Lesson video

Hello there, my name is Mr. Forbes, and welcome to this lesson from the Forces make things change Unit.

This lesson's all about Newton's third law, which describes how pairs of forces act between objects.

By the end of this lesson, you're going to be able to draw a range of force diagrams, and on those diagrams identify different types of forces and identify pairs of forces that are obey Newton's third law.

Here are a range of keywords that will help you throughout the lesson.

The first is non-contact force.

And a non-contact force is a force that exists when objects aren't touching each other, so it always exists.

Contact forces are forces that exist only when the object are doing direct contact with each other.

The resultant force is the overall effect of a set of forces.

So if you've got several forces acting on an object, the resultant force is the result of all those forces acting together.

A free body diagram is a diagram that shows you all of the forces acting only on a single object.

And Newton's third law of motion, which is the subject of this lesson, is a law that says forces come in equal and opposite pairs, which act on different objects.

You can return to this slide at any point during the lesson.

This lesson's in three parts, and in the first part we're gonna recap on what forces are and how we can describe forces as vectors, things that operate in particular direction, and we'll see a range of forces acting.

In the second part of the lesson, we'll look at something called free body diagrams, and those are diagrams that help us to analyse the forces acting on just a single object, one object at a time.

And in the final part of the lesson, we'll look at Newton's third law, and that's the law that describes pairs or forces and how objects interact with each other.

So when you're ready, let's begin with forces as vectors.

We can describe how objects interact with each other in terms of forces, and those forces can be described as pushes and pulls, so you can push an object or pull an object.

And the forces can do a range of things when they act on an object.

So for example, forces can change motion, and by that I mean you can use a force to speed something up, so a car can speed up when a force is acting on it.

Or a force can be used to slow it down.

So when you press the brakes, the car slows down, the force of the brake slows the car.

You can also use a force to change the direction of movement, so you can curve around a bend with a car when it's driving.

Or even a force can cause an object to rotate.

So when a car is driving, it might slip on an icy patch and it will start to spin 'cause a force causes it to spin.

Forces can also change the shape of an object by stretching it.

So you get an elastic band, put forces on it and make it longer.

You can squash something, so when you lie down on a bed, you'll be squashing the springs that are beneath you.

Or you can change its shape by twisting it so you can actually distort the whole shape of the object.

So forces can do a very wide range of things when they act on an object.

It's important to understand that forces are vector quantities, and a vector is a quantity that has a direction.

The direction of force axis is very important.

When you put force on an object in one direction, it's not the same as putting the same force in a different direction.

So to show that they're vector quantities, we draw them as arrows on diagrams. So you can see I've got a simple diagram here of somebody kicking a ball, and the red arrow represents the force.

The start of that arrow is placed as close as possible to the point where the force is acting.

So you can see the foot is touching the ball on one side, and that's where I've started the red arrow.

And the arrow shows the direction of the force, so you can see that that force is acting towards the right.

As I mentioned, the direction of the force shows which way the object is being pushed or pulled, and that affects how an object behaves.

The simple example of how it is, if you kick the football in different directions, I can kick it towards the left and it'll start moving to the left, or I can kick it to the right and it'll start moving to the right.

When you've got pairs or forces acting on objects and squashing or stretching them, again, the direction of the force is important.

So I've got a spring here, and in the first picture I'm squashing the spring together, so the two forces are acting to squash or compress the spring.

And in the middle one, there's no force.

And in the third diagram, I'm using the same size forces, but they're in opposite directions now on the ends of the spring so they're gonna pull the spring apart and stretch it.

I've got the first quick check for you now.

I've got a picture of a ball here with some forces acting on it.

So I've got a pair of forces acting on the ball, it's falling through the air.

Why are the arrows on this diagram incorrect? So pause the video, make your selection from the list there on the left, and then restart please.

Welcome back.

Well, the reason that that diagram's incorrect is because the forces do not start at the point on the object where they act.

The gravitational force and the air resistance arrows aren't touching the ball, so they're not actually showing forces acting on the ball.

So, well done if you've got that.

You can have a wide range of forces, from very tiny ones to very large ones.

So we normally measure forces in newtons, but if we've got a larger force, we might use kilonewtons, which means 1000 newtons, or even meganewtons, which is 1 million newtons.

