Now we’re ready for Newton’s

third law. And Newton’s third law, in some

ways, I think is the most fun because it’s– at least it

was to me, the least intuitive of all the laws. But once you really kind of

understand it a lot of things start to make sense. So Newton’s third law

essentially says– actually, I can tell you kind of what

you might have heard. A lot of people say every

action has an equal and opposite reaction. Another way of thinking about

it is, if there’s an object and it exerts a force

on another object. So let’s say I have a– oh,

let me think of something. Let’s say that I have a fist. So

let me draw the fist. So I have a fist and it is punching

someone’s face. So this is a face, and

they’re not happy. And let’s say this fist is

punching the face with a force of– I don’t know–

10 Newtons. Let’s not make it so violent. Let’s say that this is a hand

massaging the face. It’s pressing upon the face with

a force of 10 Newtons. So what Newton’s third law tells

us is that the face is also, I guess, we could

say punching the hand. Or– well no, we’re not

using the violent. The face is also pressing upon

the hand with a force of 10 Newtons in the opposite

direction. I guess you would say

along the same line. So does that make any sense? Because it seems like the hand

is doing something to the face and not the other way around. But if you think about it, when

you press on someone’s face, their face might kind of

press in a little bit, but you also feel something on your

fist or on your hand. I mean think of it, maybe a

better example would be instead of someone’s face,

imagine if it was a tree that you’ve decided to massage

or, I guess, punch. So here is the tree. That is the tree. And the same thing

would happen. If you were to press upon the

tree with– if you were to punch the tree essentially,

the tree is essentially punching back with the

exact same force. And here it makes sense because

your hand will hurt. And maybe in this case, the

face will hurt because the face kind of gives way while

your fist doesn’t. But here the tree’s not giving

way and your fist will. Another way to think about it

is if– well, and this is probably the least intuitive. If I have the earth and

here am I standing on top of the earth. So we already figured out that

the pull of the earth or the force of gravity upon me,

it’s pulling down upon me at 150 pounds. That’s the force. And you know, we could

say the Newton. But pounds is a unit of

force, it’s weight. But also, Newton’s third law

tells us that I’m actually, at the same moment, pulling

on the earth with a force of 150 pounds. And this might not make a lot

of sense to you, but you can think about it this way. When I’m stepping on– let’s

say I’m stepping on a soft surface, like sand

or something. My feet will compress a little

bit, but so does the sand. Well, depending on which

one’s softer. And another way to think about

it also is, if me and the earth are both in deep space. And I am, you could say, falling

towards the earth because the earth is

pulling on me. How do we know that the earth

isn’t falling towards me? I mean it’s kind of arbitrary. There’s no frame of reference. We’re both in deep space. There’s nothing else

to look at. We’re essentially, falling

towards each other. I’m not necessarily falling to

the earth, the earth’s not necessarily falling to me, we’re

just falling towards each other. And that’s another way of

thinking about this. So you could think about every

example where a force applies to something. And if you really think about

it, the force is going the other way as well. For example, if I were to take

I bat to this tree and swing on it really hard. So I were to swing on this tree

really hard, I have a good chance of breaking

that bat. Even though you would have

thought, hey, that bat is applying the force

to the tree. But why is the bat breaking? Because the tree is applying

an equal force to the bat. And actually, if I did it

perfectly, if I had like– let’s say I had two bats

or two swords. For some reason I think I’m

going a little too violent with these example. But I guess you know we’re

talking about forces. So maybe violence is

justified here. But let’s say I have two swords

that are completely identical hitting each other. And let’s say I keep increasing

the force at which they’re kind of going in

opposite directions. At some point, they’re

going to break. If I just keep increasing the

force on– you know, one guy is swinging in one direction,

one guy is swinging in the exact opposite direction. And the force just keeps

increasing, at some point, they’re going to break. And you could say that well–

this guy says well, I was the only guy swinging because

this guy was stationery. And the other guy will say,

well, I was really guy swinging because this

guy was stationary. Not one of them is going to

break, they’re both going to break because even though this

mauve sword was pushing on this blue sword with some force,

the blue sword was essentially pushing back with

the exact same force. So these are completely

identical swords. At some point, they’re

going to break. Another way we could think about

it, one of the swords could have just been held. You know, stationary. It could’ve been held stationary

by somebody and this other sword that– if you

were to press down on it, kept increasing the force with which

you press, at some point they both would break. Because they’re identical. If one was harder than the

other, than that one would stick around. Hopefully that gives you

a little intuition. I mean we could do a bunch

of more examples. I’m trying to think. Oh, let me think

of another one. A less violent one. Let’s say we’re in deep space

again and I have a– I don’t know– I have a basketball. Let me do it in orange. I have a basketball and

