Hello there, my name's Mr. Forbes.

Welcome to this lesson from the Hidden Forces Unit.

This lesson's all about the forces that happen when objects touch each other, such as an object resting on the floor.

And we're gonna look at what's called the normal reaction force, which is produced when those objects are touching and try and explain where that comes from and why it can change size when the objects have got different weights.

By the end of this lesson, you're going to be able to describe and explain the forces acting on objects that are resting on surfaces like the floor or a shelf.

You're gonna be able to explain where those forces come from and why they can vary in size depending on the weight of the object.

You're also going to investigate those forces and what they do to surfaces.

The key words you need to understand for this lesson are shown here.

First is gravitational force.

And you should know already that a gravitational force is a force that acts on any object inside a gravitational field, such as the one around the earth, and it produces the downward force acting on the object.

The second is normal reaction force, and this is the force produced by a surface and an object resting on it.

We're gonna look at that in detail in this lesson.

The third is elastic and the material is elastic if it'll return to its original shape and size and the forces are removed.

And the final one of these is electrostatic force, and that's the force between charged objects.

You may have seen the effect of electrostatic forces with balloons that you've charged up or other things like that.

But in this lesson we'll be looking at the charges on electrons.

And here's a set of descriptions of those keywords that you could return to at any point during the lesson.

This lesson's in three parts, and the first part is looking at the forces acting on stationary objects.

We've covered some of this material before, but we're gonna revise it and look in a bit more detail about the origin of those forces.

In the second part, we're going to look at the effect of the forces on surfaces.

So we're gonna see how surfaces behave when forces act downwards on them.

And in the third part we're gonna explain where the forces come from and how they can change in size.

We're gonna use a simple model for that.

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

In your previous look at forces, you saw that resultant forces are what cause an optic to speed up or slow down, to accelerate.

You also saw that if there's no resultant force and the object is stationary, so any stationary object doesn't have a resultant force acting on it.

So if I look at this scenario here, I've got a toy giraffe on a shelf, there's going to be an a gravitational force pulling it down, and there's gonna be an upwards force matching that.

The gravitational force is matched by upwards force from the table or the shelf there.

That upwards force is called a normal reaction force, and that acts on any object resting on a surface.

And you can see it must be equal to the gravitational force of the weight, otherwise the object would start to move, it would accelerate and start moving upwards or downwards.

So here's a question to check that you understood that.

Laura holds up a big weight.

What forces are holding that big weight steady? A, it's not moving.

So there's no forces.

Is it B, the only forces gravity pushing it into Laura's hand? Is it C, the only force is Laura pushing up, to hold it steady, or is it D, gravity pushes the weight down and Laura pushes it back up? So I'd like you to pause your video, make your selection, and then restart.

Okay, welcome back, hopefully you chose D.

Gravity pushes the weight down and Laura pushes it up.

You can see the two forces shown on the diagram there.

And there should be equal size for the object to be stationary, for it to be steady.

So well done if you chose that.

When you've got an object resting on the ground and it's stationary, the forces on it must be balanced.

So if I've got a small object like the one on the left here and it's got a small weight, the upwards force must be a small reaction force, otherwise the object would rise up or sink downwards.

In the second example, I've got a medium sized object.

So that must have a medium upward force, a medium reaction force.

And finally, if I've got a large object with a large weight, I've got a large upward force there.

So you should see the greater the weight, the greater the upward force has to be.

And those must always be balanced if the object is stationary and resting on the ground or a shelf or something like that.

I've got another check for you here.

All of these objects are stationary and resting on a floor, so they're not moving, which will have the greatest normal reaction force acting on it? So pause the video, make your selection, and then restart.

Okay, hopefully you chose the middle one.

B, the eight kilogrammes of wood.

The forces on them all are balanced, but those forces will depend on the mass.

So the one with the greatest mass will have the greatest weight.

And if you put values for weight, there're approximately this.

So the wood has a weight of 80 newtons, so there's an 80 Newtons upwards force on it.

A polystyrene rock, even though it's much larger in volume, has got a much smaller weight.

So it's not that one.

Well done if you chose B.

Okay, it's time for your first task now, which has a practical element to it.

I'd like you to physically explore the forces acting on your hand when you're holding objects.

So I want you to think about the forces that you need to keep something still such as a metal block in your hand.