So some examples, we can use a newton metre to measure small forces.

That newton metre there can measure forces between a range of zero to 10 newtons.

An apple has a weight of about one newton, so again, we can use a newton metre to measure that.

But there are some very large forces, a single rocket engine on this space shuttle taking off produces a force of up to 2.

5 million newtons, so that's 2.

5 meganewtons.

Another check now to see if you can understand values we use for forces.

So which of those values represents the greatest force? Is it 5,000 newtons? 500 kilonewtons? 500 millinewtons? Or 0.

05 meganewtons? So pause the video, make your selection, and restart please.

Welcome back.

Hopefully you chose option B, 500 kilonewtons.

If we convert each of those to newtons we'll get these answers.

So the first one's obviously 5,000 newtons, 500 kilonewtons is 500,000 newtons.

500 millinewtons is just not 0.

5 newtons, and 0.

05 meganewtons is 50,000 newtons.

So answer B was the correct one.

Well done if you chose that.

There are wide ways we can describe forces, but we generally put 'em into two groups, contact and non-contact.

So let's have a look at those groups.

When two objects touch each other, they put forces on each other and those type of forces are known as contact forces.

So for contact forces you have to have contact, things need to be directly touching each other.

Some examples of that include friction.

When two surfaces move together, a frictional force happens, and that only happens when those are in contact with each other.

Air resistance is very similar.

When you're moving through the air, you are pushing the air out of the way, you're in contact with the air, there is a force that acts on you.

And a normal reaction force, when you stand on any surface, that surface pushes back on you and that's a contact force, it only happens when you are touching the surface.

When you're drawing contact forces on diagrams, you should try to put the force arrow starting at the point where the two objects touch each other.

So if I've got a picture like this of a football resting on the ground, if I wanted to show the force of the ball acting on the ground, I should put it here, exactly where the ball is touching the ground.

Similarly, if I wanna show the force of the ground acting on the ball, I should put it where the point of contact is.

Another example here, I've got somebody kicking the football, and if I wanted to draw the force of the foot on the ball, I'd have to draw like this, exactly where the foot touches the ball.

And the force of the ball on the foot, I'd have to draw again, starting at the same point.

Each of those arrows shows the direction of the forces as well.

If you are finding it difficult to draw those arrows exactly at the right point because they overlap in some way, you can offset them very slightly, and you'll see that on some of my later diagrams. Okay, let's check if you understood that.

Which of these diagrams correctly shows the upward force acting on the book resting on the top of the table? So pause the video, make your selection, and restart please.

Welcome back.

Hopefully you selected option C.

Option C shows a force on the point of contact between the book and the table where they're actually touching each other, so that must be the right one.

So always look for the point of contact with contact forces.

Well done if you got it.

So we've seen that there are contact forces, but they're also non-contact forces, forces that operate whether or not the objects are touching each other.

So if you've got something like an apple falling from a tree, we've got the force of gravity that pulls on that apple.

The apple doesn't need to be in contact with the earth for the earth to pull it towards it, so we've got that as an example contact force.

Sorry, non-contact force.

Another non-contact force is the electrostatic force, the force you get from charged objects.

So if you rub a balloon against your hair, both of your hair and the balloon will become charged, and they will attract each other even if they're not in contact.

In this picture, some of the hair is in contact, but some of the her isn't and is still being attracted towards the balloon.

And a third example is the magnetic force.

If you've got a pair of magnets, they don't have to touch each other to actually exert forces on each other, and magnets can also attract some other metals as well.

So non-contact forces happen whether or not the object is touching.

When you're drawing a force diagram involving a non-contact force, we usually draw the forces acting from the centre of the object.

So gravitational forces act from something called a central mass, the centre of an object.

If it's a sphere like the earth and the moon, large balls of rock basically, the forces are drawn like this.

You can see both of my arrows start in the centre of the objects and they show the direction of attraction, so those forces are attracting each other.

The earth and the moon attract each other through a gravitational force.

Electrical forces are also drawn in a similar way.

If we've got two charged objects, maybe two electrons, both negative charges then repel each other, and I draw the force arrows starting from the centre of those objects as well.

For magnets, sometimes we draw the forces acting towards the end, because magnets are described as having poles where the forces are greater.

