Newton’s First Law of Motion


Good morning. It’s February already. I’m back from my hiatus. I was so burned out doing all
those SAT problems. But now I’m ready and I will start
doing some physics. So we had done a bunch of
projectile motion, what happens you throw something
in the air or drop it from a cliff. But now I want to introduce you
to is how do you actually affect the acceleration
of an object? And to do that I’m going to
introduce you to Newton’s three laws. To some degree what we were
doing before was derivative of what I’m going to do now. But this is kind of the backbone
of classical physics. So Newton’s three laws. And you’ve probably heard
of these before. Newtow’s three laws. Sometimes they’re called
Newton’s Laws of Motion. I’ve actually looked this up on
the web just to make sure and see if there’s any correct
way of writing it, but every website seems to have
a different paraphrase of the laws. But hopefully, I can give
you an intuitive sense of what they are. So the first law is an object
at rest. An object at rest tends to stay at rest. And
an object in motion tends to stay in motion. This is what I learned when I
was a kid and now when I look at Wikipedia and things, there
are some paraphrases. And we’ll go over those
paraphrases because I think they’re instructive. Stay in motion. And you might say, Sal,
this is obvious. Why does Newton get
so much credit for stating the obvious? Obviously, if I look at my sofa
for example, it is an object at rest and if I keep
staring at it, it tends to stay at rest. Likewise, when
I look at a car crossing an intersection– that’s not a red
light, that’s crossing an intersection, it’s an
object in motion. And then, I don’t know– 10
seconds later, it’s still staying in motion and of course,
it will stay in motion unless you press the
brakes or whatever. So you might say, well
Sal, this is the most obvious thing ever. This doesn’t even need
to be written down. But let’s say you were Newton
and you came to me– it was in the 17th century. And you said, Sal, I have
these new laws. And the first is an object at
rest tends to stay at rest, and an object in motion tends
to stay in motion. And I would say Newton, I can
already disprove your law. Let’s say I have an apple and
I’m holding it up at let’s say my– I’m holding it up with
my arm, so it’s roughly my shoulder level. So I’m holding an apple. This is an apple. Looks like a heart,
but it’s an apple. So I’m holding it with my hand,
I’m drawing my hand. I don’t know if that makes sense
to you, but I’m holding it with my hand. And what happens when I
let go of that apple? So while I’m holding it with
my hand it’s an object at rest, right? But then when I let
go, what happens? It falls. Falls to the ground. So I’ll say, Newton, I just
disproved your first law. Because this was an object at
rest. And I did nothing to it. I just let go. I didn’t apply, I didn’t push
it, I didn’t pull it. I didn’t throw it. I didn’t do anything. And when I let go it just
fell to the ground. It started moving without me
doing anything, even though it was an object at rest. And then Newton will say, oh,
well that’s because there’s a thing called gravity. And it’s the force of gravity. And I would say, Newton, you
need to start to learn to not make up things. Just because you’re law doesn’t
make sense, you don’t need to make up artificial
forces in the universe. But anyway, he would
end up being right. And the way to think about this,
if I did this exact same experiment while I was in space
and let’s just say– I was going to say orbit because
it would look like that, but even orbit is kind of a– you’re
still kind of falling towards the earth, it’s just
you’re moving– well, I won’t go into that. I’ll go into orbit
at another time. But let’s say we were just in
deep space and me and the apple were just floating
around in space. Maybe we’re stationary. It’s hard to say. We’re floating with
respect to what? And then, if we’re in space and
I let go of this apple, what happens to the apple? Nothing. It’s not going to
fall anywhere. It’s not going to move. And so whenever you think about
Newton’s laws– and that’s why this is so amazing. He didn’t know about space. He’s living in this planet that
everything tends to fall and things start moving for no
reason because of whatever, gravity, and the wind
and whatever else. And he actually theorized that
there could be a place where there’s no forces acting on
objects where if I were to let go of this apple, it would
just stay where it is. And similarly, the
object in motion tends to stay in motion. And there again I would’ve
told Newton, well, that doesn’t make sense. If I were to– I don’t know. If I were to push a– well, I
don’t know if they had bowling balls back then. But if I were to roll a bowling
