all right welcome back to part three of Newton's first law of motion we just talked about Newton's first law of motion otherwise known as the law of inertia inertia in fact that objects tend to resist a change in their motion that means that they're not moving they want to stay not moving and if they are moving they don't want to stop okay then we'll keep moving same crease same direction everything the same until some forces cause them to change alright and so some ideas about inertia for example if you're in a car going 60 miles an hour you slam on the brake the car begins to slow down but your body is not attached to the car directly and so it is still going at 60 miles an hour and it continues to move forward and will continue to move forward right through the front windshield unless you have airbags and a good seat belt all right and that's because of your inertial it wants to keep going if you're running and you trip your feet stop because they get the rocking exerts a force on your feet but their top half your body keeps going because there's been no force exerted on it and therefore you fall flat on your face okay the same thing can happen in the other direction also if you are in a soft car and also an you floor it and accelerate you feel as if you are thrown back but what really happens is your body is still the car begins to move but your body hasn't started to move with it yet and so the car basically slams into you and carries it with you but you feel as if you're thrown back in the car but really what happens is your body stay still and the car is moving and plans into you same thing if you're in the back car top of the hill in a rollercoaster okay as you start to throw and when you drop off for a moment there at the top your body is staying still at the top you're almost still at the top but the roller-coaster is going really fast and look at you like it goes back but really the rollercoasters land into this are flowing in your body is still not starting to move as much it and so inertia is true for objects are not moving and they start moving okay and progress or not that are moving we don't want to slow down okay another example for example here is another example for example that's that alright is this if you were just take this board and put it directly on someone's head and start nailing it would exert probably a fair amount of force on your head why because the inertia of the board is not very much and it would begin to move down on your head and exert a force but if you put four or five books between the board in your head now start hammering on it the force from the hammer has to move the mass and overcome the inertia of those four books first before it can exert a force on your head and therefore you're very unlikely to feel something I don't necessarily recommend you do this at home definitely not unless it's someone you really for us but any maybe I could do that new trust builder instead of falling back into people's arms people let them nail a board on top of your head I don't know see that catching on but maybe so alright so those are some things about inertial we'll talk more about some demonstrations of examples of that when we get to class now we talked a little bit I think at the end gravity as far as the main force we talked about and gravity and all sources are measured in Newton capital N and Newton's were named in honour of Sir Isaac Newton and came up with the theory of gravity and explained the formula for it and so forth and so in honor of him all forces are measured in Newtons now question is what is a Newton okay most of us never had a Newton unless of the signature so what is the new equal C well a Newton is in the metric system and it would be the equivalent of what we call pounds okay and so one Newton is equal to or 0.5 that Nicollet 4.45 it will just all that up and talk four and a half pounds okay so someone asked how much you weigh the next time like that I weigh 200 pounds I could tell them well I weigh nine hundred Newtons and I would be a hundred percent correctly I would just be doing it in the metric system versus the English system but they won't really know what to make of that probably but that's oh good but that's base for this so if you want to convert your weight to Newtons I mean Newtons are basically we're saying that Newton's in are fairly small force okay they're about a fifth of a pound or 0.2 pounds per Newton so there are fairly small force okay let's keep that in mind we will give probably an example the foam Apple that weighs about one you just to give you an idea what that's that's kind of what it is so we'll be talking about when talking about forces we'll be talking about new just keep in mind if you divided it by four or five that would give you a rough estimate about how many pounds of course that we're talking about right now the last thing when you talk about a new formula here his first law is basically this it says when there is a net force exerted on an object what is it a net force or sometimes we say an unbalanced force okay well and net force is the sum of all the forces acting on an object okay if I have a rock so let's just do this let's just say I have a nun book sitting on a table okay book is sitting on a table now if the book is moving then we would know there must be some kind of a net force because it's changing its motion okay it's either slowing down speeding up whatever but if it's just sitting there at the same speed of nothing then and we know that it is obeying Newton's first law which means there is no net force acting with the fact that there is no net force means two things one either there is no force acting on it or the forces that act on it are equal to zero so it's a net or the difference between them is zero okay as it turns out there are forces acting on the book all right gravity acts on all objects all right and we know that and so gravity will be represented by the arrow pushing down on a book it's holding it but the book is not going down or through the table so