So I've got north and south poles on the magnets, and I've drawn the magnetic forces starting at the end of the magnets there.

So in the first case, the magnet on the left and centre magnet, they're attracting each other, and in the other case the centre magnet and the magnet on the right, they're repelling each other, and you can see that from the direction of the forces.

Okay, another check about drawing forces, which of those diagrams correctly shows the gravitational forces acting between the star, which I've drawn in yellow there, and the planet orbiting it, which is smaller and blue.

So pause video, decide which of those force diagrams is correct, and restart please.

Hello again.

And hopefully you selected picture A.

That diagram shows the forces acting from the centre of the objects and they're attracting each other, so that's the correct one.

Well done if you got that one.

Okay, it's time for the first task now, and it's all about drawing forces.

So I want you to identify any forces that you know that would be acting on the following objects, and draw some force diagrams for them, please.

So the first one is a tennis ball, and it's travelling to the right through the air, so that ball is in motion.

And the second one is a rocket lifting off from a launch pad, and again, that rocket is moving through the air.

So I'd like you to pause the video, draw some force diagrams, and restart please.

Welcome back.

Well here's my force diagram for this, let's have a look at them.

First of all, we've got the ball moving through the air, and we've got a gravitational force acting downwards on it.

I've drawn that as a purple arrow there, acting from the centre of the object.

And then they've got air resistance or drag, because the object's moving through.

In the second diagram, I've drawn thrust from the two rocket engines pushing the space shuttle upwards, so that would be two forces pushing up, and they'd be huge.

And I've also drawn the gravitational force of the earth pulling downwards.

Now this rocket would also have a small amount of air resistance, which would be acting downwards as well, which have not drawn on it, but you might have.

Well done if you got those.

Now it's time to move on to the second part of the lesson, which is all about free body diagrams. Diagrams we use to show the forces just acting on a single object.

So let's start that.

Most objects will have several forces acting on them at once, and when you've got several objects as well, you end up with an awful lot of forces on any force diagram you try to draw.

So I'm going to try and draw a force diagram for this ball rolling across the grass there.

So the first force I'm gonna draw on it is the force of the ball on the ground.

It pushes downwards on the ground, so that's its weight.

So I can draw that force fairly simple at the point of contact like this.

And there's also a force where the ground is pushing back on the ball, so I can draw that one on, and it's the reaction force of the ball on the ground, so that's pushing upwards.

The third force we can draw is the friction.

The ball is moving across the surface there, so there's a frictional force that's gonna be acting on it, I can draw that one on.

And at the same time there's gonna be a frictional force that the ball puts on the ground, so I've gotta draw that one as well.

So I've already got four forces, and I've not yet drawn the gravitational force acting on the ball.

So gravity pulls the ball downwards, that's the earth's gravitational force there.

And at the same time the ball pulls the earth upwards with a gravitational force.

So I end up with quite a confusing diagram with a lot of forces on it.

And the reason that happens is because I'm looking at the ball, the ground and the earth at the same time, I'm analysing three objects at the same time and that's difficult.

It'd be much simpler if I just analysed the forces on the ball.

So a free body diagram is used to show all of the forces acting just on a single object.

So I've isolated the object here, it's just the ball I'm thinking about, and I just want to know what forces are acting on that.

So it allows me to analyse it and find out how the ball's movement is gonna change over time.

So here's a free body diagram of the ball, I'm just going to draw the forces that act on the ball.

So if you remember, there was the force of gravity acting on the ball, pulling it downwards due to the earth, so that's one force that acts on the ball.

The ball was touching the ground, so I'm gonna draw the reaction force that was acting on the ball.

And the third and final force acting on the ball is the friction of the ground acting on it.

So this diagram just shows the forces acting on the ball and that's what a free body diagram is, it shows all of the forces acting on a single object.

So let's check if you understand what free body diagrams are.

I've got a diagram showing you some of the forces acting on a small masses held by a piece of string.

So we've got a piece of string dangling from a mass, and I've drawn four forces on that there, tension force acting in the string, the support force acting on the mass, the gravitational force acting on the mass, and the gravitational force of the mass acting on the earth.

Which of those four forces should be part of the free body diagram for the mass? So just the forces acting on the mass, please.

Pause the video, select the correct options, and then restart.

Welcome back.