ball down a– well let’s say up a hill– At some
point that bowling ball’s going to slow down. If I rolled it up a hill,
at some point it’s just going to slow down. And maybe if I got it right it
would just stop at the top if I did it perfectly. And I could say, look, this
was an object in motion. At some point it stops or it
actually turns back around. Or even if I were to roll it
this way, at some point it’s just going to stop. Right The bowling ball’s
going to stop. If I were to push something as
hard as I could, maybe it travels for a couple of feet,
but then it’s going to stop. And he’ll say, oh, well you
know, there’s these forces that you’re not realizing
there’s a force. There’s the wind resistance in
the bowling ball example. There’s the force of friction
in the example where I just pushed something. And I would’ve said, well
Newton, you’re just making up these forces again. And this is why this is
so not intuitive. Because he had to essentially
realize that there were all of these forces acting on something
when to someone at that time, you wouldn’t have
realized that and you wouldn’t have been able to even conceive
that there’s a place called space, for example,
where these things wouldn’t happen. If I push something in space,
it will keep going. It would be an object in motion
and it will keep that velocity until some other
force acts on it. So it wasn’t that intuitive. And so a more modern way to
write this is to say that there is a frame of reference,
there exists a frame of reference– and I’ll explain
what a frame of reference is. But there exists a frame of
reference where this is true. That could be the new way
of saying Newton’s first law of motion. So what’s a frame
of reference? So everything in physics–
if I’m moving, moving relative to what? Moving relative to
the observer? Moving relative to the earth? You don’t know. So a frame of reference is what
is the observer doing? So example: when I’m in space
and I let go of the apple, me and the apple are kind of in
this– I am observing the apple from what I call an
inertial frame of reference. So this is a frame of reference
actually where Newton’s laws hold. If I take the apple on earth and
I let go and it drops, the reason why this first law didn’t
hold is because I’m not really in an inertial
frame of reference. Because me and the apple are
both constantly being pulled on by this force
called gravity. So although it looks like
nothing’s going on, me and the apple are in the same–
nothing’s really acting on us. There is. There’s this force
of acceleration. Similarly, if I’m in a
car and that car is accelerating, right? So let’s say the car– looks
more like a pickup truck, so I’ll go with the pickup truck. Let’s say you have a pair of
dice hanging from your rear view mirror. This is the dice right here. What happens when the
car accelerates? Well the dice move
back, right? And so when you’re sitting in
the truck itself, it looks like the dice are just
moving back. No one’s really doing
anything to it. Let’s say the car had no windows
and you would just all of a sudden mysteriously feel–
well, you’d feel a little squeezing on your chest
too, but you would also just see these dice move back. And you’d say, hey. Newton’s first law
doesn’t hold. And what I would say is well
that’s because you’re in a non-inertial frame
of reference. To someone outside of the truck,
they would see, oh well, the truck is moving, the
truck is actually accelerating and that’s why the
dice move back. So in I guess you could say the
horizontal dimension, and I’m probably just confusing you,
but I want to give you a really intuitive feel about why
this isn’t so intuitive. In the horizontal direction,
because there are no forces of gravity or whatever acting in
this direction, and if I’m outside of the truck, I could
then– I would be in an inertial frame of reference
in at least the horizontal dimension. I mean we always have gravity
pulling down on us. But from the outside of the
truck, I could observe that oh, you know, Newton’s law holds
because the whole frame of reference, this truck is
actually being accelerated. So me being outside of that, I
would be in an inertial frame of reference. Hopefully I haven’t confused
you too much. The way to think about it is
that an inertial frame of reference is just a frame of
reference where there’s no outside forces acting on the
whole frame of reference. And a frame of reference
is just what is the observer doing? What am I doing? Am I moving with the object? Am I being accelerated
with the object? Or, are neither me nor the
object being acted upon? And that’s the way to
think about it. Oh, I already ran out of time. I only got one law done. So I’ll see you in
the next video.




