More on Newton’s first law of motion | Physics | Khan Academy

More on Newton’s first law of motion | Physics | Khan Academy

Articles, Blog , , 100 Comments

In this video, I want
to talk a little bit about Newton’s
First Law of Motion. And this is a translation from
Newton’s Principia from Latin into English. So the First Law,
“Every body persists in a state of being at
rest, or moving uniformly straightforward, except
insofar as it is compelled to change its state
by force impressed.” So another way to rephrase
what they’re saying is, that if there’s something–
every body persists– so everything will
stay at rest, or moving with a constant
velocity, unless it is compelled to change
its state by force. Unless it’s acted on by a force,
especially an unbalanced force. and I’ll explain
that in a second. So if I have something that’s
at rest, so completely at rest. So I have– and
this is something that we’ve seen before. Let’s say that I have a rock. Let’s say that I
have a rock someplace and it’s laying on
a field of grass, I can keep observing that rock. And it is unlikely
to move, assuming that nothing happens to it. If there’s no force
applied to that rock, that rock will just stay there. So the first part
is pretty obvious. So, “Every body persists in
a state of being at rest”– I’m not going to do
the second part– “except insofar as there’s some
force being applied to it.” So clearly a rock
will be at rest, unless there’s some force
applied to it, unless someone here tries to push it or roll
it or do something to it. What’s less intuitive about the
first law is the second part. “Every body persists in,”
either, “being in a state of rest or moving
uniformly straight forward, except insofar as
it is compelled to change its state
by force impressed.” So this Newton’s
first law– and I think I should do a
little aside here, because, this right
here is Newton. And if this is
Newton’s first law, why do I have this huge
picture of this guy over here? Well, the reason is is because
Newton’s first law is really just a restatement of
this guy’s law of inertia. And this guy, another
titan of civilization really, this is Galileo Galilei. And he is the first person to
formulate the law of inertia. And Newton just rephrased it
a little bit and packaged it with his other laws. But he did many, many,
many other things. So you really have
to give Galileo credit for Newton’s first law. So that’s why I made
him bigger than here. But I was in the
midst of a thought. So we understand if
something is at rest, it’s going to stay at
rest, unless there’s some force that acts on it. And in some
definitions, you’ll see unless there’s some
unbalanced force. And the reason why
they say unbalanced is, because you could have two
forces that act on something and they might balance out. For example, I could push
on this side of the rock with a certain amount of force. And if you push on
this side of the rock with the exact same amount of
force, the rock won’t move. And the only way that it would
move if there’s a lot more force on one side than
on the other side, so if you have an
unbalanced force. So if you have a ton
of– and maybe the rock is a bad analogy. Let’s take ice, because ice is
easier to move, or ice on ice. So there’s ice right here. And then, I have
another block of ice sitting on top of that ice. So once again, we’re
familiar with the idea, if there’s no force acting
on it that ice won’t move. But what happens if
I’m pushing on the ice with a certain amount
of force on that side, and you’re pushing on
the ice on that side with the same amount of force? The ice will still not move. So this right here, this
would be a balanced force. So the only way for the ice to
change its condition, to change its restful condition is
if the force is unbalanced. So if we add a little bit
of force on this side, so it more than compensates
the force pushing it this way, then you’re going to see
the ice block start to move, start to really accelerate
in that direction. But I think this
part is obvious. This, you know,
something that’s at rest will stay at rest, unless
it’s being acted on by an unbalanced force. What’s less obvious is
the idea that something moving uniformly
straightforward, which is another way of
saying something having a constant velocity. What he’s saying is,
is that something that has a constant
velocity will continue to have that constant
velocity indefinitely, unless it is acted on
by an unbalanced force. And that’s less intuitive. Because everything in
our human experience– even if I were to push
this block of ice, eventually it’ll stop. It won’t just keep
going forever, even assuming that this ice field
is infinitely long, that ice will eventually stop. Or if I throw a tennis ball. That tennis ball
