Inertia & Newton's First Law of Motion - [1-5-4]

0:01 hello welcome back the title of this lesson is called Newton's first law of motion now of course you all heard of Sir Isaac Newton he proposed and put forth his famous Three Laws of Motion in the 1600s there every bit is applicable today as they were then and they really form the Cornerstone of essential core physics that you have to learn now the good news is they're very easy to understand but the bad news is everybody has their own idea of how they work usually it's not quite right until you take a class in physics and really try to break them down so in this lesson we'll talk about Newton's first law of motion later on Newton's Second Law later on Newton's third law just keep in mind that they're all kind of intertwined they're all related to how things move and so

0:47 sometimes it'll seem like we're restating some things that we talked about before uh and so there is a little bit of overlap I guess is what I'm trying to say but we need to cover them separately so that when we're done you will see how they all fit together in a bow and describe how things move all right so I'm going to read Newton's law of motion these are my words they're probably very similar to whatever you have in whatever textbook you're using or whatever on the internet you could look up about Newton's first law of motion I'm not going to write it down because usually you never refer back to the words anyway I'm going to read it to you we're going to say it several times and by the end of the lesson you're going to understand in your bones what it actually means which is much more important than memorizing these words okay here we go Newton's first law of motion very simple to understand

1:32 objects at rest will stay at rest unless they're acted upon by an outside force that's sentence number one objects that are in motion will continue to stay in motion in a straight line unless acted upon by an outside force okay two sentences now I could write them down but it'd be a bunch of words and it doesn't matter let's just say it one more time but we're going to talk about it understand it if an object is at rest it's going to stay at rest unless it's acted upon by an outside force and an object in motion that's already moving will tend to stay in motion in a straight line unless it is also acted upon by an outside force so there's a lot of talking in this lesson not very much math and we're going to you know

2:18 draw some pictures in a second but ultimately it's a lot of talking and that's okay sometimes the deepest fundamental things that we have to understand just comes about by talking that's how we get the point across all right so let's break this down an object at rest what does that mean right so I can have a crate here and I can say it's at rest and it's when I say it's at rest this is the ground okay so we just walk up in the forest and you find a crate or some box or something an apple sitting on the ground it's not moving you stand there and you look at it all right what Newton's law is saying is that that object when it is initially at rest it's going to stay at rest unless some Force comes along and pushes it that's very simple to understand okay but sometimes the simplest things are the most core

3:03 fundamental things that we have to have in our brain so what would cause that apple or that crate or whatever it is in the forest to actually move well the wind could come and blow it that would be a force that would be moving it from its initial place at rest all right it could be an earthquake shaking the ground that would be a force that would cause it to move I could go over and I could kick it that's a force you know you get the idea a squirrel could come along and start eating it and rolling it around those are forces so if you literally never exerted a force on that object that was at rest it would literally stay at rest forever it would never move right so an object at rest tends to stay at rest that just means it won't move unless acted upon by an outside force

3:49 right that's sentence number one now I could go into deep space and I could have some kind of Cannonball or something like that and I could shoot this thing you know out of this Cannonball right like this and this thing is traveling with some velocity V it's going to the right now if we go in deep space why am I choosing deep space because there's no forces there's no wind there's no earthquakes there's no birds there's no people and there's no gravity either before indeed space I'm talking about outside of our galaxy okay you happen to float your spaceship up and you notice that there's this object traveling through deep space it's traveling with some constant velocity and it's going in a straight line that object will continue to go in a straight line forever unless acted upon

4:35 by an outside force you could literally Cruise right next to this thing and just watch it traveling with it for thousands and thousands and thousands and millions and billions of years and it will never move unless it's acted upon by an outside force what kind of force could act on that thing well I could attach I could I could bump it with my ship right if I bumped it with my ship that would impart a force and it would uh it would uh it would not mean traveling in a straight line anymore right so an object promotion will tend to stay in motion unless acted upon by an outside force I could shine a very high power laser on it that would uh we're getting into momentum here but that would impart a little Force which could push it and change its trajectory but the idea is that objects are going to travel in a straight line unless acted upon by an outside force now in our solar system

5:21 when we shoot a space probe of course the trajectory is always curved but that's because the Gravity from the Sun is pulling on it exerting a force but when we're away from any forces things always want to travel in a straight line and here's the critical thing when an object is traveling in that straight line think about that cannonball in deep space it's just cruising along and I'm cruising right next to it looking at it and there is no force acting on that thing but it is still traveling in a straight line so in other words forces are not required to maintain the state of motion there is no Force required to get that ball to continue moving once the object is moving no forces are needed or necessary the object just keeps moving forever in our universe things just keep

