 Hey everybody, welcome to Tutor Terrific. Today I'm going to look at forces again. This is our second video in the unit 4 my physics course series and we are going to look at Newton's second and third laws today. In the last video just a reminder Newton's first law discusses inertia and how a mass, well it's called the law of inertia It discusses the background on inertia saying that the object will maintain its current state of motion Unless or rest unless a net force acts upon it. So we're building off of that today and looking at a second and third laws So to prepare for his second law of motion. I really need to define mass Mass is really the measure of the inertia of an object in physics So how much an object has resisted to acceleration is a measure of its mass So it's you've got to understand a lot of people mix this up They think mass and weight are the same thing and they're not Weight is different than mass because weight depends on the gravity of nearby objects and Mass does not so things weigh different on the moon than they do on the earth But they have the same mass just look at the lunar landing videos. You'll see what I mean So the mass of this football Is decent? I mean it's more than zero, but pretty easily with the Contact force between my foot and the football get it to accelerate quite rapidly Try applying a force to Jupiter and getting it to move Good luck. It's got way more mass than the football And so it's going to be much harder to cause any noticeable acceleration to it So that's mass as directly prepares you for Newton's second law Okay, Newton's second law has to do with this idea of mass and acceleration And the force that would cause an acceleration in particular So let's look at this paraphrase of what he said the acceleration of an object is directly Proportional to the force on the object and inversely proportional to its mass Okay, again direct proportion between acceleration and net force and an inverse proportion between Acceleration and mass exists in the universe. This is Newton's most famous law and it's commonly written like this f equals m a Okay, f equals ma if you get a alone you will see that a equals f over m So direct proportion exists between a and f and an inverse proportion exists between a and m now often Sometimes we don't like to say f net external meaning external net forces Internal forces are completely ignored here We sometimes use this it said some of f now some of that makes a lot of sense because we have multiple forces acting on an object These are all vectors and they all add to give one net vector. So f net now Force okay, I'm defining force for the first time a Force is measured in Newtons. Okay, this is the quantitative measure of a force and We use the unit Newton after Isaac Newton a very fitting unit name for sure Now let's talk about this relationship. So say we we have a certain force the amount of the force is constant Let's say it's exerted by a golfer On his putting his putter He exerts it first on a golf ball. Well the golf ball is a very small mass So it's gonna during the forces execution will have a very large acceleration. I know it's not very long It's very instant in time almost But that is a lot because the acceleration of the golf ball is quite large during that time to go from zero to Maybe one or two meters per second now say he applies that same force with his putter to The truck okay, this truck has a huge mass And so the acceleration must be extremely small in order for this product to be equal to the original product F okay ma ma ma. They are inversely proportional. Okay All right now. Let's practice using this law with a simple calculation. We're gonna do an estimation here We're gonna estimate the nest force needed to accelerate something that weighs a thousand kilograms a car at one half G or Well, also we're going to see the acceleration The net force need to accelerate a 200 gram apple at the same rate. So it's a basic size apple So party we've got the 1,000 kilogram car We've got the acceleration as well. So we have the M and the a but one half G. What does that mean? Well G is 9.8 meters per second squared as we defined in previous chapters. So we have one half of that Let's go ahead and find what that is one half of 9.8 meters per second squared It's approximately since rest are eating five meters per second squared. Remember an estimating we use one sick thing as a good ballpark Okay, so now if we take that acceleration in the proper units and multiply by thousand we'll get 5,000 per force Newtons, okay, that's a lot of newtons. Okay As you would properly expect the cars engine is capable of doing that you my friend pushing behind the car or not All right part B. We're gonna apply the same force Excuse me. We're gonna apply the same acceleration To a much smaller mass 200 grams. Okay 200 grams These are the wrong units for mass. We need to convert that to kilograms Just divide by thousand to go from grams to kilograms when our point two zero zero kilograms Now we can multiply that by the five meters per second squared same acceleration as before and you get one Newton this is five thousand times less Than the car for the same acceleration So the force and mass are directly proportional. This is what this is showing you larger mass larger force smaller mass smaller force If we keep the acceleration the same Another problem a little bit more tricky So let's say we need to find the average net force required to bring a 1500 kilogram car even heavier To rest from a speed of 100 kilometers an hour with the distance of 55 meters So in 55 meters, we're going from 100 kilometers an hour to zero We're gonna find the force required to do that well in order to find the force required to do that we're gonna need to find the