will eventually stop. It’ll eventually
grind to a halt. Or if I roll a bowling
ball, or if I, anything. We’ve never seen, at least
in our human experience, it looks like everything
will eventually stop. So this is a very
unintuitive thing to say, that something in
motion will just keep going in
motion indefinitely. Everything in human intuition
says if you want something to keep going in
motion, you have to keep putting more force, keep
putting more energy into it for it to keep going. Your car won’t go
forever, unless you keep, unless the engine keeps burning
fuel to drive and consuming energy. So what are they talking about? Well, in all of these
examples– and I think this is actually a pretty
brilliant insight from all of these fellows is
that– all of these things would have gone on forever. The ball would
keep going forever. This ice block would
be going on forever, except for the fact that
there are unbalanced forces acting on
them to stop them. So in the case of ice,
even though ice on ice doesn’t have a lot
of friction, there is some friction
between these two. And so you have,
in this situation, the force of friction
is going to be acting against the direction
of the movement of the ice. And friction really comes
from, at an atomic level– so if you have the actual
water molecules in a lattice structure in the ice
cube, and then here are the water molecules in a
lattice structure on the ice, on the actual kind of sea of
ice that it’s traveling on– they do kind of bump and
grind into each other. Although they’re both smooth,
there are imperfections here. They bump and grind. They generate a
little bit of heat. And they’ll, essentially, be
working against the movement. So there’s a force of friction
that’s being applied to here. And that’s why it’s stopping. Not only a force of
friction, you also have some air resistance. The ice block is
going to be bumping into all sorts of air particles. It might not be
noticeable at first, but it’s definitely going to
keep it from going on forever. Same thing with the ball
being tossed to the air. Obviously, at some
point, it hits the ground because of gravity. So that’s one
force acting on it. But even once it
hits the ground, it doesn’t keep rolling
forever, once again, because of the friction,
especially if there’s grass here. The grass is going to
stop it from going. And even while it’s in the
air, it’s going to slow down. It’s not going to have
a constant velocity. Because you have all
of these air particles that are going to
bump into it and exert force to slow it down. So what was really
brilliant about these guys is that they could
imagine a reality where you didn’t have gravity,
where you did not have air slowing things down. And they could imagine
that in that reality, something would just keep
persisting in its motion. And the reason why Galileo,
frankly, was probably good at thinking about
that is that he studied the orbits of planets. And he could, or at
least he’s probably theorized that, hey, maybe
there’s no air out there. And that maybe that’s why
these planets can just keep going round
and round in orbit. And I should say their speed,
because their direction is changing, but their
speed never slows down, because there’s
nothing in the space to actually slow
down those planets. So anyway, hopefully you found
that as fascinating as I do. Because on some level,
it’s super-duper obvious. But on a whole other level,
it’s completely not obvious, especially this moving
uniformly straightforward. And just to make
the point clear, if gravity disappeared,
and you had no air, and you threw a ball,
that ball literally would keep going
in that direction forever, unless some other
unbalanced force acted to stop it. And another way to think
about it– and this is an example that you might
see in everyday life– is, if I’m in an airplane
that’s going at a completely constant velocity and
there’s absolutely no turbulence in the airplane. So if I’m sitting in the
airplane right over here. And it’s going at a constant
velocity, completely smooth, no turbulence. There’s really no way for me to
tell whether that airplane is moving without looking
out the window. Let’s assume that there’s
no windows in that airplane. It’s going at a
constant velocity. And there’s no turbulence. And let’s say, I
can’t hear anything. So I can’t even
hear the engines. There’s no way for me to sense
that the plane is moving. Because from my
frame of reference, it looks completely
identical to if I was in that same plane that
was resting on the ground. And that’s another
way to think about it. That it’s actually
very intuitive that they’re similar states,
moving at a constant velocity or being at rest. And you really can’t
tell whether you are one or the other.