6:08 moving and if they're already still they're still moving it's just that they're moving with zero velocity so it's really the same law right when you think about it an object of rest tends to stay at rest and object in motion tends to stay in motion unless acted upon by an outside force so you could kind of boil the whole thing down just to say that objects always continue moving in a straight line unless acted upon by an outside force it's just in the case of an object at rest its velocity is zero so of course it's going to maintain a zero velocity forever and if it's velocity over here is 15 meters per second it's going to maintain a 15 meters per second velocity forever no forces required to maintain the motion you see because in the early days before Newton people thought that forces were needed to to maintain the motion to keep

6:55 the motion going that's what people thought Newton came along and said no no no once the thing is moving there are no forces necessary to keep it moving forces just tend to change the state of motion all right so that's why I'm going to write my first words on the board here I'm going to put it in all capitals forces do not cause motion and then under this I will say forces cause objects objects motion to change triangle means change

7:41 so I'll put a little exclamation point right here this is a critical distinction all right this is actually the main uh the main impetus the main domain the main thrust uh no pun intended of Newton's first law of motion prior to Newton people thought that objects that were moving uh like on the earth here that forces were required to maintain their motion think about pulling a a box on the ground on the grass or on the carpet I have to pull that box as soon as I stop pulling the Box stops and then I want to move it again I'm going to pull it again but as soon as I stop pulling it stops so that condition people to believe that when things move you must be constantly maintaining some Force but we now know and I'll draw a picture in a minute that the reason it stops is because there's a

8:28 force acting on the box called friction that is why the Box stopped if there's no friction then the Box actually would keep moving on forever think about the case of a of an icy lake a frozen lake you grab a a a a puck or something and you slide it just with one little push you let go you're not touching it anymore of course it goes on and on with very low resistance to motion very low friction so it goes on and on and on and if you reduce the friction all the way to zero it goes forever and ever in a straight line right but in everyday uh situations when we uh pull things or push things or whatever they generally stop because there's almost always friction acting in the real life but friction is a force so the object tends to want to continue moving once we get it started unless it's acted upon by an

9:15 outside force so forces do not cause motion that just means that once the motion is going we don't need a force to continue to motion that's what that really means forces cause objects motion to change to change right if the objects hit rest a force will change its state of motion from rest it'll start it moving right if an object's already moving and I apply a force to it it's going to speed it up or slow it down or change its direction either way I'm changing its state of motion so forces cause object's motion to change but what is a change in motion right what is a change in motion what this is another way this sentence right here is another way of saying forces cause acceleration

10:06 right acceleration that's what they do because acceleration is a change in motion that is the definition of what a change in motion is doing what is acceleration it's uh the change in the velocity divided by the change in the time how many meters per second I go up or down over how many seconds I'm speeding up or I'm slowing down and also we're going to learn that this change in velocity it doesn't necessarily need to be speeding up or slowing down a change of velocity also counts if you change directions if you just if you're going this way and I and now I'm moving this way I have now changed my velocity also so forces cause acceleration that's what they do they don't cause motion to happen that or they don't cause motion to be they're not required to maintain motion forces

10:51 simply when they act on an object change the state of motion of the object they cause an acceleration and that's kind of dovetailing into Newton's second law where we write down the that force and acceleration are related to each other but right now in words forces cause acceleration they cause the object's motion to change all right so let's go back to the picture that people you know used to talk about when we did the thought experiments about this and what we'll do is we'll put a crate on the ground like this on just on some grass or rocks or carpet or whatever and you pull this way with some Force like this well if I pull it this way and let go it stops if I pull it this way again let go it stops it appears in

11:38 everyday situations then I need to continue pulling on the thing to get it to move that's why people thought forces are required to maintain the motion but now we know that what's actually happening here is the rubbing that's happening here we call friction and so there's always a frictional force that's basically running counter if I'm pulling this way the frictional force is trying to oppose the motion so you see forces do not cause motion even though it appears to happen here what's really happening is I'm pulling but you see Force objects of motion tend to stay in motion unless acted upon by an outside force as soon as I stop pulling the outside force of friction stops the block that's why it stops and that fooled everybody for long long long time until Newton codified it into his first

