hello grade 11s and grade 12s and welcome back to my channel I'm Miss Martins in today's video we're going to be looking at Newton's first law of motion we'll be looking at the definition important theory that you need to know and we'll be doing a basic calculation but watch the whole video because I will be giving teacher tips throughout the video and at the end of the video remember I set metric papers I mark up at a metric level so I know what they're looking for and what they test in the exams let's Jump Right In firstly in order to tackle a Newton's first FLW question you need to make sure that the question is a Newton's first law question now behind me I've summarized my best teacher tips to help you recognize if it's a first law question or a second law question let's focus on the first law so if the question says the object is moving at a constant or uniform velocity or speed if they say the object is stationary or still or not moving if they say the object is an equilibrium then you know you're dealing with a Newton's first door question and you can start off your question by saying fnet equals ma but because acceleration is zero fnet is equal to zero so F net is equal to zero is how we basically start off our Newton's first law calculations if you've missed the video where I go through how to use your free body diagram to help you set up Force equations you may want to check that out because that's going to be very helpful for both Newton's first law and second law I'll link that in the description box below but it's very important to first make sure you are doing the correct type of question but hold up before we get to a calculation what is Newton's first law of motion so you can see the definition at the top of the screen behind me so it says Newton's first law of motion a body will remain in its state of rest or motion at a constant velocity unless a nonzero resultant or net force acts on it so you can see in the definition that they mentioned both a states of rest so staying stationary or still or motion at a constant velocity so constant speed constant velocity uniform velocity so they say if those are the cases then we are dealing with Newton's first law of motion and a body will stay in the state of motion or state of rest unless a nonzero resultant or net force accident now what does this mean if you take a look at these pictures behind me the first picture says Newton's first law states that an object will remain at rest so you can see that that cone over there in this picture the block is not moving so the that block will stay still until when will that block stay still and you could say Well ma'am someone could come and push it over and then it'll move and you are right it will move in that scenario because a nonzero net force or resultant force is acting on it a pushing force which is a net force a nonzero net force and in the same way we can see this picture over here so it says an object moving at a constant velocity will continue moving at that same velocity or speed so if the V uh the basketball is in motion it will stay in motion and technically if the surface is frictionless it'll stay in motion forever so if I kick a ball on a truly frictionless surface it'll carry on like that forever according to Newton's first law in this sequence of diagrams it doesn't continue forever why because look at the ball it hits the cone and what happens is a nonzero net force is acting on the ball which is causing the ball either to stop slow down change direction in other words causing the ball to accelerate okay so that is Newton's first law an object or a body will remain in a state of motion or rest at at constant velocity if it's motion unless a nonzero resultant or net force accident Now teach a tip I set metric papers I mark them so I know that if you don't say non zero they take away a Mark if you don't don't say resultant or Nets you can choose they take away a mark you need to say these things in order for you to get your full marks for your definition so here's a repetition of the definition for you this definition comes from your exam guidelines so this is the correct definition you will see alternative definitions on the internet in weird textbooks weird videos on the internet but this is the correct definition according to your exam guidelines okay cool fnet equals ma a remember we technically should start all Newton's law questions like this Newton's first and second law but remember I did tell you that fnet equals z if we're dealing with the Newton's first law question and the reason why fnet equals z ultimately is because acceleration is zero CU remember the object is either not moving which means acceleration is zero or it's moving at a constant velocity which means acceleration is zero so the body is in equilibrium there's zero net force acting on it now now before I go into a calculation there is something very important that you need to know according to exam guidelines you do need to know safety considerations in terms of Newton's first law of motion so you could be like ma'am what are you talking about you need to understand why it is important for us to wear seat belts when traveling in a car now think about this you're sitting in a car you do not have your seat belt on okay you're sitting in a car the car is going at 120 km per hour that's very fast very very fast you because you're in the car you are traveling at the same constant velocity 120 km an hour now what happens if the car breaks what happens so think about it if the car breaks okay it's negative acceleration