We’ve been talking a lot about the science
of how things move — you throw a ball in the air, and there are ways to predict exactly how it will fall. But there’s something we’ve been leaving
out: forces, and why they make things accelerate. And for that, we’re going to turn to a physicist
you’ve probably heard of: Isaac Newton. With his three laws, published in 1687 in his book
Principia, Newton outlined his understanding of motion — and a lot of his ideas were
totally new. Today, more than 300 years later, if you’re
trying to describe the effects of forces on just about any everyday object — a box on
the ground, a reindeer pulling a sleigh, or an elevator taking you up to your apartment
— then you’re going to want to use Newton’s Laws. And yes. I’ll explain the reindeer thing
in a minute. [Theme Music] Newton’s first law is all about inertia,
which is basically an object’s tendency to keep doing what it’s doing. It’s often stated as: “An object in motion
will remain in motion, and an object at rest will remain at rest, unless acted upon by
a force.” Which is just another way of saying that,
to change the way something moves — to give it ACCELERATION — you need a net force. So, how do we measure inertia? Well, the most important thing to know is
mass. Say you have two balls that are the same size, but one is an inflatable beach
ball and the other is a bowling ball. The bowling ball is going to be harder to
move, and harder to stop once it’s moving. It has more inertia because it has more mass. Makes sense, right? More mass means more STUFF,
with a tendency to keep doing what it was doing before your force came along, and interrupted
it. And this idea connects nicely to Newton’s
second law: net force is equal to mass x acceleration. Or, as an equation, F(net)=ma. It’s important to remember that we’re
talking about NET force here — the amount of force left over, once you’ve added together
all the forces that might cancel each other out. Let’s say you have a hockey puck sitting on a perfectly
frictionless ice rink. And I know ice isn’t perfectly frictionless but stick with it.
If you’re pushing the puck along with a stick, that’s a force on it – that
isn’t being canceled out by anything else. So the puck is experiencing acceleration. But when the puck is just sitting still, or
even when it’s sliding across the ice after you’ve pushed it, then all the forces are
balanced out. That’s what’s known as equilibrium. An object that’s in equilibrium can still be MOVING, like the sliding puck, but its VELOCITY won’t be changing. It’s when the forces get UNbalanced, that
you start to see the exciting stuff happen. And probably the most common case of a net
force making something move is the gravitational force. Say you throw a 5 kg ball straight up in the
air — and then, yknow, get out of the way, because that could really hurt if it hits
you… But the force of gravity is pulling down on the ball, which is accelerating downward at a rate of 9.81 m/s^2. So the net force is equal to m a, but the
only force acting here is gravity. This means that, if we could measure the acceleration of the ball, we’d be able to calculate the force of gravity. And we CAN measure the acceleration — it’s
9.81 m/s^2, the value we’ve been calling small g. So the force of gravity on the ball must be 5 kg,
which is the mass of the ball… times small g which comes to 49.05 kilograms
times meters per second squared! We use this equation for gravity so much
that it’s often just written as F(g)=mg. That’s how you determine the force of
gravity, otherwise known as weight. Now, those units can be a bit of a mouthful,
so we just call them Newtons. That’s right! We measure weight in Newtons,
in honor of Sir Isaac, and NOT kilograms! Kilograms are a measure of mass! But gravity often isn’t the only force
acting on the object. So when we’re trying to calculate a NET force, we usually have to take into account more than just gravity. This is where we get into one of the forces that tends to show up a lot, which is explained by Newton’s third law. You probably know this law as “for every
action, there’s an equal but opposite reaction.” Which just means that if you exert a force
on an object, it exerts an equal force back on you. And that’s what we call the normal force. “Normal” in this instance means “perpendicular”,
and the normal force is always perpendicular to whatever surface your object is resting
on. At least, it is when you’re pushing on something
big, and macroscopic, like a table. If you put a book down on a table, the normal
force is pushing — and therefore pointing — up. But if you put it on a ramp, then the normal
force is pointing perpendicular to the ramp. Now, the normal force isn’t like most other forces.
It’s special, because it changes its magnitude. Say you have a piece of aluminum foil stretched
tightly across the top of a bowl, and then you put one lonely grape on top of it. Because of gravity, that grape is exerting
a little bit of force on the foil, and the normal force pushes right back, with the same
amount. But then you add another grape, which doubles the force on the foil — in that case, the normal force doubles too. That’ll keep happening until eventually
you add enough grapes that they break through the foil. That’s what happens when the normal force
can’t match the force pushing against it. But, what does Newton’s famous third law
really mean, though? When I push on this desk with my finger
right now, I’m applying a force to it. And it’s applying an equal force right back on my finger
— one that I can actually feel. But if that’s true — and it is — then
why are we able to move things? How can I pick up this mug?