12:24 law of motion and said no no no when objects are moving they continue moving with no forces on their own forever in a straight line there must be a force to change that state of motion in this case the force that stops the motion is the frictional force but if there was no frictional force the the then you would literally once you terminated this pulling Force the thing would move forever and ever just like it slides across the lake uh frozen lake or in deep space all right so now we need to transition to talking about the second part of this which is kind of basically related to what we've already said which is called the word inertia so I'm going to write that word down in dark show all right again I'm not going to write a definition of inertia because you're never going to look it up I'm going to

13:09 tell you what inertia is inertia is the tendency or the propensity for an object to remain at rest or to stay in motion basically inertia is the resistance to something changing its motion let's take two situations first of all let's go back into deep space let's do a thought experiment we're in deep space we're nowhere near any Galaxy or anything where literally in the middle of nowhere and there's two objects in front of you the first object is a golf ball little bitty ball about this big right next to it is another object that is a car like an automobile you pull it from Earth it's floating they're both floating in space which one has more inertia well the car has more inertia inertia is the measurement of the resistance to motion

13:56 the resistance to change its motion if I go up to that ball and push on it with some uh known force of let's say one newton It's Gonna Fly Away and you're gonna go in a straight line forever and ever even after I'm not pushing it anymore Newton's first law but then I go to that car and I push it with one newton it's gonna start moving but it's going to resist my motion just think about being in a swimming pool when you're floating and you have you know an adult try to push on them when they're floating it's hard but when you push a child it's easy to move them around right this has nothing to do with gravity there's no gravity anywhere we're in the middle of nowhere in between the galaxies thousands and millions of light years away but that golf ball when I push more than one newton force is going to start moving rapidly when I push on that automobile the car it's going to resist

14:42 my motion but it will start moving and eventually it will accelerate away and then of course it'll cost forever and ever at some known speed away from me so those two things have different amounts of inertia and what we're going to learn and what we're learning right now is that uh basically Mass gives rise to what we call inertia if an object has more kilograms of mass it has more inertia that means it's going to be harder to get it to move right if it has fewer kilograms of mass it's going to be easier to move this has nothing to do with gravity we are nowhere near gravitational field the intrinsic amount of matter inside of something tends to cause the object to resist its state of motion if it's already at rest it's going to be harder

15:28 to get it to move if it has a lot of inertia if it's already moving at a high rate of speed it's going to be harder to change its motion to get it to speed up or slow down if it has a high inertia high mass right and so what is mass anyway it boils down to protons in the nucleus of the atom neutrons they both have a relatively high mass electrons are very small Mass so electrons don't really contribute much to the mass of the object but a little tiny bit protons and neutrons in the nucleus of the atom that is most of the mass of an object so if you have something made of lead those lead atoms have more protons than neutrons so it's a more massive object than something made of carbon let's say less protons and neutrons and so it's going to have less Mass

16:14 all right so when we when we say something as high inertia we're just saying that it's harder to get it to start moving to change its state of motion or if it's already moving it's harder to deviate its motion to change its direction or to change its velocity all right the next thing we need to talk about is what we call the net Vector force it sounds uh complicated but I promise it's really not situations here is my crate resting on the ground it's at rest which means the velocity is equal to zero it's not moving at all okay and then let's take another situation right next to it again we have a maybe a second crate separate from the first and it's sitting right here as well all right uh initially initially at rest

17:00 here it is no force is acting on the thing it's going to stay at rest forever forever and ever and ever its velocity will be zero meters per second because Newton's first law of motion says that the object at rest will tend to stay at rest unless acted upon by an outside force so we expect it to stay at rest forever now let's take that same situation over here initially at rest and let's apply a force to it now if we apply a force in this direction we'll call that F1 right then of course we expect the Box uh to start moving it's not going to be at rest anymore because we're applying a force and of course the force is going to tend to move the object if we then uh remove the force then the frictional force will slow this thing back down to

17:46 rest again okay but instead of applying one force to only one side of the block let's say it's initially at rest and I apply two different forces to this block both equal and opposite and I'll call this Force number two so I'm literally going to take this box and like squeeze it between my fingers just imagine I mean I could just take this little magnet right here this thing is at rest right now okay now if I push it of course it's going to start moving right but what if I apply a force to both sides equal and opposite okay you see if this force is bigger then of course it still moves this way if this force is bigger then it's going to move this way but if the forces are exactly the same then the thing doesn't move at all so what I'm trying to say is is if

