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Course: Physics library > Unit 3
Lesson 1: Newton's laws of motion- Newton's first law of motion introduction
- Newton's first law of motion
- Applying Newton's first law of motion
- What is Newton's first law?
- Newton's first law
- Newton's second law of motion
- More on Newton's second law
- What is Newton's second law?
- Newton's third law of motion
- More on Newton's third law
- What is Newton's third law?
- Newton's third law of motion
- All of Newton's laws of motion
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Newton's first law of motion
Newton's First Law of Motion, also known as the Law of Inertia, states that an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. This law, originally formulated by Galileo, is fundamental to understanding motion and forces. Created by Sal Khan.
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- my physics textbook says that if a bus is moving then the people sitting or standing in the bus also move in the with the same speed in the same direction .but according to me how can the people also be moving? and further my book states that when the bus stops suddenly , the upper body still continues to move and the lower body tens to move turn with the bus which causes the passengers to fall forward.
but it isn't clear to me that how is the upper body in motion?
can someone explain!<its stated under the examples of inertia>(11 votes)- The people are at rest but they are in motion with respect to the ground. If the people were not in motion how will they arrive at their destination?
It is your feet which keeps u in contact with the bus ..right. If the bus stops then your feet will also come to rest. but the tendency of inertia the upper body keeps moving to avoid that sudden jerk of coming at rest.(23 votes)
- Atwhen we were talking about the plane example......we said that if the plane was moving or at rest and we were inside it, we wont realize that whether the plane is movinr or is at rest.......However even if the plane was accelerating with we being inside it......wont we still feel that we are at rest? Isn't it that both our velocity and the acceleration w.r.t. plane zero? 8:45(15 votes)
- We actually cannot say, because there is no force acting on us. We have the same velocity as the plane, which is our plain of reference. Thus unless we have a window, we cannot say for certain.(4 votes)
- I know this video was posted a while ago and people probably won't notice it but at, Sal corrected himself and said that the directions are changing. How are the directions changing? Is Earth changing direction? Someone, please answer I am genuinely confused on this subject. 7:50(5 votes)
- offcourse, earth is chaging direction because earth is continously rotating.(6 votes)
- This a question, i have been holding for a while. In a Traffic collision, between a car and a truck, which is a way bigger than the car, both will feel the impact in the same way?(4 votes)
- Both will experience the same force. But that force will result in much more acceleration for the car than the truck, because the car's mass is much less.(3 votes)
- Does Newton's 1st law also apply to "reading a book in a car?" Like when the car is moving and you're staring at a book that is "still" and you all of sudden get motion sickness?(1 vote)
- If an object is moving with constant velocity, with no unbalanced force applied, i.e. 5 m/s, can I assume that after 2 seconds , or 5 seconds, or 10 seconds, when I measure its velocity, it would still come out to be 5 m/s?(3 votes)
- If there is no net force (same as no unbalanced force) on an object it will have a constant velocity and which means that for as long as there is not net force it will measure the same velocity.(4 votes)
- when is the force of friction maximum at rest or at motion.(3 votes)
- Max friction occurs at rest. When motion begins it turns from static to kinectic which is always less. At max Friction Force = the coefficient of friction x FN(4 votes)
- Im in 6th and just doing this to kill time(4 votes)
- when the ice melts and starts to slide on the water, what is the unbalanced causing it to move?(3 votes)
- The ice will move because as it melts the frictional force will get smaller. This will cause an unbalanced force and allow the ice to move. You can calculate stuff like this by learning to draw Free Body Diagrams (FBD's) that show all the forces acting on the object (ice). The frictional force in this case would be the product of μ static which is the static friction coefficient and the Normal force which is the force pushing against the surface of the ice.(3 votes)
- I think first law should be as " A body continues its state of rest of motion unless acted by external net force t̲h̲a̲t̲ u̲n̲b̲a̲l̲a̲n̲c̲e̲s̲ t̲h̲e̲ f̲o̲r̲c̲e̲ o̲n̲ t̲h̲e̲ b̲o̲d̲y̲"(3 votes)
Video transcript
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.