And I'd like you to answer these questions about it.

What do you think will happen to each force when another metal block is added? And try and explain why that is.

And then what I'd like you to do is to try that out, get some metal blocks or some similar objects, hold them one and then two of them and describe what happens to the forces acting on your hand and acting on the object and try and explain how you know.

So pause the video, try that out, and then reach out when you're ready.

Hello again, and your answers should have been similar to these.

The gravitational force of the object will increase when you put another block on.

So there's gonna be a need for a larger force pushing up to keep it steady.

Those forces need to be balanced, otherwise the block wouldn't be stationary.

And you can experience and feel that because you can feel the increased effort you need to hold those objects up.

So well done if you've got answers similar to these.

Okay, we're ready to move on to the second part of the lesson now.

And in that we're going to investigate what happens to the surfaces that objects are resting on, How they behave when there's a force pushing down on them.

Let's get doing that.

So the first thing to understand is it can be difficult to see what's happening to surfaces when there's forces pushing down on them.

It's usually a very small change and you wouldn't normally notice it.

One object where you can see a change when you push downwards is something like a sponge.

When you push down on a sponge, it'll dip in in the middle, be like this.

The greater the push on it, the more it'll dip.

So it's quite easy to see, but it also pushes back on your hand.

The sponge is pushing back up, reducing a normal reaction force, which is equal and opposite to the downwards force you're pushing on it.

The greater the downwards force, the more compressed that sponge will get.

So as you push down, it gets thinner and thinner.

So let's see if you can apply that idea to springs.

I've got Jacob pushing the spring down here.

The spring squashes a little when he pushes down on it.

Which of these statements is correct? Is A, there's no forces at all? Is it B Jacob pushes down, but the spring does not push back up.

Is it C, the spring pushes up, but Jacob is pushing down harder than the spring? Ir is it D, the spring pushes up and Jacob pushes down and those forces are the same size? So pause the video, make your selection, and then restart.

Okay, welcome back.

Hopefully you chose the last of those options.

Those forces are equal in size.

There's a downward force and an upwards force.

But because the spring isn't moving, it's just being squashed, then those forces must be equal and opposite.

So well done if you selected that.

Let's try another question about spring.

Jacob's gonna push down on the spring a bit harder now and the spring is gonna squash a bit more.

What happens to the force from the spring on Jacob's hand? It is A, it does not push up? B, it pushes up with a bigger force than before? C, it pushes up with the same size forces before? Or D, it pushes up with a smaller force than before? Pause the video, make your selection, and then restart.

Okay, hopefully you chose option B.

There's a bigger downwards force, so there must be a bigger upwards force in the spring 'cause those forces have to be equal and opposite.

'cause the spring isn't accelerating, it's not getting faster or slower.

Well done if you selected that.

Okay, as I mentioned earlier, all materials will compress or bend a little when there's a force pushing down on them.

But it can be very, very difficult to see.

If you put a book on the desk for example, you wouldn't notice the desk bending at all.

So we're going to look at a way of demonstrating that surfaces do bend or get compressed when force is pushed down on them.

The way we're gonna do that is by looking at something like this, a ruler.

As you can see in this diagram, we've got a metre rule and I've put some metal weights on top of that ruler.

If I draw a horizontal line where the ruler would've been when those weights weren't there, it would look a little bit like this, the green dotted line there.

And if you look very carefully, you should see that that metre rule has bent very slightly because of the weight pushing down on it.

So there is some distortion of that ruler there.

So that metre rule is bent because of the force pushing down on it.

And all surfaces will do something similar to that.

They will distort or bend when forces push downwards on them.

So what I'm gonna ask you to do is to carry out an investigation to find out how much that metre rule will bend when you put different forces on it.

So you're going to position a metre rule between two objects that'll support it there.

That wooden metre rule is placed, for example on some soft cans or something.

You're then going to put a rule behind it and that rule is gonna allow you to measure how much the wooden metre rule bends.

I'd like you to place that as close to the middle as you can.

Here, I've put it off slightly to one side for the diagram, but you can put it directly behind the masses if you like to.

Then you're gonna put masses on the metre rule like this and it's gonna bend a small amount as you can see here, only a millimetre or two.

So you've gotta be very careful when you take the readings off the ruler.

And you're going to measure the change in the height on that ruler and that'll tell you how much the metre rule has bent.