the car is slowing down it's changing its velocity however the passengers so in this case me I'm pretending to be a passenger in a car because I have something called inertia because I have mass that means that I do want I want to resist any change in motion so because I have mass I have inertia which is just a property of a body it resists change in motion so because I'm traveling at 120 km/ hour I want to continue traveling at 120 km/ hour this is also according to Newton's first law Newton's first law says I'm at motion at 120 km an hour constant velocity my body wants to continue traveling at the same constant speed so when the car breaks I will continue to move at the same velocity that the car was moving at before the brakes were applied and I will fall forward I will fly forward it's not a good thing for me what does wearing a seat belt help with well the seat belt provides the nonzero resultant force and it's a backwards Force so think about if you've ever been in a car and you've been wearing a seat belt and the car suddenly com comes to a stop that seat belt pulls you back quite sharply against your seat so the seat belt provides a resultant a nonzero resultant or net force and it basically ensures that my velocity changes in the same way of the car so when the car slows down I slow down so you need to be able to use Newton's first law apply Newton's first law to explain these various scenarios so for example if a pencil lies on a desk board or a dashboard of a car that is in motion will the pencil remain stationary when the car breaks unless there's something holding on to the pencil the the pencil won't remain stationary it will want to continue at the same constant velocity according to Newton's first law so it'll stay in that state of motion at that same velocity so when the car's velocity decreases the pencil's velocity won't it'll continue at the same velocity and it'll fall off the dashboard or roll off the dashboard and you can use that same argument for all of these scenarios over here okay now that we know the theory let's jump into a Newton's first law calculation so this is an example of a Newton's first law question we've got a body so a box or whatever or a person whatever of mass 100 kg let say it's a box rests on a horizontal surface so they don't give me a picture I'm just going to draw you a picture here's my box it's 100 kg it's racing on the surface and they give me the coefficient of kinetic friction so that is this little symbol over here is 0.2 my Mass is 100 kg and look what the question want calculate the force required to slide the body at a constant velocity now the reason we know it's a Newton's first law question is because of the wording constant velocity that means that acceleration is zero and therefore remember fnet is equal to ma a but if a is zero fnet is zero and we know to Newton's first law question now I find questions like this very very difficult to do especially as I get more complicated unless I have a free body diagram they didn't ask me to draw one but I'm going to draw one because it helps me so much with my question so a box resting on a flat horizontal surface it sits on a flat horizontal surface we know we're going to have a normal force acting straight up that's FN because there's a surface involved we know we have FG acting straight down they say they want me to calculate the force required to slide the body at a constant velocity so obviously we're going to have an applied force acting in One Direction so let's pretend it's going to the right that's F applied that's the force required to slide the box I'm going to highlight that just so we can see that is what I'm actually looking for calculate the force required to slide the body I'm looking for f applied cool now they do say that there's a coefficient of kinetic friction which means that there's obviously friction present and if the Box moves to the right friction goes to the left left and is parallel to the surface so I'm going to say FK for kinetic friction now you have to always assume that there's friction present unless they say frictionless that's an important teacher tip because some people see that there's an a menion of friction in the question so even if they didn't say the coefficient of kinetic friction so even if they didn't tell me that there was a coefficient of kinetic friction we have to assume that there's kinetic friction unless they say that the surface frictionless remember if you don't understand any of these individual forces I have videos on all the individual forces also how to draw free body diagram so that's linked down below but drawing it is very important because it helps me understand and answer the question I think at least so again we know it's a first law question so we know fnet is equal to zero so I'm going to write fnet is equal to Z they want us to calculate the force required to slide the body or to move the body to the right at a constant velocity so I'm looking for f applied now in my one video where I mentioned the equations for Newton's laws and I said you must watch this in order to understand Newton's Laws I said that if we're looking for if applied or if we're looking for any force in the horizontal Direction so the parallel direction or the X Direction so in this case I'm looking for f applied which is in that direction I need to consider all the forces in that direction which would be both of these forces F applied plus FK both of those forces I need to consider and you might say why ma'am because remember the box is sliding in the horizontal Direction