Well you should have seen the clues in the phrasing of this description of the forces.

The forces acting only on the mass.

So you've got like this, acting on the mass, so option B was correct, and option C that was acting on the mass, that's correct.

The other two were acting on different objects, on the string or on the earth, so they're not part of the free body diagram for the mass.

Well done if you've got those two.

So let's have a look at another free body diagram, a more complicated one.

I've got two magnets here placed on a desk, and those magnets are going to be attracting each other.

And what I want to do is draw a free body diagram for magnet B.

So let's try and analyse that.

So first of all, the magnets are attracting each other, but I only wanna talk about forces acting on magnet B.

So I'm just gonna draw the force acting on magnet B, that's the force of attraction due to magnet A, a magnetic force there.

The second force is going to be, well, the magnet's going to start sliding across the desk or friction's gonna hold in place, one of those two.

In either case, there's this frictional force acting on the magnet due to the desk.

Next, there's gonna be a force pulling the magnet down causing its weight, that's the gravitational force acting on the magnet, and I've drawn that one on there, acting downwards.

And finally, the desk is gonna be pushing back up on the magnet, supporting it.

So I've got this normal reaction force acting on magnet B.

So now I've got all the four forces acting on magnet B.

I don't really need the desk and magnet A, that's really just getting in the way of my diagram.

So, what I'm gonna do is erase those and I'm left with a free body diagram for magnet B, just the forces acting magnet B though.

When you're drawing free body diagrams, you don't really need to be detailed in your drawing of the object.

In fact, it's usually better if you don't try and draw a realistic object.

So I've got a realistic picture here, I've got air resistance on a skydiver, I've got gravitational force pulling down, but it would take me age to draw that skydiver.

It's often much better just to simplify that object, to draw it as just a block or a simple dot or a little circle, something like this.

So there's my free body diagram of the skydiver, it's got all the important forces that are acting on it, but I've not wasted time attempting to draw a realistic picture.

A quick check for you now.

I've got a steel paperclip attached to some string, and then a magnet is held above it, as you can see in the photograph there.

I'd like you to decide which diagram is the correct free body diagram for the paperclip.

And you can see I've got the three different types of force, the magnetic force, gravitational force, and the pull from the string.

So what I'd like you to do is pause the video, make your decision, and then restart please.

Welcome back.

Hopefully you selected option C.

We've got a magnetic force pulling the magnet, sorry, pulling the paperclip upwards there, and that's shown by the big red arrow pointing upwards.

We've got the gravitational force pulling the paperclip down, that's shown by the short blue arrow.

And we've got the pull from the string that's pulling the paperclip down as well, and that's drawn as the green arrow pulling downwards.

So well done if you selected option C.

If you've got a situation where there's several objects, you can draw free body diagrams for each object individually in order to analyse the behaviour of each object individually.

So I've got someone pushing a box here, and if I wanted to draw a free body diagram for the box, I can draw just the forces acting on the box.

So first force, I've got the gravitational force pulling it downwards, so drawing that there as that green arrow.

There'll be a normal reaction force that's pushing the box back upwards, so I can draw that on the box.

And then the box is being pushed towards the right, so I've gotta put the force that's pushing it.

So I've got those forces drawn on.

Next, if I wanted to draw the forces in the box, sorry, the forces acting on the person, I've got a gravitational force pulling them down, I've got a normal reaction force from their feet pushing them upwards, and the box pushes back on the person.

So I can analyse both of those two objects separately.

As you saw in the previous diagram, the force is acting on an object aren't always in a straight line, you can have horizontal and vertical forces or in fact forces at any angle.

So let's have a look at a possible situation here.

We've got a plane, and I can look at the horizontal forces acting on it.

I've got a thrust pushing the plane forwards, that's formed by the propeller of the plane in this case.

And I've got air resistance or drag opposing the motion of the plane through the air.

So they're the two horizontal forces.

At the same time, I've got some vertical forces acting on it.

So I've got the gravitational force pulling the plane downwards and I've got lift from the wings.

So as the plane moves through the air, the force of lift on the wings keeps it in the air, and that acts vertically as well.

So I've got a situation where I've got forces that are horizontal and forces that are vertical here.

Okay, I'd like you to think carefully about pushing a book across a desk.