Then you can add more masses and that one metre rule will bend a little bit more and you'll record again how much it's bent there.

So that's the investigation you're gonna carry out in a minute.

Before you carry out the investigation, we're gonna look at a couple of predictions.

Now, in the past you've looked at the behaviour of springs and seen how springs behave when we put different forces on them.

I'd like you to choose which one of these predictions suggests that the ruler will behave in exactly the same ways as a spring.

Is it the ruler will bend more when the force is increased? The amount the ruler ends will be directly proportional to the weight on it? Or the rule will snap when the force is too great? So I'd like you to pause the video, make your selection.

Remember you're comparing the ruler to a spring.

Okay, welcome back.

And if the ruler behaved like a spring, you'd have this prediction.

The stretching of a spring is directly proportional to the weight on it.

So the bending of the ruler would be directly proportional to the weight on it.

And we'll see whether that's true during the experiment.

But well done if you selected that.

Okay, now it's time for you to carry out the investigation and I've read out the instructions here.

You are gonna place a metre ruler on two supports so it's flexible and can bend in the middle.

And this works best with a metre ruler, but you could use a plastic ruler if that's all you've got.

You are gonna place a smaller 30 centimetre ruler behind it and that'll allow you to measure the bend.

As I said before, try and place that as close to the centre as you can.

And then you're gonna put 100 gramme masses onto the ruler, two at a time.

So you'll get a noticeable bend and record how much the ruler has bent from its original position.

So you'll work that out from the readings of the smaller ruler there.

And then once you've collected that data for a range of weights, I'd like you to try and write a conclusion and compare that perhaps to how a spring stretches when you put forces on it.

So I'm gonna show you a quick video of the procedure just so you've got the right ideas and then we'll start.

So here's the video.

What you can see here is a metre ruler, balanced across two cans.

And the shorter clear ruler in the centre has got a white mark that shows the level of the ruler.

I'm gonna see how that level changes when different weights are added to it.

First of all, we're going to add five small weights.

And now five more.

Okay, hopefully that showed you exactly what you needed to do.

I'd like you to carry out that experiment.

So pause the video, carry it out, and then restart when you've got your data.

Okay, welcome back.

Well, here's the data I collected when I carried out the experiment.

As you can see there, when the force goes up, the bend and the ruler does increase.

So my conclusions were the greater the force, the more the ruler bends.

But I look carefully at that data and I can see it's not linear, it's not increasing by the same amount each time.

It goes from naught millimetres to two and then when I increase the weight more, it goes to four.

So that has doubled.

And then it goes to five.

So it's not double there and it goes to six and six and then seven.

So it's not a direct proportionality, it's not linear, it doesn't follow Hookes law.

So it's not actually behaving here the same as the spring.

Well done if you collected that data.

Okay, we're moving on to the final part of the lesson now.

And in this part we're going to explain how the force from a surface increases when you put a larger object on it.

And to do that we need to look at the structure of the atom again and to look at electrostatic forces.

So let's get doing that.

Now to explain where the normal reaction forces come from and why they can change size we need to know about the structure of an atom.

So I've got a picture of two atoms here and you should already know that an atom contains a nucleus and it's got some electrons surrounding it.

We've got the nucleus here in the middle containing protons and neutrons and that has got an overall positive charge.

And around the outside I've got electrons and those electrons have drawn in simple orbits.

It's a bit more complicated than that, but it'll do for now.

And we've got negative charges on those electrons.

So those electrons are both negatively charged.

And you should remember when you look at electrostatic forces that when you've got two objects with the same type of charge on each other, they're going to repel each other.

So there's going to be some electrostatic repulsion.

There's gonna be forces pushing those two electrons apart.

So they don't like being near each other at all.

The electrons in nearby atoms will repel each other because of the electrostatic forces between them.

So we've always got repulsion between those two nearby atoms if the electrons are near each other.

And if I push those atoms closer together, the electrons get closer to each other, that electrostatic repulsion increases.

The force of the repulsion depends on the distance between them.

And when it's a smaller distance, there's a greater force pushing those atoms apart.

So the closer the atoms get, the greater that repulsion.

So I'd like you to try and imagine that the forces between the atoms in a material are a little bit like springs 'cause we've looked at springs in past lessons.

So I've drawn some atoms here.

