 Hey everybody, welcome to the Tutor Terrific, starting a new chapter in my physics course, chapter 4. It's all about forces. Today, I'm just going to give you a real intro into the topic of forces, how it plays into so many different related topics in physics. It's super important to understand what's going on here. We're going to look at the types of forces. We're going to look at Newton's first law, and Newton has three laws, but we're going to look at his first law as sort of a culmination of all we do today, so let's get going. So Dynamics is the study that we're going to do, and that's where forces lie under, which is study dynamics. That's what we're studying is forces. I want you to understand how it fits into the larger picture of all of the studies in physics. Here we go. So here's physics, and if you break it down, there's study like optics studies, there's mechanics, there's electricity, there's heat, and many more, sound, light, lots of different things. If you go under mechanics, and you break that down to its various categories, kinematics is what we've done so far in chapters 2 and 3, studying motion, just the characteristics of motion specifically, how to model that motion. Dynamics is what we're doing now, and that's studying the causes of motion, not describing it, but understanding its causes. So that's what we're going through right now. And dynamics also discusses changes in motion, not just the motion we see, but how it can be changed. Okay? So just for a definition perspective, dynamics is the study of the causes of motion, namely the causes of the acceleration that causes changes in motion. Okay. So, of course, that begs the question, what causes acceleration or the changes in motion? What do you think, guys? Think about it for a second. What do you think is the cause of acceleration? I told you the causes of changes in velocity was acceleration, but it didn't discuss what causes acceleration. Well, it's forces. Forces lead to acceleration, and I'm going to talk a lot about how that works in this video. Basically, in antiquity, you study this in junior high most likely, there are four fundamental forces in our universe. One of the main ones is gravity. Gravity can act at long distances is, excuse me, it can act at long distances, and it is quite weak. Then we have the force of electromagnetism, which can act at distances, but much shorter, and it's much, much stronger than gravity in short distances. The weak nuclear force is something that happens in the nucleus of an atom, and it deals with different types of decay, alpha decay, beta decay, as we see here, and the strong nuclear force, which is what keeps the neutrons and protons next to each other, and it's by far the strongest force at the very small distances of the size of the nucleus of an atom. We're talking picometers here. Very, very small. All right, so those are the four fundamental forces. We're going to look at the top two in detail in this course. Now, let me explain to you, through a couple videos, how things can get so screwy when it comes to people understanding forces. Veritasium, I give credit to this next video about to show you. Veritasium is a wonderful channel on YouTube, and it's an educational channel like mine. He does great work. He was in Oxford, and he discussed this, he did like a sort of interview of students on campus about forces. So let's go ahead and take a watch of this video right now. What is a force? It seems to be an idea that a lot of people are confused about. Like, Star Wars is the only source of knowledge. What is a force? Like, force is how you raise X-wings out of spots. You just concentrate. What would you say a force is? But outside of Star Wars, a force is energy and mass. So, you have a huge thing moving really fast. You have a lot of force. Something that exerts energy on the whole other matter. The movement of something in one direction and something that exerts energy on all of the students. I don't know. It's sort of this indescribable thing that kind of keeps us from flying off into space. Nobody really knows what it's about. It's just theoretical. That should teach me to interview philosophy students. Contrary to what you said, there were some people who knew what force was about. Something that makes something move from its original position. How you tell something or how you get something to move or how to shape it. Contrary to what you said, there were some people who knew attraction or direction or something that makes something move. Yeah, push, push or pull. Okay, it took some coaching, but force is essentially just a push or a pull. So, then I asked people what forces are acting on you right now. Most can identify a downward gravitational force towards the center of the earth. But then things got a little bit hazy. Can you tell me about the force in Nanyed's moment? Well, probably there's a big force down to the center of the earth, which is gravity. Well, the gravity of earth? How does it act a little bit hazy? Well, it keeps me on the earth. I don't fly away. I would be standing on the earth and it's pulling me down and making sure that I'm stuck to the earth. What other forces are on you? Are there any other forces on you? Well, there are other air pressure forces around me. There's my own muscular forces keeping me on. Well, it keeps me on the earth. And also, then, different, say, weather or its magnetic conditions. In terms of what's happening, whether it's surfing or just normal gravity. This is what's keeping me on the ground. It's like gravity. So, there's a gravitational force pulling you down? Yeah. Are there any other forces that are acting on you? Like, on me right now? On you right now. I can't think of anything else to describe it. Only one force. I mean, surely there's, like, a lot of small ones. Let me put this to you. If there's only one force on an object, what does it do? It goes towards it. But there's another force acting on you, which is pretty important. There's a force acting down, right? That's a gravitational force. What about a force acting up? I can't think of anything else to describe it. So, what forces are acting on you right now? If there's only one