# Newton’s Third Law

Hi. It’s Mr. Andersen and this AP Physics
essentials video 41. It is on Newton’s third law which is sometimes referred to as the
law of action and reaction. In other words when you launch a canon ball out this side
of the canon you are applying a force to that canon ball. But the canon ball is exerting
an opposite and equal force on the canon. And that is going to cause recoil. And that
is why these giant ropes are on this canon so it does not just go shooting off across
the deck. And so according to Newton’s third law if an object exerts a force on another
object, then that other object is going to exert an opposite and equal force on the first
object. We call these action reaction pairs. This is the action and this is the reaction.
And those action-reaction pairs remember will always be equal in magnitude and opposite
in their direction. Now these are sometimes hard to identify. So you can use a free body
diagram to identify where those forces are and figure out the overall acceleration. So
you can use a free body diagram to identify where those forces are and figure out the
overall acceleration. So imagine pushing into a wall, applying a force to a wall. You are
applying an action into the wall and the wall is actually applying an opposite and equal
reaction on you. The wall is pushing on you. Now you do not go anywhere and the reason
why is you apply a force into the ground, an action, and there is a reaction force from
the ground pushing back on you. And so the net force on you is the same. You do not accelerate
anywhere. Now let’s put you in space and do this same scenario. And let’s just make sure
that that wall does not move anywhere. So now you apply a force into the wall. It applies
an opposite and equal force back on you. And so now since there is no ground to push against
you are simply going to quickly accelerate and then coast away from the wall. Let’s look
at another scenario. You are pushing a bowling ball. And we have kind of frozen it at the
point at which you are going to push the bowling ball. So you apply a force into the bowling
ball and it is going to apply an opposite and equal force back on you. Now the ball
is going to move, but you are not going to move. And the reason why is you are applying
a force against the floor and it is applying a force back on you. So you do not move but
the ball is going to quickly move away like that. Let’s do this scenario again in space.
What happens? You apply a force into the bowling ball. It applies an opposite and equal force
back on you. What is going to happen now? There is no ground. You are both going to
accelerate away from each other. Why do you not move as fast as the ball? Because you
have a greater mass. Let’s look at just walking or running across the floor. You apply a force
into the floor and it applies an opposite and equal force back on you. That does not
make sense. But if you think about it in space, what happens if there is no floor? There is
no action. You apply an action, nothing happens and there is nothing to push against. You
are simply stranded there. And so these action-reaction pairs make sense but it is sometimes hard
to figure out what is pushing on what. And so we use a free-body diagram to do that.
And this is the first time we have used free-body diagrams. And so what do you include in a
free-body diagram? The body. So you are going to include the body. And you are going to
include any external forces on that body. Now it is more important what you do not include
in a free-body diagram. You do not include any other bodies. No internal forces, you
for example pushing on yourself or parts of an object pushing on other parts of the object.
And also you do not include any of the forces exerted by that body. That is the one mistake
that students tend to make. And so let’s do a free-body diagram, a few examples. And so
if you have a person standing right here on the ground, let’s do a free-body diagram of
their body. So we start, I usually draw a square like this. This represents the body.
What are the forces that are acting on that body? Well gravity is clearly acting on it
in the vertical direction. But the person, since they do not accelerate down into the
earth, there must be an opposite and equal reaction. That is the ground pushing back
on the person. And we call that the normal force. And so those are going to be the two
forces in our free body diagram. Let’s change it up. Now we are looking at this person pushing
So gravity is pushing down. That is an external force. What else do we have? We have that
normal force, it is not moving down into the ground. What else do we have? Let’s start
identifying those action-reaction pairs. So you are pushing against the wall. But that
is a force exerted by the bodies. So we do not include it. We do include the wall pushing
back on you. The force of the wall. If we look at the ground, remember, you are applying
a force on the ground, but there is a frictional force in the opposite direction. And so there
is no net force there. No net acceleration. If we put this scenario in space what do we
have? Now we have the body again. But there is no gravity. And therefore there is no normal
force. We are applying a force into the wall and it is applying a force back on us. So
that is going to be your free-body diagram. Where is the net force? It is to the left.
And so where is the acceleration? It is to the left as well. Let’s breakdown these two
objects. So we have object one which is the person. And so we have gravity and normal
force. Those are easy to do right away. You are applying a force to the bowling ball and
so it is going to apply an opposite and equal force back on you. And then again you are
applying a force in the ground and it is applying an opposite force on you. And so we have that
frictional force. Now let’s shift to the bowling ball itself. So we again have the object.
Is gravity acting on the bowling ball? For sure. But you are quickly going to let it
go. There is no normal force. You have just kind of frozen this picture in time so there
is no normal force. What is the only other external force? It is going to be the person
pushing on it. And so where is the acceleration going to be? Down and to the right. And so
did you learn to construct explanations using Newton’s third law or action-reaction? Can
you identify those action-reaction pairs? And then finally, can you use a free-body
diagram to identify not only the object but all the external forces acting on it? I hope
so. And I hope that was helpful.