it weighs 1 kilogram. And let’s say that I weigh 50

kilograms. So let’s say I push– so my hand pushes on this

ball with a force of– I don’t know– let’s say I push

on that ball with a force of 10 Newtons. 10 Newtons to the right. What Newton’s third law tells

me is that essentially, that basketball is going to push on

my hand with an equal and opposite force. So it’s going to push on me with

a force of 10 Newtons. So what’s going to happen? So we’re touching. I’m pushing on 10 Newtons on

the basketball and we’re in deep space. There’s no gravity from random

stars, et cetera. And then the basketball’s going

to push on me with the force of 10 Newtons

simultaneously. We know F equals ma. So the basketball, so 10 Newtons

is equal to 1 kilogram times acceleration. So acceleration is going to be

10 meters per second squared to the right. So as long as we’re touching,

the basketball’s going to accelerate at 10 meters per

second squared to the right. And simultaneously, I’m going

to accelerate at a certain acceleration to the left. And what’s that going to be? 50 kilograms. We know that the

force to the left is also going to be 10 Newtons. That equals 50 kilograms

times acceleration. So here, the acceleration is

going to be 1/5 meters per second squared. So we’re both in deep space

floating around and I push on this 1 kilogram basketball with

a force of 10 Newtons. As long as I’m pushing on it,

it’s going to accelerate at 10 meters per second squared. But simultaneously, while I’m

pushing on it, it’s exerting an equal and opposite force

on me of 10 Newtons. So I’m going to actually move

back a little bit at a slower acceleration. That’s just because

I have more mass. At 1/5 meters per

second squared. Another example you could

think of is if someone shoots a gun. There’s that– I forgot

the term because I don’t shoot guns. But your shoulder jerks back

as I’ve seen in the movies when a bullet is shot. That’s because the gun is

exerting a force on that bullet and the bullet is

exerting an equal and opposite force on the gun, which

kind of pushes back on your shoulder. And the reason why the bullet

just goes a lot, lot faster forward than you and the gun go

backwards is because your mass is much, much, much

larger than the bullet. Hopefully that gives you a

little bit of intuition on Newton’s third law. And this is kind of

non-intuitive. So look around you in the world,

look at all the forces that are being applied, and I

want you to think about when one force is being applied in

one direction, why does it make sense that another force,

an equal and opposite force, is being applied in the exact

opposite direction? I’ll see you all in

the next video.

Thanks a lot for this video! Had trouble understanding the third low,. this vid explained it to me 🙂

hahahah massaging the face

jeez just stick with punching man. dont be so timid about it, its easier to relate to

Where exactly is this video located on the KA website?

It is not here:

https://www.khanacademy.org/science/physics/forces-newtons-laws

It is not this:

https://www.khanacademy.org/science/physics/forces-newtons-laws/newtons-laws-of-motion/v/newton-s-third-law-of-motion

And I've been unable to locate it on the KA site through the Search function.

Where is it?

you may consider (if its right) using the example that if you were to punch the tree and it didn't exert a force back, you would expect your arm to just extend straight through the tree and keep going. However that doesn't happen because the tree exerts a force back on you, pushing you backwards

he sounds like he got chicken stuck in his throat

OMG I CAN DO MY DEMONSTRATION FOR MY SCIENCE PROJECT! You are the only person who made a video without involving a balloon to demonstrate newton's third law!! 😀

What will happen if someone pushes an object heavier than him with the force of 10N, will the opposite force still remain the same?

"violence is justified here" lmao

how come that guy explains so well!!!!!!! i mean i just got it in that 9 min i impressed

Thanks foe explaining this because I didn't understand it from my teacher

When action and reaction are equal and opposite then how does the body moves?

I HAD NO PROBLEM WITH PHYSICS UNTIL I STARTED TO THINK ABOUT IT

WHAT

if there was a satellite orbiting Jupiter, the satellite would feel more inertia compared to Jupiter since jupiters mass is greater right ?

Massage the tree 😀

I stopped watching when he said no nvm lets massage the face…

its called recoil – gun's reaction

Okay but what if the basketball pressed back with a force of 7N? Or is it always the same exact force?

Good job! Great videos!

Is it possible that when two swords of the same mass and material hit each other, only one sword breaks.

So how would Newton's third law explain when a martial artist brakes a brick with his hand? It's hard to picture that the brick, which shatters, is still applying an equal and opposite force on the hand.

weight is not a form of force?? 2:54?

so are you saying that if a throw a pen, that pen sorta throws me back or exerts a force on me too??

Thanks for helping me with school work!

Redo please, lose the whole anti viloence. Balls and sports works better. Or hammer and nail verses

……

its recoil for the gun