force on an object, what does it do? Cool. So, we'll go over those throughout the course. But he's talking about a very important upward force called the normal force. And many of you probably were like, ah, come on, students, get it. But we'll go through that. Don't worry. I want another video. The Novel Learning Center has a wonderful channel on YouTube. And they've got a really good introductory video on forces. And so, I'd like to discover some everyday forces by watching this video. A force is a pusher of pull. We can't see a force, but we can see the effects of a force. A force can move the stationary object, stop a moving object, or change its direction. A force can change the speed of an object. It can also change the shape of an object. A force is measured using the force meter. The SI unit of force is moving. There are two kinds of forces, contact force and non-contact force. When we push a pull, an object, we have applied force to the object. The object will move in the direction of the applied force. And applied force is an example of contact force. Friction is also a form of contact force. Friction goes against motion. Even though friction slows things down and makes me more difficult, there are many things we cannot do without friction. Friction holds us all around. And allowing us to walk without slipping. Friction causes a heart to break in time. So that's it. Friction is also a form of contact force. The amount of friction of force depends on the texture of surfaces in contact. When the surface is smooth, there is less friction. The force that makes things fall to the ground is called gravity. Gravity is an example of non-contact force. Gravity pulls an object towards the ground. The force of gravity also exists on the moon. But it's not as strong as it is on the surface. When the object rests on the surface, there will always be a reaction force. When we're standing on the earth, the reaction force balances the weight of our body. So we don't have to worry about the pain. The reaction is also called normal force. Magnetic force is non-contact force. Magnetic force only acts on magnetic objects. Iron, steel and metal are magnetic materials. The force of magnetic attraction pulls the magnet, magnetic object towards the magnet. The interaction of forces makes the world we live in the way it is. The cycle on a bicycle experience the interaction of different forces. Gravity pulls the weight of our bicycle and the ride towards the earth. While an equal reaction force acts upwards to balance gravity. As we ride, the sectional force between the wheels of the bicycle and the ground acts against the thought of the word force in the different parts. As a fish swims forward, wateriness acts in the opposite direction. While gravity pulls the fish towards the bottom of the water, the up thrust exerts an upward force. In level flight, the upward lift applies to the wings of an aeroplane, which is the downward weight of the plane. As a fish swims forward, wateriness acts in the opposite direction. As a fish swims forward, wateriness acts in the opposite direction. They interact with each other and us dictates how it is to live in our world and how it is to as a baby learn to walk and learn to interact with the environment. Getting back to Veritasium's main point is that a force is most fundamentally defined as a push or pull on an object. They can, but don't necessarily cause acceleration. Now I'll get into that because multiple forces can act on an object. They can cancel each other. While a force is the cause of acceleration, forces don't necessarily lead to a non-zero acceleration in every instance. One thing you've got to get used to about a lot of the quantities I introduce in this course is that they are vectors. Forces are no exception. Forces are vectors. If you look at the image, this guy is pushing up on this box. It's an applied force. It's a contact force on the box in this direction. Multiple forces can act on a single object in the video. The upward thrust. I love his little British accent. We've got weight downwards. We've got the gravity acting on the plane. We've got the lift due to the shape of the geometry of the plane and the wings. We've got the forward thrust due to the jet engines in this case. We've got drag due to the air resistance. Those are all forces acting on the plane in the air. Some more about the vector nature of forces. We have to be very careful. Forces act on a particular object, but they also have a source object. We've got to talk about forces that act on an object by another object. Let's take the same image from the last slide. This is a force on the box by the person. The box we call the recipient of the force. That's where the force is drawn. It's drawn on the recipient. The agent is the person exerting or the object exerting the force. We can add forces as well. Multiple forces can add, like vectors normally add. These two ladies are the agents of a force on this recipient here, this person. They're acting at right angles to each other. You would add those force vectors by either method. The tail to tip method was used here. The total force is at this 45 degree angle. This free body diagram is a picture of all the forces acting on a particular object. We're going to talk about that in a later lesson in this chapter. That's just a preview. Forces can cancel. If you have two force vectors that point in equal and opposite directions, such that they're anti-parallel and equal magnitude, the net force, which would be the sum of all the forces, would be zero in this case. The book would not move. Also, forces can act at an angle. We can talk about the direction of forces as some angle with respect to some other direction, this being this force at some angle, like 30, 40 degrees, with respect to the horizontal. We're going to look at that for sure. Now, let's talk about some examples of forces. Here are two examples of contact forces. This was in that video, the novel Learning Center Contact Force. It's a force that acts through the physical contact between objects. We have to have actual physical contact. Now, that is a gray area because that has to do with the electromagnetic