18:32 the force number one the vector number one we call Force when we add it to the vector Force number two if that equals a zero then it stays at rest so I guess I wanted to point out because the the law of motion says an object at rest will tend to stay at rest unless acted upon by an outside force and then a lot of people say well wait a minute there's forces here Why didn't it move well it's because when we say acted upon by an outside force what we're saying is if multiple forces are acting it's the net force acting on the outside of the thing if an object has a net force different than zero then it will change its state of motion so what we could do is rewrite Newton's law uh you know we could take some creative license and we could say an object at rest will tend to stay at rest

19:18 unless acted upon by a net force a net external Force so if we have two forces the net force here is zero when you add these vectors one is in One Direction one's in the other direction they're negatives of each other it's zero so the thing stays at rest because the net force on the outside is zero so it's not so much that there's a force on the outside it's that there has to be a net force that is different than zero then of course the thing won't be at rest anymore now let's take the same sort of situation but instead of being at rest let's take this object and let's uh you know what let's see here let's do it in deep space let's not put it on the ground

20:05 let's not let's not do that let's let's do it like this and then let's say that this object is just traveling uh forward at a velocity of 10 meters per second so it's moving this way so we know from Newton's law of motion news first law this thing is going to go at 10 meters per second forever in deep space unless acted upon by some kind of outside force so it's just going to keep on moving so let's draw this thing again some snapshot in time later where it's now over here it's moved from here to here and what is the velocity it's still 10 meters per second because you know no forces have acted now let's say that I I flowed up to this and my spaceship and I'm on the

20:51 spacewalk or whatever and I apply a single Force to the outside like this single Force what's going to happen I'm going to accelerate this thing to something faster than 10 meters per second because it was traveling at a constant rate of speed now I apply a force like a booster through like a rocket booster on the back and that speeds it up okay but if so of course that's a that's an external force a net external Force that's going to change its state of motion okay um but what if I actually do the following what if I put a rocket booster on the front and a rocket booster on the back in other words I have a booster pushing it this way and then I have a booster on the front pushing it in exactly the opposite way right then the same exact thing happens if Force number one

21:37 plus Force number two is equal to zero and when I say plus I mean the vector sum of these forces if this is uh the same then it stays in motion in constant motion 10 meters per second so the way to illustrate this is if I have my little ship right here and it's you got a mat you got to use your imagination let's just say that it's moving at a constant velocity this way and I float on to it and I start pushing on both sides it's still going to maintain it's 10 meters per second because the net external force is still zero so the second part of it an objects in rest tends to stay at rest and less active defined by an outside force an

22:24 object in motion tends to stay in motion unless acted upon by outside force it's really unless acted upon by a net force on the outside it's not just that there's a force here there has to be a net force a net Vector sum in this case there's a zero Force as a vector sum so nothing changes so you have to have that's why in rocket engines are always on the rear end of the rocket because you want to cause an acceleration of the thing so the net force acting on an object is actually what matters right so this is a long long lot a good bit of talking to describe what we now consider to be very simple but you got to remember that before Newton put these laws down there was no space flight no one saw pictures of people floating in space you have all

23:10 seen pictures of people floating and throwing pencils and pens and you can easily understand how things move in space right and so it's easy for you to visualize what's happening but before we had they had this all they had was things on Earth you throw a ball uh and you stop the force of your hand and the thing comes crashing down the motion stops hits the ground the motion stops you pull something across the carpet stop pulling it stops you're running you stop exerting that force it stops so it appeared for a long time for thousands of years that forces are required to keep motion moving but Newton came along and said no no no motion is something that always happens it always things are always wanting to move we're always wanting to move in a straight line with a constant velocity if they're at rest

23:56 it's just that their velocity is zero so it always wants to stay at rest it's really the same law really with the velocity being zero if the thing is it rests it always wants to move and whatever velocity it was forces do not cause Motion in the sense that they're not required to keep the motion happening forces cause the motion to change which we have a word for forces cause acceleration and so that is what uh Newton's first law says inertia The more mass something has the harder and the more it resists your motion and so mass is basically a measurement of how how much it resists the motion so we're going to wrap these ideas up and kind of uh continue working with them when we talk about Newton's second law of motion uh in the follow-on lesson so for now

24:41 watch this a couple times make sure you understand it follow me on to the next lesson we'll continue talking about Newton's laws of motion