## 76 thoughts on “Newton’s Third Law”

• ### Sophia Reyes Post author

FIRST COMMENT

• ### Hezar5166 Post author

Mr Anderson (i'll never get used to that)

• ### kotpx3 Post author

How come the outcomes are different for the bowling ball and the wall when their free body diagrams look completely identical? Is there something I'm missing?

• ### Majestic Turtle Post author

Have I ever told you the definition of awesomeness? Oh… Well science of course!

• ### Siavash Gharehkhani Post author

ur channel is awesome ! definitely the best experience i have ever had on Youtube !

• ### Jeff Burnz Post author

There is gravity in space, just not much. We need to be careful about such things, people often make this mistake, and I see it repeated often.

• ### Cho co Post author

To be more precise when you push a bowling ball ( on earth) the ball moves or accelerates but you do move too in the opposite direction because the earth will accelerate too on the opposite direction even if it is so substancial !!!

• ### marvDE Post author

You're awesome m8, helped a lot !

• ### Scraps The Wolf Post author

no you did not help me

thankyou!

• ### Saadz RS Post author

Amazing Video!

• ### Kiwimations Post author

Thank you this helped me SO much!!! Give dis man 1,000,000 subs!

• ### Sonali Gupta Post author

Holy guacamole I've been going through textbooks/articles/youtube videos for the past 4hrs and NOW I GET IT! more importantly how to apply it, THANK YOU!

• ### Abhishek Khandare Post author

here is my question any one can answer this according to Newton third law every action has equal and opposite reaction the how can I displace a cupboard on the ground suppose I have to displace the cupboard I applied 10 N from left side than I should get 10N force from right side due to friction the more I apply force the more I get reaction force than how can the cupboard move

• ### thawne Post author

I don't get it, if I push a cart for example I use 60 N there is reactions 60 N but how does the cart move if the net force is 0?

• ### ahmad kamal159 Post author

you are awesome 🙂

• ### Aman Rubey Post author

Good Explanation!

• ### ShakaTheNightWolf Post author

tnx really needed this writing a test later.

Cool

• ### Ein Hacker-Typ Post author

Thanks! Prior to this vid, I had no idea on how to use the "Free Body Diagram".

thanks

• ### Brooke Allen Post author

This was very helpful… My Physics teacher told me to watch this and I am glad she did. I understand them way more!!

• ### Uncle Zen Post author

need help translating ?

• ### Sheikh Abdul Raheem Ali Post author

This really helped me understand the intuition behind the Third Law. The 'don't include forces exerted by the body' was a really nice piece for the puzzle. Thank you so much for these videos!

• ### Celaena S Post author

this doesn't apply for unbalanced forces

thanks man

• ### Abdulnoor Muro Post author

best teacher ever

• ### Ryan Melton Post author

this was really boring

• ### James Simpson Post author

Thank you soooooo much. This just helped me understand what was going on in science class

• ### Pooja Rawat Post author

The forces aren't really an action and reaction pair as we understand by those words.Both occur simultaneously so basically it's not like reaction force acts after the action force.Anyone of the force can be called action or reaction.

• ### Charismatic Batman Post author

is everything in this newton 3rd law creates the karma-like reaction with same amount of forces applied?what about gun recoil?the speed of the bullet after firing is 240 mph but the gun recoils at 2mph

• ### Joseph Radd Post author

Did he just say that pushing a bowling ball in a frictionless atmosphere will move faster away then you from you?! "Because you have a greater mass?

• ### Saksham Sahgal Post author

Thank u Very much for ur nicest explanation <3 from India!

• ### Peter & Pete Post author

The content of the video is misleading. It's better to understand Newt's 3rd Law by stating, for every action there is an equal and opposite resistance to that action (reaction). This is why rockets cannot work in space because there is no resistance!!.

• ### Kenny Johnson Post author

Paul Anderson — 2 in the field; the 1 is taken…big flood commin'…

F = —F
A B

Every action has an equal (2 in the field)
and opposite reaction.

All forces are interactions such that:
the force with no equal (God)
does not exist (the 1 is taken).
—————————

Beyond motion law, Paul is teaching world peace. The third law precludes the "non-equalled force" of God. The third law is literally a "2 in the field; the 1 is taken." Learning the third law of motion becomes the full equivalent of learning that Jesus was a "2 in the field" atheist. Third law Paul wakes us up and we gather the wheat…

…"and the other left" is evolution…

The kingdom of heaven IS:
like to a grain of mustard seed,
which a MAN took,
and sowed in his field:

Which indeed is the least of all SEEDS:
but when it is grown,
it is the greatest among herbs,
and becometh a TREE,
so that the BIRDS of the air come and lodge
in the branches thereof.

—a seed became a tree so that a bird

• ### Hasini Nettikumara Post author

how would u apply Newton’s Third Law to examples of equilibrium situations?

• ### Maximus Prime Post author

What did u just say

• ### steve wittwer Post author

Of we are supposed to be weightless in space how can mass make a difference. That translates into weight. Anyway, theee is no space, gravity. Just density.