acceleration and I don't just have the acceleration in my given information. So I'm gonna have to use kinematics Okay, and during that time. I've got this hundred kilometers an hour velocity. That is not SI units I need to convert that first So I'm gonna show you that here 100 kilometers per hour Convert the kilometers to meters by multiplying by thousand over one cancel the kilometer units Then the hours I'm gonna multiply by one over 3,600 to get rid of hours and Convert to seconds because one hour is 3,600 seconds and When I do that a hundred times a thousand divided by thirty six hundred I get twenty eight So that's the initial velocity of the car the final velocity is zero and I have the change in position the displacement 55 meters which of the kinematic equations do I have to use? The time independent one, okay The time independent one will be required here. So That is the road map for this particular problem. We have to use the time independent equation now Solving for the acceleration would require subtracting v not squared and then dividing by two times x minus x not then a is alone The final velocity is zero and the initial velocity is 28 meters per second So we'll go zero squared minus 28 squared over two times the difference in displaced difference in position So 55 meters. That's final minus initial 55 meters and this comes out to negative 71 7.1 meters per second square. What why is it negative? Why did I not just look at the magnitude? Well, it's because velocity and acceleration point in opposite directions and by default I chose my velocity vector to be in the positive direction Acceleration as we saw in a previous lesson in chapter 3 if it points backwards Excuse me chapter 2 if it points backwards from velocity, then that means it's negative if we choose the direction of velocity as positive So that's why I'm keeping the negative here. So it means a negative force were result when I use F sum of f equals ma So I've got the mass and now I have the acceleration which is negative when I multiply those two together I get negative 1.1 times 10 to the 4 Newtons remember forces unit Newtons whenever you've got kilometer excuse me kilograms times meters per second squared that is Newtons, okay for force It's a negative force since it also points backwards from the velocity Okay, if you haven't already seen that the direction of the net force Is the same as the direction of the movement, okay, what I mean by movement is acceleration So the direction of the net force will be equal to the direction of the acceleration that the object undergoes Okay, a little bit more particular information Look at this box. Okay. It's on a table. There's lots of forces on it. There's a gravity force downward There's this upward force counteracting gravity that I've kind of talked about through the veritasium video and normal force Friction which I haven't talked about much at all except to first define for you That points backwards and I'm pushing so fp forwards with this red vector is you can tell the net force Will be to the right the normal force and the gravitational force cancel when something's resting on a surface So those two cross out the friction forces Substantial but it's way less than that push force or the force applied So the net force points to the right. So the acceleration will also Point to the right. This is how it works that force direction equals the acceleration vectors direction Okay, so that's newton's second law now. Let's look at newton's third law I want to prep you with asking you if you've ever heard the following statement before For every action there was an equal and opposite reaction Yeah, many of you have well, guess what that was coined by a very important person to us Isaac Newton Okay, same Newton is the one I've been talking about with his laws of motion But that's not the formal definition of his third law. Although it's quite related His third law reads like this as one object exerts a force on a second object The second object will exert an equal but opposite force on the first object Okay, wait, that's all mouthful. So we've got two objects one object will exert a force on the other If that happens then the other will exert the same force in the opposite direction on the first Interesting so it is like a reaction and an equal but opposite reaction is occurring with this just gets a little more into it Notice how the forces emanate from different objects and act on different objects I'll try and visualize that for you first. I want to show you this The mathematical equivalent of this statement with the double subscripts. So we've got a force by object one on Object two if you look down here, this is how the subscripts are read f a b means force exerted by a On b so the agent is listed first and the recipient is listed second So this is a force by object one on object two will be equal to and opposite the force by object two on Object one so these forces vector wise would point in opposite directions and have the same length Okay, here's an example This person is pushing on this brick wall. Okay These are a Newton's third law pair of forces, okay, they are the forces that are Guaranteed by his law if one happens the other will happen So this first one I can label based on my previous slides nomenclature We are going to say that this person that's 100 Newton force to the right will be this force of the person So this is by the person on the wall. So person first Wall second p first w second in the name. What about this force that the wall exerts on the person? Yes, the person feels that at their hands Well, that would be the force by the wall on the person. So f w p So this is an example of a Newton's third law pair You can always tell it's a Newton's third law pair when the subscripts