and we'll talk more about this on Newton's next law our third law there must be another force pushing back against it okay and so otherwise it would be moving or changing its feet downward okay and so there must be another force pushing up so it turns out that all objects that are sitting on a surface as well as the gravity pushing them against the surface the surface pushes back up against them with a certain force okay and it's going to be the same force that gravity pushes down on with and this is called either the normal force okay and it's called normal because normal and mathematical means we would argue that math is not normal but at any rate in math normal means perpendicular to because you notice if gravity is pushing straight down then this is perpendicular to the surface and pushing in the opposite direction perpendicular surface and is pushing and the opposite direction drag that is the normal force now the book and some place may also call this the support force but you may see both of these words and so you need to be aware that they are the same thing all right so normal force or the support force knot and needs to be the same now if I add more books on here what would happen to force of gravity well it would increase okay because it acts on the mass the objects the more mass the more it would the support force would also increase now if you put enough books up here the force of gravity would be more than the cable could push up against and eventually it would break the table or collapse it okay but as long as it an object is sitting on surface and is not collapsing then that means the surface is pushing back with an equal force to the force of gravity okay and so the net force on this book would be zero these two forces have to cancel it out they are going in opposite directions so they would subtract from each other let's just say that this book weighs 5 Newtons that would be its weight or the force of gravity pushing it down and so that means the support force but it also has to be 5 Newtons but since it's in the opposite direction we could put a little negative sign to show this in opposite direction and so how you could add together these and get zero or you could not use the negative sign to say since they're in opposite directions we would subtract them there'd be five months five the net force would be zero okay when the net force is zero we say that the object is in equilibrium okay because we have equal forces acting on it from both directions I may not make it all the way to the word equilibrium but any of that means they have equal forces acting up in both directions and its movement is remain the same either don't look for start moving right equilibrium just means when there are no net forces sometimes it's called a state of steadiness since usually it involves motion we call this mechanical equilibrium okay so it's an equilibrium of motion but not moving at all of course is a state of steadiness you're staying the same going at a constant speed or constant velocity is also at a state of steadiness okay it's staying the same and when that is true one says the net force is equal to zero or we use the summation symbol say the sum of all the forces equal zero and basically that tells us that if the net force is equal to zero the motion of the object is not changing notice it doesn't mean that there is no motion it just means the motion the object is not changing not another example very similar type thing if we have a bag of sugar spring scale 90 gravity is exerting a force of 9 Newton's downwards but the scale is also earning a force up of 9news and usually first reading the rope in a tension force there's a tension force of 9 pulling upwards on the sugar and there's the gravity for counterweight of the sugar going down for 9 Newton so an object can be in equilibrium if it's motionless like the book and the sugar that's hanging there here it can also be in equilibrium if it is moving say for example if this airplane the engine is providing thrust and pushing it forward it's also moving through the air it will maintain a constant velocity and therefore that's an equilibrium even though it's moving okay now if something is not moving like a sugar or the book and it's just sitting there we call that static equilibrium a static means it's not moving on the other hand if it's moving at a constant velocity that's still at equilibrium but it's different because it's moving so it's called dynamic equilibrium so either one of these would be considered equilibrium or a net force of zero now one quick question yes an object is acted on by zero forces can it be in equilibrium and the correct answer is yes if there are no forces acting on an object then it would be an equilibrium if you have more than one source for example we've demonstrated here with two forces could it be an equilibrium yes it could be okay so that they cancel each other out if I have a ball and it's being pulled in three different directions by three kid is it possible for those three questions to be an equilibrium in the ball not move yes it is okay as long as they are the right angles and at the right amount of force if they cancel each other out you can still have equilibrium or four forces or five forces are basically any number of forces however the only way you cannot have equilibrium is with one force if you have just one force there's nothing to balance it therefore let's force a gaddafi and there's a lot of times people just work and say well gravity's pushing it down and yet it's just sitting there it's just one force acting on no that means there must be another force acting on it which is why we point out there is the support force or the normal force is pushing up on the book as well and that way they cancel each other out that's one of the main things that Newton says that forces have to cancel each other out all right that's basically it for our content we're gonna look at a couple problems in one last video and then we will be finished with the notes for reading first law