Lesson video

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 meter.

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

Here are some keywords we're gonna be talking about in this lesson: Newton, newton meter, 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 recaps using a newton meter.

And then, there is a first practical element where you can go and practice that skill of using a newton meter.

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

So, looking in a bit more detail about how to use newton meters 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 is 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 1 newton is about, roughly speaking, the force needed to lift an apple.

So, every apple is gonna be slightly different, but roughly speaking, a force of about 1 newton is about the force needed to lift an apple.

Something that's about a kilogram, like a bag of flour or a bag of sugar that's a kilogram that would need 10 newtons of force to lift it.

So, that gives you a sense of roughly how big 1 newton and 10 newtons of force are.

So, this is a newton meter.

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 meter.

Within the newton meter, 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 meter 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 meter, 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 meter that's been set up on a clamp 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 millimeters now.

So, let's look at the newton meter on the left of the screen, the red one that's on the left of the screen.

That newton meter, the marker is directly in line with the 6 line on the scale.

So that newton meter is reading 6 newtons, 6.

0 newtons.

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

And the same for the other newton meter on the right of this slide, the green newton meter 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 0 line and the 1 line.

So, it must be halfway between 0 and 1, so it's 0.

5 newtons.

But, of course, you need to be able to read the newton meter whatever bit of the scale the marker 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.

5, 1, 1.

5, 2, 2.

5, 3, and 3.

5, and the last line that's marked on the diagram is 4.

But what about the next scale along?

Well, that's going up in steps or intervals of 2 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 steps in intervals of 2 newtons.

So, to read a scale accurately, firstly, make sure you know what steps, what intervals it's 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 0 and you divide that by the number of intervals, the number of steps, to get to that number from 0.

So on that first scale diagram that's on the slide now, the first number marked after 0 is 1 and there are two steps to get there.

So, what each step where you have to divide that 1 newton step into the two little steps it takes to get there.

So, of course, the scale goes up in 0.

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 marked after 0 is 4 newtons and there are two steps to get there.

So, 4 newtons divided into 2.

So, that scale goes up in 2 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 0, 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 meter, look at the picture please, how many intervals are there between 0 and 1 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 0 and 1 newton?

Well, there's 5 intervals there, there's 5 gaps between the 0 and the 1 line.

So therefore, how much force does each interval measure?

Question two; well, there's 1 newton divided into 5 gaps.

So, 1 newton divided into 5 gaps gives you 0.

2 newtons for each interval.

So, this scale goes up in 0.

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 meter read?

It might be worth pausing the video on this one.

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

Off you go now.

Pause the video if you need to.

Right, let's check your readings.

The first newton meter looks like it's exactly on the 1 newton line.

The second newton meter picture looks like it's exactly on the 6 newton line.

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

And the third newton meter looks like the marker is on the second line after the 8 newton line.

So, the scale goes up in 0.

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 practice now.

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

So firstly on this newton meter, how many intervals are there between 0 and 1 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 0 and 1 newton.

Therefore, how much force does each interval measure?

Well, 1 newton is broken into 10 intervals.

So, we've gotta divide with the 1 newton into the 10 intervals.

Therefore, each interval is 0.

1 newtons.

So, well done If you've got that.

That scale goes up in .

1 newtons.

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

What does the newton meter 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 0.

5 newtons, half a newton.

The second one is exactly on the 3 line.

So, again, that should've been really easy.

That's 3.

0 newtons.

And the third picture, it's on the second line after 1 newton and this scale goes up in 0.

1 newtons.

So, that is just 1.

2 newtons.

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

What does this newton meter read?

So, I've given you- you just need to choose a, b, c, d or e.

What does that newton meter read?

So, look carefully at the scale.

Work out the interval.

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

Pause the video if you need to.

The correct answer is d.

This newton meter reads 6.

4 newtons.

That's because between 4 and 6, that's a chunk of 2 newtons.

But the gap between 4 and 6 is divided into 5 intervals.

So, the 2 newtons between 4 and 6, 2 newtons is divided into 5 intervals, 2 divided by 5 is 0.

4.

Therefore, each interval represents 0.

4 newtons.

So, that marker is at the first interval after 6 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 meters.

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 practiced in the previous checks everything that you could do from photographs that are just done again in this task but with a real newton meter.

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 meter, a small spring and a 300-gram mass.

And secondly, work out what are the intervals on the scale of your newton meter 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 centimeters, and the force needed to lift 300 grams, 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 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 meter, 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 practice readings from newton meters 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 meter.

The intervals on that newton meter would be 0.

1 newtons because the scale goes up in 0.

1 newtons 'cause there's 10 intervals between 0 and 1.

So, that 1 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 on my newton meter represent 0.

1 newtons or whatever they represented on your newton meter 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 force to lift a pencil case, well, that's gonna depend on the mass if 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 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 benchtop, 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 it's important to do repeat measurements, to check for mistakes.

You really need to 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 centimeters.

Now, if everybody in your classroom used identical or fairly identical springs, everybody should get fairly identical results 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 centimeters if everybody used the same style of spring.

And the same for the force to lift 300 grams.

Now, on earth, the force needed to lift 300 grams slowly should be about 3 newtons, but somewhere between 2.