the double subscripts are switched as far as their order Okay, here's another one. We've got a force They hammer exerts on the nail which is this right pointing vector And there's the force that the nail exerts on the hammer They might not have thought that existed because the the nail seems to Lose the battle, but he still exerts that force you feel it every time you contact your hammer with the nail So it makes it hard to push the nail in I'm going to talk about why Something moves like this nail in another slide, but let's name these forces So this force here the second force I named that's the force by the nail on The hammer, okay, and this is the force by the hammer on the nail so f n h f h n That's a newton's third law pair, too Can you name the third law pair forces in this example? So this balloon has been blown up It's put on this rack, and it's allowed to slide along the rack and then the knot at the end is cut So the air pressure inside which is high Escapes and that ends up propelling the balloon forward. So what are the two forces? Well, there's a force by the balloon on the air now. That's often Misunderstood, but that's true. The balloon has to be pushing on The air net outwards push and the the pressurized air inside the balloon Well, it's gonna push the balloon forward. So this is the force of the air on the balloon Okay, now if you might think you might be thinking well, okay These forces are equal and opposite so they should cancel right if we added forces to that Together because they're vectors and they're anti-parallels. So they point opposite directions and they have equal magnitude They should cancel right well Yes, if they acted on the same object So you might be thinking how can I get anything to move? Well, first you have to understand that you have to understand that the Newton's third law pair forces Always act on different objects not the same object. So they do not cancel when you look at particular objects Okay, so this man pushes on the fridge the force for it. So this would be f p F so force of by person on fridge and the fridge will push back on the dude So it'll be the force By the fridge on the person so you think well those will cancel nothing moves Well, not really because you're supposed to look at one object at a time to determine that force not all the forces in a situation Okay, you've got to remember that so Let me explain how you can get this stupid fridge to move because I'm sure you've moved a fridge or caused a fridge to slide in your life So we need to look at all the forces acting on the man and the fridge In order to understand this. Okay, first you're gonna draw a man and a fridge and then you're gonna draw all Not just the ones you care about but all the forces acting on the man then all the forces acting on the fridge Don't worry. We'll get a lot more practice with this There are actually four forces acting on the man. Okay stick figure man here downward. We've got gravity pointing down Upwards we do have that force that counter-extra gravity that normal force Now if he's pushing on the fridge then the fridge pushbacks on him so f By fridge on man exists to the left Now, why isn't the man just fly? Why doesn't the man fly backwards then well He's got shoes or he doesn't and he's sliding But let's pretend he has some nice shoes on the ground with contact and they keep him Steady at one spot. Well, that's the type of friction force and that has to point to the right So the man's not really going anywhere. Okay, because all these forces in pairs cancel each other out, but the fridge Maybe not so So first of all the fridge has a gravitational force downward and a normal force upwards and those two cancel the fridge also Exerts an equal but opposite force on the man So the man exerts an equal but opposite force on the fridge this force that points to the right f Mf stands for the force by the man on the fridge. It's the equal and opposite force do That's on the by the fridge. Excuse me on the man back here that points to the left So those are Newton's third law pair forces But the fridge fights back with friction which we talked about earlier due to the uneven Paved surface or the uneven surface between contacts. So the fridge will have experience of friction force but the Man may overcome that. Let me explain. So Let's look at the forces that cancel try and simplify the situation So on the man we talked about the normal force and gravity cancelling. That's fine when the fridge We talked about those same forces cancelling and now We're going to identify the third law pair forces That would be ffm and fmf Man fridge fridge man. So those are the third law pair forces. Then we have friction. Okay We're going to talk a little bit more about friction But friction cannot just be any value friction actually has limitations on what values it can be The force by the man on the fridge can actually overcome Friction, okay, it can be greater than friction thus causing an imbalance that we need to look for forwards to the right and so the fridge can actually start accelerating to the right if the man with Fmf can overcome through exerting that force friction. Alright, so the net force on the fridge would be to the right because The force by the man on the fridge is greater than the friction. It's actually called static friction. We'll talk about that a little later Between the fridge legs the little pegs that the fridge has in the ground So he's overcome that cause the net force forward thus the fridge will move forward. Alright guys That's enough of those diagrams will definitely look at more of those next time They have a special name and a huge significance when it comes to more math in this unit But for now, thanks for watching so much guys. This is Falconator signing out