Or how can a reindeer pull a sleigh? Basically, things can move because there’s
more going on, than just the action and reaction forces. For example, when a reindeer pulls on a sleigh,
Newton’s third law tells us that the sleigh is pulling back on it with equal force. But the reindeer can still move the sleigh
forward, because it’s standing on the ground. When it takes a step, it’s pushing backward on the ground with its foot — & the ground is pushing it forward. Meanwhile, the reindeer is also pulling on
the sleigh, while the sleigh is pulling right back. But the force from the GROUND PUSHING
the reindeer forward is STRONGER than the force from the sleigh pulling it back. So
the animal accelerates forward, and so does the sleigh. So, one takeaway here is that: there would
be no Christmas without physics! But, now that we have an idea of some of the
forces we might encounter, let’s describe what’s happening when a box is sitting on
the ground. The first thing to do — which is the first
thing you should ALWAYS do when you’re solving a problem like this — is draw what’s known
as a free body diagram. Basically, you draw a rough outline of the
object, put a dot in the middle, and then draw and label arrows, to represent all the
forces. We also have to decide which direction is
positive — in this case, we’ll choose up to be positive. For our box, the free body diagram is pretty
simple. There’s an arrow pointing down, representing the force of gravity, and an
arrow pointing up, representing the force of the ground pushing back on the box. Since the box is staying still, we know that
it’s not accelerating, which tells us that those forces are equal, so the net force is
equal to 0. But what if you attach a rope to the top of
the box, then connect it to the ceiling so the box is suspended in the air? Your net force is still 0, because there’s
no acceleration on the box. And gravity is still pulling down in the same way it was
before. But now, the counteracting upward force comes from the rope acting on the box, in what we call the tension force. To make our examples simpler, we almost always
assume that ropes have no mass and are completely unbreakable — no matter how much you pull
on them, they’ll pull right back. Which means that the tension force isn’t
fixed. If the box weighs 5 Newtons, then the tension in the rope is also 5 Newtons. But
if we add another 5 Newtons of weight, the tension in the rope will become 10 Newtons. Kind of like how the normal force changes,
with the grapes on the foil. But in this case, it’s in response to a pulling force instead
of a push. The key is that no matter what, you can add
the forces together to give you a particular net force — even though that net force might
NOT always be 0. Like, in an elevator. So let’s say you’re in an elevator — or
as I call them, a lift. The total mass of the lift, including you,
is 1000 kg. And its movement is controlled by a counterweight, attached to a pulley. The plan is to set up a counterweight of 850kg,
and then let the lift go. Once you let go, the lift is going to start accelerating downward
– because it’s HEAVIER than the counterweight. And the hope is that the counterweight will
keep it from accelerating TOO much. But how will we know if it’s safe? How quickly
is the lift going to accelerate downward? To find out, first let’s draw a free body
diagram for the lift, making UP the positive direction. The force of gravity on the lift is pulling it down, and it’s equal to the mass of the lift x small g — 9810 Newtons of force, in
the negative direction. And the force of tension is pulling the lift UP, in the positive direction. Which means that for the lift, the net force is equal to the tension force, minus the mass of the lift x small g. Now! Since Newton’s first law tells us that
F(net)=ma, we can set all of that to be equal to the lift’s mass, times some downward
acceleration, a. That’s what we’re trying to solve for. So, let’s do the same thing for the counterweight. Gravity is pulling it down with 8338.5 N
of force in the negative direction. And again, the force of tension is pulling
it up, so that the net force is equal to the tension force, minus the mass of the counterweight
times small g. And again, because of Newton’s second law,
we know that all of that is equal to the mass of the counterweight, times that same acceleration,
“a” — which is positive this time since the counterweight is moving upward. So! Putting that all together, we end up with
two equations — and two unknowns. We don’t have a value for the tension force,
and we don’t have a value for acceleration. But what we’re trying to solve for is the
acceleration. So we use algebra to do that. When you have a system of equations like this,
you can add or subtract all the terms on each side of the equals sign, to turn them into
one equation. For example, if you know that 1 + 2=3 and
that 4 + 2=6, you can subtract the first equation from the second to get 3=3. And in our case, with the lift, subtracting
the first equation from the second gets rid of the term that represents the tension force. We now just have to solve for acceleration — meaning, we need to rearrange the equation to set everything equal to “a.” We end up with an equation that really just
says that “a” is equal to the difference between the weights — or the net force on
the system — divided by the total mass. Essentially, this is just a fancier version of F(net)=ma. And we can solve that for “a”,
which turns out to be 0.795 m/s^2. Which is not that much acceleration at all! So, as long as you aren’t dropping too far
down, you should be fine. Even if the landing is a little bumpy. In this episode, you learned about Newton’s
three laws of motion: how inertia works, that net force is equal to mass x acceleration,
how physicists define equilibrium, and all about the “normal” force and the “tension”
force. Crash Course Physics is produced in association
with PBS Digital Studios. You can head over to their channel to check out amazing shows like: BrainCraft, It’s OK To Be Smart, and PBS Idea Channel. This episode of Crash Course was filmed in
the Doctor Cheryl C. Kinney Crash Course Studio with the help of these amazing people and
our Graphics Team is Thought Cafe.