9 and 3 newtons.

So, again, that would be a good one to check with all the people around you to check that everybody got about 3 newtons for that force.

Well done for your effort on doing that task.

Hopefully, it gave you some really valuable practice on using newton meters.

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

So, when there is no force applied, newton meters should read 0.

And this newton meter does that.

There's nothing on the hook, and you can see it's reading exactly 0.

The marker is exactly in line with the 0 line.

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

However, this newton meter should read 0, 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 0, but it reads an 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 0, 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 should read 0.

6, it's gonna read 0.

8 because every measurement starts off 0.

2 newtons too big.

So, every measurement becomes 0.

2 newtons too big.

What about this newton meter here?

Now, this newton meter does the opposite.

It's gonna make readings about 0.

2 newtons too small because the marker is starting at a position about where -0.

2 newtons would be.

The marker is 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 0.

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 newtons more force than the newton meter actually reads.

So, this problem is called a zero error, when the newton meter should read 0 but doesn't.

So, let's do a quick check of that.

No force is applied to this newton meter.

The newton meter 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 meter 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 a go at answering all three.

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

It reads 0.

3 newtons when it should read 0.

So, will the measurements made be too big or too small?

Well, right now, the measurement is too big because it should read 0, 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 the zero error?

Well, the first thing to do is check for it.

So, when you first pick up a newton meter, 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 0 when it should read 0, when there's nothing on the hook.

But if it doesn't, if there is a zero error and the newton meter doesn't read 0 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 need to do quite a few turns before you notice the marker moving back to the correct 0 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 newton meter back to 0 when it should read 0.

The following video will show a demonstration of this.

<v Presenter>On this newton meter,</v> the scale does not line up with the 0 mark and this is called a zero error.

If we take measurements with the newton meter 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 0.

To adjust for this, we need to turn this adjustment screw at the top and move up the scale until it reaches 0.

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 meter.

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

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

So, this is a 0 to 5-newton newton meter 'cause it can measure from 0 newtons to 5 newtons.

So, the range of each newton meter goes up to the greatest force it can measure; in this case, 5 newtons.

Whereas this newton meter goes from 0 to 10 newtons.

So the range is 0 to 10-newton newton meter.

And this one goes from 0 to 20 newtons.

So, it's a 0 to 20-newton newton meter.

So, have a look at this.

The same force, 0.

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

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

4 newton steps.

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

This newton meter 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 meter.

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 meter has a different scale interval as well.

This scale goes up in 0.

1 newton steps and that's what 0.

5 newtons looks like on that newton meter.

Now, I think it should be fairly clear which newton meter 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 goes up 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 meter 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 newton meter has a range up to 5 newtons and the intervals are 0.

1 newton.

This next newton meter, 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 meter 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 meter to use is the one who scale goes up in the smallest intervals, but the range is still large enough.

So, out of the three, that first one's gonna be best for forces up to 5 newtons because it's got the smallest intervals and it can measure forces up to 5 newtons.

What about the next newton meter?

Well, actually that's the one that's gonna be best for forces between 5 and 10 newtons.

Because anything below 5 newtons, you should use the first newton meter, the green one on the left of the screen.

But anything between 5 and 10, you should use the yellow one 'cause that's got the next best interval for measuring forces above.

Well, it's got the best interval for measuring forces that are above 5 newtons that the first newton meter can't measure 'cause the range isn't big enough.

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

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

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

Choose the most suitable newton meter, newton meter a, newton meter b, or newton meter c, to measure each force of those forces, forces 1 to 6 on the left of the screen there.

So, remember you are looking for the smallest interval possible, but the newton meter 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 1 to 6.

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 newton meter a 'cause it's got the smallest interval.

Unless the force is too big for a to measure, then we should 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 the last 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 1 is 0.

2 newtons, a can't measure it but b can.

So, let's go with b.

Force 2, 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 meter c.

Force 3 and force 4 are both below 5 newtons.

So, 3 and 4 can both be measured by newton meter a because they're below 5 newtons and we want to use a if we can.

It's got the smallest interval.

Force number 5, 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 6 is below 5 newtons.

So, we can use a again which has 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 be provided with a selection of newton meters and objects, which you can measure.

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

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 meters.

So, you will need to choose the most appropriate newton meter for each object by considering the scale on the newton meters.

And remember to check for a zero error every time you pick up a newton meter and you're about to use it and check for a zero error by checking it read 0 with no force 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 meters on the screen so you can still practice the skill of reading a newton meter.

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 meter 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 meters that you could pause the video now and attempt to read what force each newton meter is measuring.

So, you've had to go at practicing the skill of reading a newton meter before I go through the answers.

Pause the video now to do that if you'd like that extra practice at reading newton meters.

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

So, the first three pictures all show the same newton meter, the yellow newton meter 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 6 newtons exactly, it's exactly on the 6 line.

The second picture shows 5.

4 newtons because the marker is on the second line below 5 newtons.

So it's 5.

2, and then 5.

4 newtons.

The third picture, it's on the third line below 3 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 marker is 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 meter.

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 3.

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 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 meter, which is pictured.

Newton meters should be checked for zero errors before use by checking they read 0 with no force applied, and then adjusting them if necessary.

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