100 thoughts on “More on Newton’s first law of motion | Physics | Khan Academy

  • Yusuf TANA Post author

    Hi Sal again,
    Since we need more and more force to accelerate or decelerate or change the direction of any object in space more and more, this means we will need an infinite amount of force to accelerate any object to an infinite velocity or to decelerate it to a zero velocity. Can you confirm it.

  • Juana Navarro Post author


  • Yusuf TANA Post author

    Hi Sal, this is comment 3 on this video:
    I suggest the following experiment to determine the actual velocity of any object in space:
    If we take (for example) a 1 unit mass object and give it 1 unit force it will be accelerated 1 unit velocity. So, an object with 2 units mass will need 2 units force to accelerate 1 unit velocity. If two objects require the same amount of force to accelerate 1 unit velocity they have the same mass. So, to know the actual velocity of any object in space … comnt4

  • Yusuf TANA Post author

    Hi Sal, this is comment 4 on this video:
    So, to know the actual velocity of any object in space we take the reference object with 1 unit mass which is given 1 unit force to accelerate it 1 unit velocity and call its initial velocity (before we gave it 1 unit force to accelerate it 1 unit velocity) the reference actual velocity in space. If this reference object have a faster initial velocity, it will require less force to accelerate it 1 unit velocity. Hope to hear from you Teacher Sal.

  • MegaGangsta4life Post author

    My physics teacher doesnt give us textbooks. He gave us a piece of paper with your website on it XD He knows we aint gonna read. Unfortunately no one in my class except me watches ur vids. Poor poor FOOLS XD

  • Freddie Frillman Post author

    What program did you use? It doesn't look like Windows paint.

  • lionelthefox Post author

    The probe fired from the ISS would be constant travelling 'uphill' against the Sun's gravity. Without using its thrusters or using other planets' gravity to accelerate, the probe would eventually slow down and fall back towards the Sun.

  • Ivana Egenti Post author

    thanks Khan academy you just helped me with my homework

  • HawtTurtle Post author

    what program did you use to draw on the pictures

  • Ninosław Brzostowiecki Post author


  • khadija malick Post author


  • Mr Pregnant - Atelston Fitzgerald Holder 1st Post author

    “Newton’s Third Law of Motion,” a law in physics that has stood the test of time. With exception ofcourse to the quantum world of improbability and ambiguity. Newton’s first law of motion states that the uniform state of an object remains in motion unless disrupted by an external force. Second law states the relationship between object, mass and acceleration, (F is F = ma). The third law states; for every force in nature there’s an equal opposite reaction. A propagating law in physics that you can analogically and metaphorically cross-platform into mutually exclusive fields; "figuratively," and would definitely stand the test of time. Visit my website, I do a magnificent correlation between propagating laws in physics, zoology, linguistics and it's relationship to humor.

  • Sheikh Tadeeb Post author


  • TheTrainIsCominggggg Post author

    dat last example doe!!!!!

  • Keshav Khanna Post author

    But if the ice is just sitting on ice, wouldn't it just slide and move? Like there would be so little friction to stop it

  • nat1XP Post author

    So when you push something over a carpet, and want it going at constant velocity while overcoming resistant forces, do you apply the same force continuously? I thought this would give an acceleration.
    I keep getting mixed up with applying the same force constantly (once overcome friction), whether it will always accelerate or if there's a time when it travels at constant velocity. Thanks, hope it's a clear question. It's weird how something so obvious we experience all the time is confusing me aha
    These videos help with a lot, thanks.

  • MrKazoozles Post author

    I now hate physics just a little less thanks to you, Sal. 

  • nino tiso Post author

    that is crazy that a person can come up with a theory about anti gravity movement when they didn't know anything really about space

  • Yusuf TANA Post author

    If we slow down the velocity of any object in space to zero, then the rest of the objects in space will go past the slowed object extremely fast. Because all the objects in space are going round the gravitational center of the universe. So, can we be even theoretically sure we can slow any object to zero.

  • xnOrpix Creative Post author

    awesome ^..^

  • nadiya 17 Post author

    You saved lot's of people, man 😊 I have a test tomorrow and I watched the few videos of yours, and I think I understand everything, I'm super happy, thank you 😃

  • Tal Moore Post author

    Every BODY, not everybody, lol.

  • Jhanavi K S Post author

    Well, you said that an object changes its state of rest or motion when an unbalanced force acts on it. What is the force that makes the earth or any other planet for that matter, revolve around sun?