And I've got a set of forces drawn on here, and like you decide which of those forces would represent a force acting on the book when it's being pushed and it's moving across the desk? So pause the video, make your decisions, and then restart please.

Welcome back.

Well, it's in fact all of them.

All of those would be forces on it.

C would be the force acting on the book due to the hand pushing it.

D would be the frictional force acting between the book and the table.

A would be the normal reaction force, and B would be the weight of the book acting downwards, or the gravitational force of the book acting downwards.

So well done if you spotted that.

All of those were correct.

Okay, time for the second task of the lesson.

I've got a diagram showing a person pushing a broken down car along a flat, straight road.

I'd like you to draw a free body diagram showing the forces acting on the car as it's pushed, and a separate free body diagram showing the forces acting on the person pushing the car.

So pause the video, draw those diagrams and then restart please.

Welcome back.

Here's the free body diagrams you should have drawn.

So we've got first, situation one, the forces on the car.

I've drawn a fairly simple version of the car there, but it's probably a bit overly complicated, you could have drawn a simpler one.

I've got the push from the person acting towards the right.

I've got frictional forces on the road acting towards the left.

I've got gravitational forces acting on the car, pulling it downwards.

And normal reaction forces acting upwards where the wheels touch the ground.

Well done if you've got that one.

And in the second one, I've got the person there, again, I probably could have drawn a simpler person.

But I've got the gravitational force acting on them, pulling them downwards, normal reaction force pushing them upwards, a person pushing, sorry, the car pushing on the person backwards, and a frictional force of the road where their feet are gripping on the road.

So well done if you drew those diagrams. And now, we've reached the final part of the lesson, which is about Newton's third law, which describes how objects interact with each other with force pairs.

Isaac Newton devised three laws of motion that describe how forces affect objects.

You may have heard of some of these laws before, I'll mention all three of them now, and then we'll just look at the third one.

The first law of motion describes what happens when an object has no resultant force acting on it.

The second law motion describes what happens when there is a resultant force acting on an object.

And the third law describes how forces always act in pairs, one force acting on one object and another equal and opposite force acting on the other object, and that's what we're gonna concentrate on in this section.

Newton's third law of motion explains interactions in terms of pairs of forces.

So the third law says that any interaction can be explained by a single pair of forces.

These forces act between two separate objects so they affect each other's behaviour.

Those interactions could be due to contact forces or non-contact forces.

So pairs of those, and those pairs, we sometimes call them third law pairs of forces.

So the law can be stated like this.

If object A exerts a force on object B, then object B will exert an equal size force in the opposite direction on object B.

Those pairs of forces are always the same size as each other, they always act in opposite directions, they always act between just two different objects, and they're the same type of force.

So I've got an example here.

If we've got some magnets, magnet A attracts magnet B with a magnetic force, magnet B will attract magnet A with a magnetic force as well.

So magnet A attracts magnet B, magnet B attracts magnet A.

And it's the same size force, same type of force, and in the opposite direction.

To see if you've understood that concept, I've got an example here.

I've got a ball hanging from a piece of string, as shown there.

We've got the ceiling, we've got some string, and we then we got the ball hanging anything, which of these statements are correct? And there's more than one.

So the string is exerting a force on the ball.

The ball is exerting a force in the string.

The forces are equal in size and act in opposite directions, or there must be no other forces acting on the ball? So pause the video, decide which must be true, and then restart please.

Welcome back.

Well, you should have selected the first three.

The string exerts a force in the ball, it must be doing that to hold the ball in position, so it doesn't fall.

That must mean that the ball is exerting a force on the string, and those forces have to be equal and opposite in size.

There can be other forces acting on the ball, in fact, there must be, the ball is being pulled down by a gravitational force, which we've not mentioned.

So option D was incorrect.

Well done, if you selected the first three.

Okay, let's have a look at another example of Newton's third law.

I've got the moon orbiting the earth in this diagram here, and I've drawn the forces acting on them, and we want to decide if this is a third law pair, if it's due to Newton's third law? So I've got the gravitational force of the moon on the earth and the gravitational force of the earth on the moon, shown as those two arrows.

Now this is a third law pair, because the forces follow those rules we mentioned earlier.

The forces are the same size as each other, they act in opposite directions, they act on just two different objects, the earth and the moon, and we can see that it's just the earth and the moon mentioned in the description.

