About this video
Forces arise from interactions between two objects, with or without contact. Newton's third law says that when object A puts a force on object B, B puts an equal and opposite force back on A, so forces always come in pairs. These pair forces never cancel because they act on different objects; for example, an apple pulls up on the Earth just as hard as the Earth pulls down on the apple.
You'll be able to
- explain that forces come from interactions between two objects.
- state Newton's third law: equal and opposite forces on the two interacting objects.
- identify the force pair in an example by switching the words around.
- explain why third-law force pairs don't cancel out.
Key words
- interaction
- Two objects acting on each other, which is what produces forces.
- Newton's third law
- When object A puts a force on object B, object B puts an equal and opposite force back on object A.
- force pair
- The two equal and opposite forces from one interaction, each acting on a different object.
- contact force
- A force that exists only while two objects are touching.
- non-contact force
- A force, like gravity, that acts without the objects touching.
Transcript
- [Instructor] When you drop an apple, we know that the Earth pulls on the apple and makes it fall. We call this the force of gravity. But the big question is, does the apple also put a force on the Earth? Let's find out. Let's quickly recap, what exactly is a force?
Well remember, the force is just a push or a pull. But what exactly is needed for an object to experience a push or a pull? The answer is some kind of an interaction. For example, when you hit a ball using your bat, the bat is in contact with the ball and it pushes the ball.
It exerts a force on the ball. We call this the contact forces. Here, the bat is interacting with the ball. But you don't always need a contact. For example, if you consider the force that the Earth is putting on the apple, for example, and making it fall, look, there is no contact over here.
That's why these are called non-contact forces. But there's still an interaction. And the key thing is that you need two objects. An apple cannot put a force on itself. You need, for example, an Earth to exert a force on the apple. Similarly, a ball, for example, cannot put a force on itself.
In this particular example, we need a bat to push on it, to exert a force on it. So look, the forces arise from interactions between two objects or two particles, you can say. But the big question is this. When the bat interacts with the ball, it exerts a force on the ball.
But does the bat experience a force as well? Let's see. And to think about this intuitively, imagine you're not hitting a baseball, imagine you're hitting a bowling ball. I know it sounds weird, but just think about it. You take a bat and you try to whack that bowling ball.
You put a force on that bowling ball, what's gonna happen? Oh, you can feel it in your bones! That bat is either gonna break or it's gonna fly backwards, isn't it? So just from that intuitive feeling, you can see that there must be a force acting on the bat as well, and that force will be backwards.
So yes, indeed, there is a force acting on that bat. So what you notice is that when the bat puts a force on the ball, the ball puts a force back on the bat. But how big is this force? Well, let's think about it. If you were to put a small force on that ball, if you were to gently tap that ball with a very tiny force, then you wouldn't expect your bat to break.
You would expect a tiny force back on your bat, isn't it? But if you put a giant force on that ball, if you try to whack it, then there is a very good chance that bat is gonna fly off or the bat is gonna break. So this means, and we've done careful experiments to arrive at this conclusion, but this means that when a bat puts a force on the ball, the ball puts an equal, but look, opposite force back on the bat.
And this is true in general for all forces. For example, imagine you and your friend are skating and you push your friend to the left. What's going to happen? Well, surely your friend is gonna start moving this way, but you will also get pushed backwards, which means your friend will also end up pushing on you.
And the force lasts as long as there is contact. The moment the contact is lost, the force is gone. So this is always true. And we give a name to this, we call it the Newton's third law. So in general, we could say when object A puts a force on object B, object B puts an equal and opposite force back on object A.
For example, if the bat puts a force on B, on the ball, sorry, the ball puts an equal and opposite force back on the bat. Similarly, when this girl puts a force on this boy, the boy puts an equal and opposite force back on the girl. And by the way, you're completely free to choose which one is your A and which one is your B.
It doesn't matter. The force only lasts as long as there's interaction, and over here, the interaction only exists when there is a contact. So when the contact is gone, the force disappears. And since this is true for all forces, we can now answer our original question.
The Earth is putting a force on the apple, pulling it down. So according to Newton's third law, the apple must also put an equal and opposite force back on the Earth, which means the apple is also pulling back on the Earth with the exact same force. That is not intuitive at all.
I mean, think about it. This apple is so tiny and it has such a small mass compared to the Earth, and yet they exert exactly equal but opposite forces on each other, which is pretty awesome if you think about it. So according to Newton's third law, forces will always, always come in pairs.
But that raises another big question: if forces always come in pairs and they're always equal and opposite to each other, shouldn't they cancel out? Why don't they cancel out? Well, because they're always acting on different objects. For example, over here, one force is acting on the apple, the other force is acting on the Earth.
Similarly, if you go back to our previous example, look, one force is acting on the ball, the other force is being exerted on the bat. One force is acting on the girl, the other force is acting on the boy over here. So these pairs of forces are always acting on different objects or on different particles, so they can never cancel out.
Remember, when you're determining whether forces are balanced or not, you're talking about forces acting on one single particle or one single object. All right, finally, the cool thing about Newton's third law is if you know one of the forces and you want to identify the other force pair, then just switch the words around.
For example, a fish uses its tail to put a backward force on the water. Therefore, according Newton's third law, the water at the same time puts a forward force on the fish. Consider jumping. When you're jumping, you put a downward force on the ground. Well, according to Newton's third law, at the same time, the ground puts an upward force back on you.
And if you consider the example of an apple or a ball falling down, for example, the Earth pulls on the apple, pushes a downward force, exerts a downward force on the apple. According to Newton's third law, the apple exerts an upward force on the Earth. So long story short, forces arise due to interactions.
Some of these interactions will require contact. Others, like gravity, for example, don't. But you always have two particles or two objects interacting with each other, and as a result, forces always come in pairs. These forces are always equal and opposite, but because they're acting on different objects, they will never cancel out.
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