  • Bill Paterson Post author

    Rock is not at rest since it's attached to the earth which is moving round and travelling through space. Is that facetious or what? Nice presentation though, like the drawing gizmo, will have to try it out.

  • newgamer85 Post author

    With the plan analogy would you not feel the plan decelerate?

  • Cloud Shadows{back up alt} Post author

    So what if the incredible hulk pushed on the left side of the ice. (To me it would break and moe to the right)

  • Mukundan Rangarajan Post author

    The law States that an object at rest or uniform motion in a straight line unless compelled to change by an applied force .

    Galileo told that this law will only obey when the object is in vacuum or friction free environments

  • Jasneet Singh Post author

    Thanks sir

  • Vinod Venkatesan Post author

    Great video. Another good example of not feeling uniform motion is the rotation of the earth on its axis which we can't feel. Strictly speaking it isn't EXACTLY uniform and does have an acceleration but it is so small that it can be considered uniform for most cases. If the rotation were to stop it would be like a high speed car suddenly halting, only it would far more drastic stop which would lead to everything being destroyed.

  • DeeDa Ak Post author


  • Kagichu Maina Post author

    He picked Galileo's nose.

  • Never drink and derive Post author

    I like how he repeats everything

  • Courtney Tanguay Post author

    I do find that as fascinating as you do! Awesome 🙂

  • Sunnyvale Post author

    How could someone possibly dislike any one of this gods videos

  • Zach Warner Post author

    All of this is wrong

  • Los Evilas Post author

    What program is used to draw/write everything?

  • Murat Atay Post author

    If there are creatures that live outside the universe, these videos are universe 101 😀
    there are some things that can move on their own, they are called animate objects and there are things that do not, they are inanimate objects
    An outsider to the universe might ask why isnt gravity upside down(maybe it is if you think about it) and why do things drop down when you release them from above instead of going up
    This is like explaining minecraft to a beginner, things wont fall down if there had been a block under them before unless they are sugarcanes or something and a person could easily say it doesnt make sense but it's consistent in its own logic
    That's how i think of the universe, an outsider to our universe would think our universe doesnt make sense.Why the fuck cant everything move according to their free will.But it's consistent within itself

  • ArtyPreventsCamping Post author

    I love khan academy keep up the awesome work guys

  • Ethan Powell Post author

    I'm sure what he's trying to say is exactly what he said. Any moving body will keep moving in the same direction with the same speed indefinitely, unless it is compelled to change it's state by an outside force. Yes your car wouldn't keep moving (on earth) because of the many outside forces on earth, other bodies such as Nitrogen and CO2 in the air and also gravity. But if you were to push your car in a vacuum and out of the influence of any gravitational force then it's state will never change.

    Now I don't claim to be right but this is just my intuition of the subject. I am open to any and welcome any proof that I'm wrong. Also I love the videos! They have helped me a lot.

    One question, does anyone else only watch the video if Sal is teaching? I find myself avoiding all others.

  • Ukkasha Farqaleet Post author

    you have been amazing for me

  • kegan Post author

    I think another good way to think about the second part is that objects on Earth that are in a state of rest are also moving uniformly forward due to the Earth's constant rotation. Amazing videos man!

  • Isaac Pender Post author

    What I don't understand is that if you were in a car and moving at a constant speed, that would mean the net force is zero. But how could the car be moving if the net force is zero?

  • W Post author

    I'm not even in school and I'm watching all his videos lol

  • PoseidonKross2009 Post author

    bump and grind… lol

  • Tim Tommy Post author

    someone forgot about the existence of space…

  • KATORI BLAUSE Post author


  • Athif Mohamed Post author

    "Perpetual movement."

  • Sean Yu Post author

    I love his videos. They help me soo much. Love the website as well! BTW, how can I guy be so good at writing with a mouse. My letters turn out like spaghetti. He is so smart and talented with the mouse. XD

  • Davis John Post author

    Sir Isaac Newton is hot.