And they're the same type of force, they're both gravitational forces, so they fulfil all the criteria, they are Newton's third law pairs of forces.

Let's see another example.

We've got a book resting on a desk, and I've drawn a pair of forces acting on it here.

We've got a normal reaction force of the table on the book upwards, and the weight of the book acting on the table downwards.

And we'll see if this is a third law pairs as well.

Well the forces are the same size, they're acting in opposite directions, they act between just two objects, we've got table and book, so there's only two objects mentioned here.

And they're the same type of force, they're both reactive contact forces of the object touching.

So this is another example of Newton's third pair.

Let's have a look at a non-example, an example that doesn't match the criteria, so isn't a Newton's third pair of forces.

We've got a book resting on the desk again, we've got the normal reaction force and a gravitational force here.

Are these the same? Well, they would be the same size, in this diagram they are, so that's okay.

They would happen in opposite directions, and again, that's okay, in this diagram they're acting in opposite directions.

Then the third thing, they would act between just two different objects, and they're the same type of force.

Well, they're not acting between just two different objects here.

We've got the same size, that's correct.

We've got opposite directions, that's correct.

The two different objects rules broken, we've got the table, the book and the earth, so there's three objects here.

So these are not Newton's third law, and they're not the same type of force either.

The normal reaction force is a contact force and the gravitational force is a non-contact force, so they're different types of forces.

So this is not a Newton's third law pair of forces.

Okay, let's check if you understood those examples.

Look at the forces in the diagram I've shown and decide which of those four statements there match, and that'll help you decide whether or not this is a Newton's third pair of forces.

So pause the video, decide which of those statements that match, and then restart, please.

Okay, welcome back.

Well, in fact, all four of those were correct.

The forces are the same size, they act in opposite directions, they're just acting on two objects, magnet A, magnet B, there's no other object involved, and they're the same type of force, magnetic forces.

So those pairs of forces there shown are in accordance with the third law according to Newton.

So well done if you selected all of the options.

Okay, now it's time for the final task of the lesson.

And I've got a diagram here showing some ring magnets.

So a ring magnet, well, I've got a photograph there, a ring magnet has a north pole on one side and the south pole on the other side of it, and a hole through the middle so it can be mounted on a vertical stick.

And I've placed some magnets on a stick there as shown in the diagram.

And as you can see, magnet B hovers above magnet A when you place them on the stick, because of the forces acting on it.

What I'd like you to do is to describe the two force pairs that involve magnet B.

So just the force pairs that involve magnet B, please.

Then I'd like you to draw a free body diagram showing the forces acting on magnet B, so just forces acting on magnet B.

And finally, a free body diagram showing the forces acting on magnet A.

So pause the video, answer those three questions, and restart please.

Welcome back.

Well, for the first answer you should have written something like this.

We've got a gravitational force acting on magnet B, pulling it downwards.

You've also got a gravitational force from magnet B pulling the earth upwards, so that's a force pairs there.

Then you've got the magnetic force of magnet A acting upwards on magnet B, pushing up, making it levitate.

And similarly you've got the magnetic force of magnet B pulling on magnet A, or pushing on magnet A, I should say.

So well done if you've got that.

And your force diagrams should look like this.

We've got, in the first case, the free body diagram for B.

We've got the magnetic force of magnet A, acting upwards on magnet B, and the gravitational force of the earth acting downwards on magnet B.

And for magnet A, you've got the normal reaction force involved as well, which is worked in contact with the bottom of that stand that it was on.

And you've got the magnetic force of B pushing down, and the gravitational force of the earth pulling down.

So well done if you've got that.

Okay, we've reached the end of the lesson, and here's what we've covered.

Several forces can act on an object at once, so you can have one object and you can have five or six different forces acting on it quite easily.

These can be a combination of contact and non-contact forces and contact forces only happen when things are touching and non-contact forces happen even if they're not touching.

The overall effect of the combination of forces acting on the object is represented by a resultant force, and we'll see more about resultant force in future lessons.

Newton's third law of motion states that when object A exerts a force on object B, and object B exerts an equal and opposite force on object A.

And I've got an example there with those magnets, where you can see there's equal and opposite forces between the two objects.

Well done for reaching in the lesson.

I'll see you in the next one.