very well done if you've got every single one of those correct, that would be very impressive.

So well done for making it to the end of the lesson on measuring forces.

Here is a summary.

The size of a force is measured in units called newtons.

The size of a force can be measured with a newton metre, which is pictured.

Newton metres should be checked for zero errors before use by checking they read zero with no force applied, and then adjusting them if necessary.

And finally, forces can be measured more accurately by choosing a newton metre with a scale with smaller intervals, as long as the range is still large enough to measure the force you're trying to measure.