• ### steve wittwer Post author

Dishonest. You are pushing theory as fact. Prove it is gravity and not density. Gravity has never been elevated to fact. It has also been proven false by density. Space is fake, it doesn't exist. Show some proof, some real photos /videos, not CGIs. Bet you cant find any, because there are none because there is no space, satellites, ISS, hubble, moon landing, trip to mars.

• ### Ikleel Nabi Post author

How is there a Action-Reaction force in space? The spacecraft is not acting against anything! LMFAO.

probably not

• ### Johnny AppleStead Post author

Thrust can NOT produce motion without resistance to push against, PERIOD! This is why you will NEVER see the ISS thrusters in action! The thrusters are just as fake as the ISS. Lol!

• ### The md Post author

keep up the good work!

• ### Edinburgh Scotland Post author

one of his best videos. very clear.

• ### dumbcreaknuller Post author

when you use your arm in space to push on the ball the arm act as a force transfer medium and means that if your arm movement was replaced by a gas, it would expand between you body and the ball to create motion. rockets dont have this in space so there is nothing that drives them forward. think of it this way, you stand in space, there is no ball but you try to push youself backward by trowing your hands forward, what happends is that your arms goes forward and your body bacward but you moved nowwhere. that is what happens to a rocket in the vacuum of space.

I know this has no relation to the video but i think a good question do you believe in god or are in sciencific

love

• ### Ranjith Thatsme Post author

How it is different from elasticity..?

• ### Manas Raj Post author

Does a rubber pulled with equal force from both side an action reaction pair?

• ### 8bitsorcerer Post author

hello .. I need people's opinion about this… A \$10.00 bet is on the line.. here is the question

QUESTION:a person is driving a car forward with his friend that is in the passanger seat. the driver suddenly jerks the steering wheel left….. does the window hit the passanger's head or does the passanger's head hit the window?

PERSON A : the window pushes into the passanger's head F=MA.

PERSON B : BOTH happen. the passanger's head hits the window AND the window hits the passanger's head. they both hit each other. newton's 3rd law applies (every action has an equal and opposite reaction..so the answer is NEITHER FULLY: the head hitting the window or the window hitting the head as both must happen ..

now who deserves the \$10.00 and why?

• ### asleepinthealley Post author

This is nothing but disinformation attempting to validate the lies NASA needs to explain How rockets might work with unequal action / reaction. This is bullshit. You failed to mention that the cannon and cannon ball are affected by the pressurized air in our Atmosphere. That is a purposeful omission in order to hide the fact. Neither the wall nor the ground are pushing back, they are stationary. The reason the ball and you move away from each other is simply because you and the ball are similar in mass. Rockets in Space cannot work because as Newton's 3rd law states the action/reaction must be equal. NASA must convince people of these lies in order to get their other lies to work.

hi

• ### Rajat Raichandel Post author

Thank you so much you cleared my concept:-)

• ### sara muhamed Post author

I love your explaination and way of presenfing the info I TOTTTAAAALLLLYYY DO thank you sir…..

• ### Aquamarine 245 Post author

Didnt understand it so well from any other video around here.

• ### Gikkens - Clips Montager Post author

5:53 of course it was

• ### Angel C Post author

Mano ya cobraste

• ### Juan Sebastian Nava Post author

mano ya cobraste x2

• ### Angel C Post author

Mano pon esto de outro https://youtu.be/bcHTl9h7TWI

cmon sasha

• ### ColaCoca? Post author

why doesnt earth's gravity have an equal and oppsite force, it only pulls shit from space?

• ### Huzaifa Rashid Post author

best expaination.

• ### P Bhaskar Babu Post author

Plz tell slowly …! Sir

• ### noureddine bouras Post author

Great, Simple, Awesome, Explanation Thank you sir .

• ### Moto Shark Post author

My question is, why did the planes on 9/11 sliced right through steel and reinforced concrete flooring without breaking a piece? Not to mention, that plane is way weaker than the tower. The plane literally flew inside the tower without damaging itself, this is impossible according to the 3rd law, right? To this day I’m very confused about hollow aluminum planes slicing steel.

• ### DPCH Gaming Post author

good video upvoted +1

• ### David Smulson Post author

Mr. Anderson is the best!!!

• ### David Goliath Post author

Now explain why that guy would sink if placed on mud not concrete. Does mud not have an equal and opposite force?

• ### Stephanie Ishihara Post author

Oh my goodness I've watched so many videos related to physics but your explanation is the best. It's very clear and easy to understand. Thanks for the hard work making the video. It did help a lot.

• ### Thomas Huth Post author

This „law“ is least convincing.

• ### Deepak Khundrakpam Post author

Why wasnt it applied to the movie "gravity"… The man could push his jetpack weighing almost 20 kg n he could have been saved……

• ### tyler harris Post author

why does he look like Matt Damon

PERFECT!!!!!

• ### ELHAM&SIMA SULTANY Post author

thanks very much sir