Comments
  1. No one is forcing you to watch this stuff. Some people need help with their school work and this is great stuff for that. Sal has a very clear way of explaining things.

  2. thanks dude i searched it up on google but it was to hard to understand you made it simpler. nice vid man. and i finished my homwork : D thanks times 2

  3. lololol i love this one. Newton you need to stop makingn up things. Just because your law doesn't make sense doesn't mean to make artificial forces in the universe

  4. Actually no. Newton's First Law only indicates that a mass which isn't affected by any force will stay at rest or keep moving in a straight line at a constant speed.

  5. I find it interesting that no matter how much we revolutionize physics, and step BEYOND classical Newtonian physics, Newton's three laws of motion stay the same. Crazy.

  6. I love how he "tried" to disprove Newton's first law and starts back-sassing Newton.

    "Look, I didn't do anything to it."

    "It was gravity, gravity was the force that makes it accelerate.."

    "Stop making stuff up."

  7. Can someone please answer something for me?

    Laws exist – FACT

    Laws comprise, are behind, or are underneath other laws – FACT

    Because laws comprise, are behind, or are underneath other Laws they are the reason other Laws exist's – FACT

    Are any of these false?

  8. uhhh hey guys, yeah i loved the first episodes on projectile motion but im really looking for a better explanation on inertia anddd such…cuase you see. "suh eyeeezik" should really be praisedd more often. not so much for his rape accusations but more so his theory's on gravity and MASS

  9. Friction causes objects to stop. In a frictionless enviroment an object in motion would continue to move until acted upon by a net force. This is called inertia

  10. Look, I know your videos are good but the part where you dropped the apple, and you said it was disprove of newton's first law, i advise strongly that you watch the Eureka Episode on inertia, just type eureka inertia on youtube

  11. his voice charms me, before i go to sleep i open my laptop then watch this videos because his voice are so charming……uhhhhh

  12. kindda confused me.. i guess all i needed to know was that it stays in motion or stay at rest unless acted upon by an external force.. the inertia bit I dont get and confused me since I don't know what it means.. :/

  13. bahaha "newton i just disproved your rule"
    "oh well there's a thing called gravity…"
    "Newton! you need to not make up things!"
    I love how I always get a kick out of all your videos.

  14. When your in a vehicle, the dice doesnt move for the outside viewer, it stays stationary, while the car moves. same if you put a book on the dashboard, and take a sharp turn, it appears to slide sideways to the people in the car, but from the frame of outside the car, the book isnt moving, it simply stays stationary, while the car moves

  15. thank you so much! you're very good at explaining this, i will definitely be looking towards your other videos for physics help.

  16. hey men this mass is in the line of zero,and all started from zero,unit or gravity is zero,so when the mass is at rest it will stay atrest,when you spin it it will stays in motion,at the line of zero

  17. If F=ma and E=Fd so isn't kinetic energy; KE=mv^2? of course this is not the accepted formula, can anyone explain why?

  18. @mdias5907699 I hadn't learnt of calculus yet when I posted this, since posting I discovered the reason why, but thanks anyway. The fault lies with those who promote the following incorrect statements: v=d/t a=v/t
    Are these usually taught at university undergraduate level?

  19. hey this video is basically the law of linear inertia. everything gets crasy when you talk about rotational inertia and the angular counterpart of mass. A video on rotational inertia would be greatly appreciated!

  20. hey, can u explain how the pair of dice move backwards, when the car accelerates forward, isnt the dice supposed to go in the same direction of the acceleration of the 'pickup truck'??

  21. I have an idiotic question but someone please refresh my mind

    If you apply a constant force of 10N to a mass of 1kg, it will accelerate 10m/s^2 until the application of the force ceases, in which case the velocity of the mass will be whatever it was when the force stopped being applied. 

    Now to the question. If you bench press for instance (a small weight that you can do pretty much whatever you want to) you're applying a force to the weighted bar, right? However, the weight is not necessarily accelerating. Say you intentionally press the bar from your chest all the way up with a constant velocity (ofc t(0) won't be the same as after 2 seconds but anyway) of 0,2 m/s, what is the catch in that situation? A force must be applied constantly or the weight would drop on your chest, why isn't it accelerating?

    thanks a lot for answers!

  22. I see a strong correlation between Hegelian's Dialectics; Thesis-Antithesis-Synthesis, and Newtons Laws of Motion. Antithesis is the contrast or opposition of an idea or concept, and synthesis is the unified whole of mutually exclusive elements, or the reconciliation of contradictory ideas where a new idea is formed, and this concept has been perfectly demonstrated within the medical context. Our perception of the world is constructed around polar opposites, a notion demonstrated in Issac Newton's Three Laws of Motion, a law in nature that laid the foundation for classical mechanics.

    Primarily the "Third Law" which states; every force or action is contingent on an equal reactive force. Meaning if you push a cup on a table, it's motion and velocity should be perpetual, but what hinders or constricts is movement is an external force; like the friction on the table applied contrastingly to the cup in motion, hence it stops. Or the "First Law" which states; an object in uniform motion stays in motion unless acted upon by any given force.

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