I've got two layers of atoms and they're stable distance apart and I've drawn springs between them to represent the forces.

If I try and move that top layer of atoms further away from the bottom layer of atoms and they become further apart, then I'm stretching those springs a little bit.

And the electrostatic forces, the forces between the atoms, will act to pull those atoms back together.

They'll reapproach each other and try and get back to that stable distance.

And the same thing happens when I try and push the atoms a bit closer together.

I'm compressing the springs this time and this time the springs will try and push the atoms further apart.

They'll repel each other a bit and they'll try and return to their original stable distance.

So they like to be a certain distance apart and if they get too close, they push each other apart and if they get too far apart, they'll pull each other back together again, until to get back to that stable distance.

So let's have a look at how that idea allows us to understand how surfaces react when you put something on top of them.

I've drawn the atoms in a surface here in purple and there's lots of those and I'm gonna put an object on top of it.

I'm gonna put a lightweight object.

So place that object there.

And what happens is that object will produce downward forces on the atoms that are beneath it and squash them together a bit.

But because there's electrons in that object and electrons in the surface, they're gonna repel each other and that's gonna produce a force that acts upwards on the lightweight object.

So those squashed atoms beneath are gonna be a bit closer together and they're gonna push against each other and try and return to original size, but that's gonna produce a force acting upwards on the object that's resting on the surface.

So you can see there the atoms in the surface have been squashed together very slightly if you look carefully at that dotted green line.

So the electrostatic forces are causing the atoms to push back upwards and they're gonna push back on the object and that's producing the normal reaction force.

Now let's have a look what happens if I put a heavier object and that's pushing down on the surface.

So if I put a light object on, I get a slight distortion on the surface as it's compressed downwards and that produces a force that pushes back upwards, that electrostatic force, that supports the lightweight object.

So we get the atoms squashed together slightly there, on that surface, if I put a heavier object, it produces a larger downward force 'cause it's got a greater weight due to gravity and that's gonna compress the surface a little bit more.

It's gonna be squashed together a lot more.

So that heavier object is going to compress the surface more.

The atoms are gonna be a bit closer together and that means the electrostatic forces are going to be larger 'cause the atoms are closer together.

And that will give us a larger upwards force, a larger normal reaction force.

So the heavier the object is, the larger the normal reaction force that's produced because the atoms are squashed a bit closer together in the surface.

Right, so let's check if you understand that idea.

I've got a light crate and a heavy crate, both placed on the same type of surface, the same floor.

Which of these statements is correct about the situation? Is it each crate pushes down on the floor with the same size force? Is it the floor pushes up on each crate 'cause it's squashed a little? Or is it the floor pushes up on each crate with the same size force? So pause the video, make a selection, and then restart.

Okay, welcome back, the answer to that one is the floor pushes up on the crate 'cause it's squashed a little and it will push up with a greater support force on the heavier crate because it's being compressed more beneath it.

So well done if you selected that.

Now let's look in a little more detail about what's happening in the surface.

And as we've used springs as an idea, we're gonna look at arrangements of springs.

So a mattress in a bed will behave a little bit like atoms in a surface.

So if you looked inside a mattress, you might find it's got lots of feathers and things or maybe foam, but also it's got mattress springs.

The mattress contains thousands of those springs and if you lie on it, those springs become compressed.

The heavier the person that's lying on the mattress, the more weight they'll have, the more they'll compress those springs.

So they'll be squashed more and more.

The more those springs are compressed, the bigger the force they'll produce and push back up with.

So the mattress will get squashed a little, but it will also support the person that's lying on it.

And if a large person lies on it, it'll produce a large upwards force.

And if a small person lies on it, it'll produce a small upwards force.

Those forces will always match each other.

So let's see if you understand what will happen to a mattress when there's different forces acting on it.

So these images show identical springs in identical mattresses, which mattress is producing the greatest upwards force.

And I've drawn a picture of the uncompressed springs for you there as well to help.

So I'd like you to select which of those is producing the greatest upward force.

Pause the video and then restart.

Okay, hopefully you selected mattress C.

The springs in that mattress are the most compressed.

They squash the greatest, so there must be the greatest downwards force acting on it.

And that means that the mattress must also be producing the greatest upwards force.

The other two are gonna be producing smaller forces.

As you can see, the one in the middle, the smallest force of all.

So well done if you selected C.