so the two forces that affect that motion would be F applied and FK that's why I need to consider both of them I need to add them together when I add them together it must give me zero why because the box is sliding so the sum of these forces adding them together must give me zero y zero because it's a Newton first law question acceleration is Zer so fnet is zero and my students are ask me ma'am must I put a plus in between because if applied and friction are going in opposite directions you always start off your equation with vector addition which means you always start off with putting a plus sign in between when we substitute in values that's when we'll put in a minus if necessary and in this case it is necessary because I'm going to choose to the right as my positive direction so I'm looking for f applied F applied will be going to the right that's my positive direction so when I substitute in a value for friction I will substitute that in as a negative but first I have to find friction how do you find friction friction kinetic friction is equal to the coefficients of kinetic friction multiplied by the normal force now yes they do give me the coefficient of kinetic friction they give me that that is 0a 2 but do they give me the normal force no they don't however I can find the normal force look at the normal force here the normal force is in the same direction as FG there are no other forces acting perpendicularly or up or down so what that means is if I take those two forces FN and FG and I add those two together it must also give me zero which actually technically means that FN and FG have the same magnitude think about it like this the box is moving in this direction not not this direction so FN and FG those are the only two forces acting in the perpendicular Direction the Box does not move in the perpendicular Direction so it moves in the horizontal Direction and it moves at a constant velocity which means in the horizontal direction fnet is zero because it's moving at a constant velocity but it's not moving at all in the perpendicular Direction think about it the box is going like this it's not moving in the up down Direction which which means that F net in the up down direction is also equal to zero so technically the correct way to write that in order to find the normal force is as follows we technically should do the following I'm going to use the two blue forces FN plus FG must give me zero now let's say up is positive FN FG is going down do you see how it's pointing downwards so I'm going to say minus how do you work out FG in the previous videos we discussed that it's Mass so 100 time gravitational acceleration 9.8 that must equal zero so take it over FN equals then it becomes a positive 100 * 9.8 and it would will be 980 Newtons upwards and I know what you're thinking I know what you're thinking grade 11s and grade 12s ma'am that equation is so unnecessary because the box is on a flat surface so that means that FG which is 100 * 9.8 is the same as the normal force so why can't I just say 100 * 9.8 get the answer and that's my normal force you can that is true but remember this is a simple example you may get an example where FN and FG are not the only two forces acting up or down and then you have to do it like this so I want you to practice doing it like this so that when the more difficult examples come you know what to do okay because if there's another vertical force acting like if applied perpendicular or something you can't say if n is equal to FG because it's not okay so we found the normal force which is 980 Newtons up and remember why did I want to find the normal force well because I was looking for FK remember FK frictional force is equal to the coefficient of kinetic friction times the normal force so it is 0a 2 given in the question time the normal force 980 why do I want friction because I need friction in order to work out the applied force so friction is 196 Newtons to the left so I'm going to say minus 196 here in my formula again why is it minus because I chose to the right as positive and friction is going to the left that means that my applied force required to keep moving the box at a constant velocity is 196 new new to the right or in the positive direction so knowing how to do these Newton's Laws questions properly is very very important remember in your exam we do allocate marks for things such as do you start off with vector addition and I know in some memos they don't but when we Mark at a final metric level we do sometimes look for that you need to start with vector addition you need to choose your positive direction then substitute your your answer in you need to be able to use a free body diagram to work out these little equations that you need in order to do Newton's Laws so I hope that this example was helpful I will be doing a lot more if I have not already so this is a living and breathing playlist which means that I am adding to it all the time so I have past paper videos I have videos going over examples so remember to click the link in the description box below I also have this that I'm busy working on if it's not out already my website is linked in the description below as well for more information on that so hopefully all of these things will help improve your physical sciences marks and I can't wait to see you guys in another video where we tackle either Newton's Second Law or more difficult Newton's first law questions whichever video you end up watching first I love all of you thank you for your support and I'll see you in more videos bye everyone