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 obey Newton's Third Law.

Here are a range of keywords that'll 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, that's where those exists.

Contact forces are forces that exist only when the object are in 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 the forces acting only on a single object and Newton's Third's 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 a particular direction, almost in 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 th 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 of 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 an 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 brakes slows the car.

You can also use a force to change the direction of movement.

So you can curve around a bend a car whilst 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 where the 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, make it longer.

You can squash something.

So when you lie down on a bed, you'd 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 a force acts is very important.

When you put a 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 are vector quantities, we draw them as arrows on diagrams. So you can see we've got a simple diagram here of somebody kicking a ball and the red arrow represents the force.

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

When you got pairs of forces acting on objects and you're 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 size forces, but they're in opposite directions now on the ends of the springs.

So we've got to 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 we've got a pair of forces acting on the ball and 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 is incorrect, is because the forces do not start at the point on the object where they act.

The gravitational force in 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 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 one 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 of 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 5000 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 you convert each of those to newtons, you get these answers.

So the first one's obviously 5000 newtons.

500 kilonewtons is 500,000 newtons, 500 millinewtons is just 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 a wide ways we can describe forces, but we generally put them 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're 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're 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 it like this, exactly where the foot touches the ball and the force of the ball on the foot, I'll have to draw again, starting at the same point.

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

If you're 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 upwards 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 at 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 there are 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 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 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 the hair is in contact, but some of the hair isn't and it's still being attracted towards the balloon.

And a third example is the magnetic force.

If you 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 the centre of mass, the centre of an object.

Now, if it's a sphere, like the Earth and the Moon, large balls of rocks 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 attract each other.

The Earth and the Moon attract each other through a gravitational force.

Electrical forces are also drawn in a similar way.

If I've got two charged objects, maybe two electrons and they're both negative charges, they'd repel each other and then draw the force arrow 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 greatest.

So I've got north and south poles on a magnet 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 a star, which we've drawn in yellow there, and the planet orbiting it, which is smaller and blue? So pause the 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 would need to identify any forces that you know that would be acting on the following objects and draw some force diagrams from 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 launchpad 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 diagrams. So let's have a look at them.

First of all, we've got the ball moving through the air and then we've got a gravitational force acting downwards on it and I've drawn as a purple arrow there, acting from the centre of the object.

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

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.

And this rocket would also have a small amount of air resistance, which would be acting downwards as well, which I've 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 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 very 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.

And 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 going to 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 we end 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 analyse the forces on the ball.

So a free body diagram is used to show all 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 and 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 when a small mass is held by a piece of string.

So I've got a piece of string dangling from a mass, and I've drawn four forces on there.

A tension force acting in the string, a 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've seen the clue's in the phrasing of this description in the forces.

The forces acting only on the mass.

So we've got act 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 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 the 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 is going to start sliding across the desk or friction is gonna hold it 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 on magnet B there.

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 realistic objects.

So I've got a realistic picture here.

I've got air resistance on the skydiver and I've got gravitational force pulling down.

It would've took me ages to draw that skydiver.

It's often much better just simplify the 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 there 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 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.

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 its 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've drawn.

So we've got for 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've 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 and we get downwards, a normal reaction force is acting where the wheels touch the ground.

Well done if you got that one.

And then the second one, I've got the person there.

Again, I probably could've drawn a simpler person, but I've got the gravitational force acting on them, pulling downwards, normal reaction force pushing them upwards, the car pushing on the person backwards and the 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 or I'll mention all three of them now and then we'll just look at the third one.

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

The second law of 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 an object A exerts a force on object B, then object B will exert an equal sized 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 and magnet B will attract magnet A with a magnetic force as well.

So magnet A attracts magnet B, magnet B attracts 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.

You got the ceiling, you got some string and then we've got the ball hanging beneath it.

Which of these statements are correct, and note there's more than one? So the string is exerting a force on the ball, the ball is exerting a force on 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've selected the first three.

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

And 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 were 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 follows those rules we mentioned earlier.

The forces are the same size as each other, the act in opposite directions, they act on just two different objects, the Earth and the Moon, and we can see that there'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 let's see if this is a third law pair 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 are both reaction contact forces of the objects touching.

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

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

We've got a book resting on a desk again.

We got a 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 act 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 direction, that's correct.

There's two different objects, the law is broken.

We've got the table, the book and the Earth, so there's three objects here.

So these are not a 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 shown and decide which of those four statements they 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 they match and then restart, please.

Okay, welcome back.

Well, in fact, all four 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, we're on to the final task of the lesson now and I've got an example involving more magnets.

So I've got circular, disc magnets here with north poles on the top and south poles on the bottom and you can stack them onto a vertical stick, because there's a whole through the middle of the magnet, as you can see in the picture.

So when those two magnets are placed on a vertical wooden stick shown in the diagram, and you can see there's magnet A and magnet B and I've color-coded the sides for you there.

The top magnet floats on top of the bottom magnet, so it hovers in the air.

I'd like you to describe the two force pairs that involve just magnet B, please and then draw a free body diagram showing the forces on just magnet B.

So you don't need to do anything about magnet A in that diagram.

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

Welcome back and here's the description you should've done about the two force pairs that involve magnet B.

The first one is due to the gravitational force acting on the magnet.

So there's a gravitational force acting downwards on the magnet and there must then be a gravitational force acting upwards on the Earth due to the magnets.

So that's one of the pairs of forces.

And a second is the magnetic force, we've got a magnetic force on magnet A pushing up on magnet B and so magnet B is got magnetic force pushing down on magnet A.

Well done if you got those.

And if you drew the free body diagram for magnet B, it would look like this.

We've got a magnetic force from magnet A pushing upwards on it and you've got a gravitational force of magnet B gets gravitational force.

And if you drew your free body diagram correctly, it should look like this.

We've got the magnetic force on magnet A acting on magnet B, pushing it upwards and we've got the gravitational force of the Earth acting on magnet B, pulling it downwards.

And those are equal in size, but they're not a Newton pair, 'cause they're different types of force.

So well done if you drew that force diagram.

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 we 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 a combination of forces acting on the object is represented by the 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, then 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 the end of the lesson, I'll see you in the next one.