  • Gloria Pujante Post author

    People that dislike this kind of videos are MORE LIKELY have no interest in knowledge, there's nothing to lose when u learn. I cant even—

  • Btstrash Post author

    this video was amazing, just amazing

  • Brian Patterson Post author

    i do not believe an object will go forever in space. there are atoms in space therefore friction and pressure.
    eventially this will stop the object.
    there is not a vacuum in space. this terminolgy is like saying the surface of the earth is a vacuum to its center core where great magntudes of pressure exist.

  • Roberto Sanchez Post author

    Great, but you go so slowly and you take so long to make it to your point!
    Taking too long = waste of time!

  • Noel Sandoval Post author

    at around 5:00 you said that things will not go forever actually they will but the out side force is gravity and friction

  • Zona D Kira Y algo Mas Post author

    what program you use for do this video that black board

  • Archie Moore-moore Post author

    make more. videos

  • Luis Serrano Post author

    There's a chance the rock and ice could break apart if the forces are too great.

  • Action VideoGames Post author


  • WEE3EEB Post author

    why would anyone dislike this video

  • LookListenFeel Post author

    Friction be like, I know you don't see nothing wrong baby, with a little bump n grind.

  • thebutter flyNL Post author

    why does friction cause heat?

  • Brett Shorette Post author

    I knew it! The earth is flat!

  • 이건열 Post author

    Since the ball has the same inertial on the earth and the moon it will require the same force on both places to stope a flying ball? True?

  • Lun Haolai Post author

    about this law… can the astronauts shut down their engines while they are in space, considering there is no gravity, atmosphere or any force. perhaps save fuel… Hahahah

  • MrTrackman100 Post author

    Rock is "LYING on the grass." Sorry, but an instructor should use correct English.

  • Nirab Ahmed Post author

    3:39 the ice would broke

  • pathways Egypt Gehad Tariq EL-Gamey Post author

    These videos are a great work.

  • Delta Side Post author

    When an object is pushed in space, say a ball is thrown, it will never stop moving. Unless acted upon by and unbalanced force such as gravity, an asteroid, or anything else. If it even hits in to any of those things knowing how big space is. Point being, yes on Earth we intuitively know that you must keep adding force to make an object go. But because I knew about how some physics in space works, it was intuitive. So for me, that is the best example for law 1.

  • Delta Side Post author

    And of course gravity is acting on the ice cube pulling it down creating even more friction.

  • Delta Side Post author

    I just wanted to say Thanks to Sal Khan!

  • Delta Side Post author

    Kind of like the fact that we are moving 67,000 miles and hour around the Sun yet feel no force acting against us. Kind of like if you throw something up in the air in a car. It travels at the same speed as you.

  • Kamila Mereke Post author

    if the car is moving in the vacuum, will it stop eventually? or keep moving?

  • Harnath Ram Post author

    Newton's three laws are the steps of motion.. It is not motion law.. these are ideological principles.. not adjust with Physical Nature.. so Newton's laws are wrong.. I can it wrong And rewrote new motion law for science

  • Harnath Ram Post author

    Newton's three laws are the steps of motion.. It is not motion law.. these are ideological principles.. not adjust with Physical Nature.. so Newton's laws are wrong.. I can it wrong And rewrote new motion law for science .by Harnath Ram teacher from Sumerpur dist pali, Rajasthan

  • Harnath Ram Post author

    Newton's three laws are the steps of motion.. It is not motion law.. these are ideological principles.. not adjust with Physical Nature.. so Newton's laws are wrong.. I can it wrong And rewrote new motion law for science .by Harnath Ram teacher from Sumerpur dist pali, Rajasthan

  • Harnath Ram Post author

    Newton's three laws are the steps of motion.. It is not motion law.. these are ideological principles.. not adjust with Physical Nature.. so Newton's laws are wrong.. I can it wrong And rewrote new motion law for science .by Harnath Ram teacher from Sumerpur dist pali, Rajasthan

  • Harnath Ram Post author

    Newton's three laws are the steps of motion.. It is not motion law.. these are ideological principles.. not adjust with Physical Nature.. so Newton's laws are wrong.. I can it wrong And rewrote new motion law for science .by Harnath Ram teacher from Sumerpur dist pali, Rajasthan

  • Andressa Teodoro Post author

    Esse vídeo é ótimo… Obrigada…Me ajudou bastante.