100 Comments
Camille Loredo
what is the law exerted in cable cars?
gabriel esqueda
I forget the whole thing. My rise was approximately at the beginning. What a remarkable creature. So well spoken.
Om Karmakar
Solution:
Given: u= 20 m/s; F= 500 m/s; m = 100 kg
By newton's 2nd law of motion,
F=ma;
or, 500= 100 a
or, a = 500/100
therefore, a= 5 m/s*s
By equation of motion,
v= u+at
or, 0=205t
or, 20=5t
therefore, t= 4 s
By equation of motion,
S=ut+1/2*at*t
or,S=20*4+1/2*5*4*4
or S= 80+40
therefore, S= 120m
Ans : The car would stop after covering a distance of 120 m.
Jersey Dayao
OMG THE MIGHTY DUCKS
kevin pavicevic
No Xmass without Newton, a crash course in spirituality as well !
Kill Me
newtons 3rd law example: a gun fires, there is a recoil
vale martinez
Hate physics
Soham Singh Maruwada
a kid at rest will remain at rest until yelled at by their mom
The Extreme Gamer Aayat
This is the amount of laws that Newton made

V
Jesse Woodby
Hate to be “that guy” but does anyone else think all this stuff can be explained more plainly?..
Isaac C. Guevara
8:31 You mean to say Second Law is F(net)= mass * acceleration not First Law. You correct yourself the next time you mention it.
Great video! Easy to understand and appealing to look out.
KawaiiPotato
I laughed so hard when the lady got hit by that bowling ball😂
Zara M
nice accent
Brainybutnotgeeky.
I can’t figure out who was more brilliant,Sir Isaac Newton or Albert Einstein.
Pineapple Panda
Newton= a×p+c
A awesomeness
P pysics
C curiosity
Pineapple Panda
What is your favourite field in pysics mine is quantum mechanics
nicole escobar
Imagine being her and being that intelligent.
You're amazing 😢❤
Everything Bulb
🙂
S Km
Newton's 4th Law of Motion – "A Rubik's cube at unsolved position will continue to be in unsolved position unless it is acted upon by a external effort to change its state of being."
Daniel Lindsay
The 3rd law is like trying to punch through a board but can't punch through it. The reaction is going to break your hand
Daniel Lindsay
Lady if you're that smart then why haven't you figured out how to attack a viruses ability to mutate?
Daniel Lindsay
Theoretical physics is good but again it's just theoretical. Use your intelligents to help humanity in a real applicable way
Daniel Lindsay
Intelligence sorry. Stupid autocorrect
Mohd. Danish
Thanks mam, I live lndia
Emiliano Gomez
love these vids, helping me through college one at a time! thanks for all that you folk at crash course do!
WyArYUGei N1
The 1st and 3rd leas are easy but I can’t understand anything about the 2nd
Eyobed Rezene
You suck at teaching wasted my time😤😤😤😤😤😤😡😾😾.
Rajiv Malhotra
8:32
*Second law
Not first law (Inertia)
SLOGAN PLAYS
you women looks so hot
SLOGAN PLAYS
what is your name
SLOGAN PLAYS
can I have your number
Michail Petrov
Raindeh
Niofer
Ap physics 1?
Hailey Brown
She pretty asf
Helen Tesfay
Img
Helen Tesfay
I looooovreeee
Fujin Yumi
Hank green is better at this. Why didn’t they just use him?
The Bicycle Engine
What's happening when the top of the World Trade Centre is crushing the bottom of the World Trade Centre?
dean odean
why is she so hott
The LOL Minecrafter
What's with the "Snake in a Box" reference?