So let's look at what happened when you remove the downward force acting on the surface.

Well, the electrostatic forces between the atoms were producing the upwards force.

When you remove the downwards force, that upwards force will cause the atoms to move apart again.

So it's a bit like a mattress, again, we've got the springs before the objects was placed on it.

When you put the object on it, those springs become compressed because the downward force acts on them and they produce an upwards force making the object balance there.

And then once you've turned that object away, the springs return to their original size.

And that's what happens with many materials.

If you take the force off a floor, the floor will unbend it or return back to its original shape and size.

Not all materials do that, but most do.

If a material is elastic, it returns to its original shape.

So we use the term elastic to mean a material that will return to its original shape when you've removed the forces.

Some materials don't do that.

We tend to call those materials plastic.

They don't return to their original shape or size.

If you walk across something like a snowy field, that snow is not gonna spring back up and resume its original shape.

Let's see if you understand the difference between plastic and elastic materials.

An object is placed on a wooden beam which bends.

When the object is removed, the beam becomes straight again.

Which of these statements is correct? Is it A, the beam is behaving plastically? Is it B, the beam is behaving elastically? Or is it C, the beam produces an upward force when the object is removed? So pause the video, make your selection, and then restart.

Okay, welcome back and hopefully you chose B.

The beam is behaving elastically.

It's returned to its original shape and size, therefore it's an elastic material.

Well done if you've got that.

Okay, we're onto the final task of the lesson.

And in this task you're gonna compare a surface to a mattress with springs in it.

So some mattresses contain a lot of springs and some pupils are using a mattress to think about objects resting on the floor.

And you can see there in the diagram, I've got the floor, it's being represented, so two surfaces connected by load springs just like a mattress.

I've got two crates on it.

They're thinking about a light crates and a heavy crates sitting on the floor and they want to explain the forces acting on the floor and on the crates.

I like you to think about how that diagram and how the idea represents the forces produced by a surface.

And I'd like you to state three ways, in which that's a good representation of the crates resting on the floor and three ways it's not accurate, it's not a good representation of the crates on the floor.

So I'd like you to pause the video, think about that for a while, and write down your answers and then restart when you're ready.

Okay, welcome back and here are some of the possible answers or suggestions you could have given for the first question here.

We've got the springs are squashed by the weight of the crate in the same way as the floor is squashed.

Each spring pushes up when it's squashed.

And because at microscopic level, a tiny level, it's springy, the floor pushes back on the object with an equal force in the opposite direction.

Springs has squashed more by heavier weight, so they push back more and a heavy object rests on them.

And that's the same as parts of the floor.

So they're the ways that the representation is a good one.

The model is a good picture of what's happening to the floor.

Well done if you've got those.

And here are the ways that the model isn't a good representation of what's happening on the floor.

So your suggestions could have been these, there's no actual springs in the floor.

The electrostatic forces between the atoms are stopping the crate from falling into it.

So they're the things producing the support forces.

Squashing the floor is actually too small to see.

If you had actual springs, you'd see them squash, but you won't normally see the floor bend because you put some crates on it.

Big movements in the mattress springs can affect other parts.

So if you put a big weight on a mattress, it will dip across and you won't normally see those sorts of dips when you put something on a floor.

And a very heavy object would squash the springs flat.

And that sort of thing doesn't happen with floors.

They might break, but they don't dip substantially like a bed or bed frame would.

So well done if you've got those.

Okay, we've reached the end of the lesson now, and here's a quick summary of the ideas we've covered.

When you've got an object resting on a surface, there's a gravitational force producing a weight that acts downwards.

That compresses the material under that object.

So if you put something on the floor or on a shelf, it produces a force that acts downwards and compresses the surface.

The atoms in the material of that surface will be squashed closer together and the electrostatic forces will cause them to push back.

And that's what produces a normal reaction force that supports the object.

So that's the origin of that upwards force.

When you remove the weight from the surface, the material stops producing the upwards force.

And if it's an elastic material, if it behaves elastically, it will return to its original shape.

And we looked at the idea that atoms kind of a stable distance apart and if they get too close and the forces between them will produce forces that push them apart and return to that stable distance.

And we used the idea of springs between atoms to help picture those electrostatic forces.

So that's the end of lesson.

Well done for reaching the end of it and I'll see you in the next one, goodbye.