 The story so far is that Newton took his first law from Galileo who said that if you don't apply a force to something It doesn't change its velocity and then he developed his second law Which makes the first law really just a special case of his second law Which tells you exactly how things accelerate when you apply forces to them So the mass times acceleration is equal to the sum of the forces acting on something So if you want to know how something moves or you need to know is all the forces and Newton's second law is correct Provided you don't go too fast so that relativity becomes important Well, you don't look at things that are too small so that quantum mechanics becomes important So it's a very very powerful tool for calculating things but you do need to know all the forces that apply to things and One of the ways to help that is to look at Newton's third law, which tells us an important rule that forces have to obey Fundamentally all forces come from interactions So a force arises where you have something here. I have something a and something else Be and they have some kind of interaction Now we only know of a few fundamental kinds of interactions. We know about gravity. We know about electromagnetism We know about the nuclear strong force and the nuclear weak force But all forces no matter where they come from all follow Newton's third law a Newton's third law says that all forces come in pairs So the force on particle a due to particle B has to be paired with an equal and opposite force on particle B due to particle a So in other words, you've got one force going one way one force going the other way They're exactly the same magnitude so if you to add them up you'll get zero and that's Newton's third law So you can get examples anytime you have any kind of force and indeed every time you have a force You can do an example of Newton's third law. Let's start with if you're lifting a book So if you lift a book There's a force by you to push up the book and there's a force by the book pushing down on your hand It's a handbook interaction or you might have a plane if you have a plane's propeller There you have the plane pushing on the air the propeller pushes the air and also the air pushes the plane That's how the plane goes forward It's a propeller air interaction. You might have a swimmer in water The reason the swimmer pushes on the water is in order for the reaction the water pushing on the person to push them forward It's a person water interaction Well, you might have a cricketer and if the cricketer hits a cricket ball Then the ball experiences a big force But the bat experiences an equal and opposite force and that's why she has to hold it very firmly and also hit the ball Right the middle so that it doesn't twist in her hands due to the bat ball interaction So in each of these cases all you have to do is figure out what two things are interacting and that tells you how to apply Newton's third law And the force between the two things is going to be equal and opposite Now there are some very common misconceptions that people fall into when they look at Newton's third law The first is when a big thing interacts with a small thing that the big thing applies a bigger force For example, if you step on a blade of grass Then yes, there's a very large force on that blade of grass from the your body weight But in fact, there's an equal and opposite force from that blade of grass up on you But even though that force is equal the same force would do a lot more to a blade of grass than it will do to a foot And so the real way this works is that big things respond to forces less sensitively than small things The next misconception is that a force is something that an object can have for example a big magnet People might say has a big magnetic force or the earth they might say has a large gravitational force But the reality is that all forces come in pairs and they're the interaction between two objects So a big magnet doesn't have a big force a big magnet and something else can exert an equal and opposite force on each other Which may or may not be big and similarly the earth doesn't have a large gravitational force The earth and some other mass together can exert a mutual equal and opposite Gravitational force on each other the third misconception is that balanced forces on an object exists because of Newton's third law So for example if I'm standing on the earth and there's a force of gravity on me Pulling me down and there's a force due to the ground pushing me up is often called the normal force and The fact that I'm neither accelerating up into the sky nor down through the earth means that the normal force is exactly equal and opposite To the gravitational force and that's why I stand still But those are not the Newton's third law equal and opposite forces because they're both acting on me In fact, they come from two different interactions the interaction from the gravity is because of the existence of the earth And it's enormous mass and me it's a gravitational Attraction between me and the earth so the equal and opposite force from Newton's third law for that is My pull on the earth so I'm pulling the earth up Gravitationally just as the earth is pulling me down and the interaction for the normal force is the existence of the ground If I was instead standing on water then I would find that the gravity would indeed accelerate me downwards And so the Newton's third law pair for that is my force on the ground if I was standing on thin ice Then that force would break the ice and so the normal force is about the interaction between me and the actual surface I'm standing on and the gravitational force is between my mass and the earth's mass So because Newton's law describes forces on two objects And there's only ever one of those forces on each object they can never balance so they never balance on an object If they could then what Newton's third law would be saying is that there's always an equal and opposite force on that same object And so there'd be no such thing as force. They'd always cancel But they don't always cancel if I'm pushing on someone someone's pushing on me We both get pushed we get pushed in equal and opposite directions, but there's definitely pushing happen on each of us