Penguin Bros
Khan Academy vs. Crash Course: Who wins?
em
Don’t listen to him tho he died a Virgin
SPUDZ_ Nitro
Basically the 3rd law equals to friction
The Magic Potato
brain hurt
Daniel Hamilton
Go talk Tycho and Kepler 🙂
housta ghani
physics tyest tmrw so hardddddddddddddddd
Minion Man#Kevin
why do you have 3 of the same book
ThePinStripes
She's so good at what she does. Could listen to her forever
爱漫画Love manga
Im still soooooo confused
Анна Алексеенко
damnn crashcourse! its like they got MGS stock animation for dayss, just sprinkle some in that video and in this video lmao
Kristy Tran
u suck
Kristy Tran
what is this
Kristy Tran
why is my teacher showing me a video that was made in 2016
Stephen Ferris
Where is Hank
Javsisbest _
Like my comment for no reason
Nafis Ayazi
Did she just read my mind about the reindeer?
Jaden Gaming
/:
Jaden Gaming
Thx
Lekhashree Balakumar
oh my god lol everyone suggests crash course but i really always do think that these people talk so fast i have to put on the subs lmao
Shaud Tutor
My teacher says 9.81 what make it negative instead of positive?
Jayde Dinnoo
Sexy. Science.
Jane Doe
her: algebra
me: hold on jus a moment
Poonam Singh
Kya Boli samaj nhi aaya
Chris Gillett
Thanks I'm a scientist too a like toxic gases
order and chaos
3rd law i always reference it as
"If you shoot a gun in space the bullet will go forward but the gun will go backwards"
Ian Armijo
Gotta appreciate that metal gear reference.
Cpt Slow
Im in love
BOB Builder
It's a 100% that I'm going to fail my exam because I don't know anything about maths wish me luck bros
Eric Green
Where is John Green 🙁
Javsisbest _
Trash video
Martin Zanichelli
This video is from 2016. Are these Newton´s law still valid or this they are now considered racist, fascist, contrary to gender equality, cultural appropiation of aboriginal traditions or antifeminist? Does the new inquisition approves all this stuff considering that Newton is white?
Ace Valenzuela
sasabog na utak ko jusqoo
Riley Lowe
This just confused me more
🙁
Jermey Dawoj
Hi mom
Sebastian Carbajal Gomez
this teaches me more then my actual science teacher
Sarah Beitel
Did anyone else notice that she made a mistake at 8:34?
She said that Newton's first law tells us that Fnet = ma… but that's actually Newton's SECOND law.
Im that Guy
so impotant…..lol
GodakuriOPTC
Thank you
Little Moth Big Wings
Enem
mike tayolar
Good but in my opinion, too fast for people to study off of
Shankar Ghimire
How do airplanes fly?
Betim Krasniqi Odal Resursskolan
Great video. Thank you veary much
The Game Rules of our Universe
I just started out making videos on physics too, intuitively explaining it's most interesting and essential ideas in a series called
"The Game Rules of our Universe".
In case you are interested, I would really appreciate you checking out my channel.
James
When you're crushing flat earthers online and need cartoons to simplify things 🤣
Thedecisive Leader
you have stolen these information literally from the book called Fishbane – Physics for Scientists and Engineers with Modern Physics 3e
and this is a crime and must be punished
Nappie Smiles
•Newton’s 3 laws of motion•
First law: 1:01
Second law: 1:41
Third law: 3:47
Your welcome
Carlos Glaves
5:11 maybe cuz u got hands,🙄
kerji
this was soooooooo helpful!!
Collin Sinec
No Christmas = No Physics
So you're saying
No Santa = Isaac Newton isn't real?
Muammar Ssamad
Teacher in crashcourse: The normal force is special.
Me: Then why it is called a normal force?
Family: :/
Muammar Ssamad
5:52
The teacher says: There would be no Christmas without physics.
Me: Why he doesn't put a flying reindeer?
super porky
i fuk woman
MehruinNisa Zeeshan
poor rane dear
Mirayt beta
she's making me more sleepy than the teacher does
Marc Schneider
More mass doesn't mean more stuff 😁 but never the less i've never seen a video explaining Newton's laws better than this one 👌🏻 the best hands down
xXallyXx
HUH?!
xXallyXx
So what you're saying is, I'm going to fail my test tomorrow
Drey DoodleYT
totally dont have a physics exam tomorrow…
lareen Alashrafi
The accent of the girl is very noisy
tupac shakur
karma has an equal and opposite reaction