  • Francesco De Geronimo Post author

    great stuff 🙂 it really help me with my grades. thankyou for producing useful contents

  • Mark Moody Post author

    Scientists are the problem. They are considered mainstream only if their theories are based on phenomena.
    Example: Gravity is considered a phenomenon. It is considered a phenomenon because we cant explain it. Because it doesn't work and it doesn't exist. If it did exist, we would certainly have discovered it by now. However, instead of making the Logical conclusion that we need a real Force. We continue for 200 years with this force. Gravity does not even explain Gravity. Lot alone anything else in our universe.
    We do not correct the error. What we do is add another phenomenon to explain the first phenomena such as dark matter. Then when that failed they created another phenomenal dark energy.
    That is science now?
    You cannot use a phenomenon to explain the phenomenon that explains phenomena.
    That it's not science under any circumstances or any definition. At best it is witches and Fantasies.
    This is why they're not finding the truth. They are taking us further away and destroying anyone's chance of seeing reality.

  • samir hafza Post author

    Granted, in space, there's no gravity, no air, therefore no friction. But what about extraterrestrial (space) dust? Doesn't that create friction, however minute? The little physics I had in college was a long time ago. So forgive my ignorance.

  • Nilaya Deshpande Post author

    You are wrong atoms do not touch each other their electron interact this is stated by physics

  • Junyi Li李俊逸 Post author

    he taught very well,eth will stay on the normal when there is no unbalanced force

  • Antonio Shells Post author

    You stated the first law of motion beautifully. Bravo good sir, keep up the good work.

  • orgami technology Post author

    Are you Muslim

  • Genral SaharóÒ Post author

    If there is nothing in human nature that does not stop why doesn’t the earth stop it’s movement

  • Jasil KM Post author

    Hope Disliked persons can give more clear explanation on this

  • Adnan Abbas Kanchwala Post author

    Sir What force keeps the planet rotating and why it does not collide with sun due to force of gravity that pulls the earth?
    Thank you for those beautiful explanation!

  • Brandon Wright Post author

    My name is Brandon Wright. I have defeafed all three of Isaac Newton's laws of motion. I have proved gravity is momentum and it is not the law of acceleration. I can email scientific proof of my claims to you or you can find it on my Facebook page at
    Isaac Newton should be removed sir. Feel free to build it for your self. (niveus in verba)The video demonstrations are also on my Facebook page too, in my albums, under Hook vs. Newton. Thank you.
    We have an opportunity together to correct this error in physics. To better tomorrow, as a community and as a society. Again thank you and enjoy the defeat of Isaac Newton's laws of motion.

  • Gunnar Schmidt Post author

    at 7:06 its not true its just gravity

  • Chris Vodden Post author

    I’m in school with Jackson and Michael

  • Jade Broun Post author

    U know, everybody just LOVE your videos on Russian 💓💓
    Cause they are so natural and easy to understand ! Thank u:)

  • Don’t Know Post author

    why do all these planets keep revolving indefinitely? shouldn’t the planets be slowly moving closer to the sun due to the gravitational force of the sun?

  • Elijah MC Post author

    Thanks Thor! Btw, please kill Thanos.

  • Lennon Sliger Post author

    This basicly means kinetic energy can be turned into potential energy with an outside disturbance, as well as an object formulated in potential energy can be renewed as kinetic energy with an outside disturbance.

  • Lennon Sliger Post author

    If you may solve this problem you are a genius. Why is Tesla wrong?

  • TsunaXZ Post author

    I have report tommorow about Law of Inertia

  • Nashi Allen_West Post author

    I have a test tomorrow on Newtons Law in science and we have learned it for four days now.We barely spent time on Yhe third law of motion.I am good with the first law,but not the other 3.

  • Yorum Adam Post author

    Thanks for Turkish Subtitle. .

  • Rumki Das Post author

    So why does the Moon revolve around the Earth forever even if Gravitational Force of Attraction is present between the two of them ??

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