force, repulsion between electrons. We're going to pretend like we're actually touching something to make that occur. Here are some examples of those. Then there are non-contact forces. These are forces that can act at a distance. They do not require this physical contact. Now, the students at Oxford, I believe it's Oxford, I'm not entirely sure, what forces are acting on you right now? That's the last question that Veritasium channel creator had for you. Well, everybody there was able to discuss gravity acting downwards. Then we saw in the novel Learning Center video, there is... First, let's talk about the agent, be the Earth. The Earth is exhibiting this force on me. It's exerting the force on me and I am the recipient of it. There's an electromagnetic force due to the very small distance between the electrons in the bottom of my shoe and the electrons in the carpet on the floor or the wood on the floor wherever you're standing. That creates an upward thrust, an upward force that counteracts gravity, which is why you don't move. The agent would be the carpet or the floor that you're standing on and you would be the recipient. Again, I'm just drawing all the forces acting on me. Friction is an example of a contact force that acts between two contact surfaces and these surfaces may be moving with respect to each other or not. They don't have to for friction to occur and it opposes the motion of other applied forces. If someone's pushing this purple box or indigo box to the right, the friction force will act to the left opposing its motion. If someone's pushing a lawnmower to the left, the friction force will be felt to the right and that's posing the direction of motion as this little guy says here. There are two types of friction. Static friction is the first type. It's friction between contact surfaces not moving with respect to each other. If I push on this purple box and it's not moving but I still feel some resistance, that's called static friction. Then there's kinetic friction which is the friction between contact surfaces moving with respect to each other. If I push this purple box and it would start to move, you might hear some sound. You'll still feel friction opposing you but now it's kinetic friction because the contact surface between the table or whatnot and the box, they're moving with respect to each other. Tension is another type of contact force. It's exerted between a long top ropes, so ropes that are taught, not filled with slack or strings. It's a very interesting force because tension at one end of the top rope is different than tension at the other end because they're equal in opposite directions. The force to feel the magnitude is the same but the direction is the opposite. It acts along the direction of the rope as you see here. Pulleys are very important when it comes to tension. We're going to be looking at a few examples of situations that involve pulleys in this chapter. A pulley is a wheel that spins freely. This rusty one probably doesn't but let's pretend it does. It has a track that can be filled with a rope like this. What do pulleys allow us to do? Why are they used in the industries? They allow us to change the direction of the tension force. That's the point. Now the tension force on this ball here points upwards whereas the tension for me points at this angle. They're not equal in opposite anymore. In this case, they're pointing the exact same direction. You can get them to point the exact same direction with the pulley. A single pulley like this allows a change in direction of the force. We've set up multiple pulleys to actually alleviate the magnitude of the force as well but we're not going to discuss that in this course. That's more of an engineering course. Now we're going to look at the laws of motion. We're just going to get started but these laws will be our foundation pieces for all of our mathematical analysis in this chapter. We're going to start with someone named Aristotle in the height of the Greek Empire. There were many great philosophers. Aristotle was one of them. Aristotle thought, he theorized, that a force was required to get an object to maintain a certain speed and that no forces acted on objects that were not moving. A force was required for an object to be moving at a certain speed. The greater the force, the greater the speed. This is the understanding for a long time. Then this man, Galileo, Galilei comes on the scene in the Renaissance. He stated, among other things that we've already talked about in the last chapter, that no, this is not the case. Forces are not required to maintain objects at a certain speed. The greater the speed, that is incorrect. Forces are required to change the speed. If no net force is acting on an object, it will maintain its current speed forever. That's in stark contrast to Aristotle's statement. It turned out that Galileo was correct. His understanding led to this man's first law, Isaac Newton. Now we're in the Baroque period. We're past the Renaissance. He's super famous for his contributions to the mathematics of calculus and to physics equally. He stated that what Galileo said was true, and he reframed it slightly. What I'm going to do is I'm going to prepare you for that first law with a definition, inertia. Inertia is a property of all objects with mass. It's the tendency for an object to remain at a state of uniform motion or rest. Objects like to maintain their current state, whether it's moving with respect to you or not moving with respect to you. The more mass an object has, the more inertia it has. Inertia is a way to measure an object's resistance to acceleration, which would be changes in its motion. This little pebble here, for example, will have some inertia. It will resist my hand maybe picking it up very slightly, compared to, for example, Jupiter. Jupiter has a lot more inertia than that pebble, and so it's going to resist changes to its current motion much stronger than that little pebble, and that has to do with directly the fact that Jupiter has more mass than the pebble. And now, let me, I'm prepared now with all of that to state Newton's first law, which really reframed Galileo's understanding and rebuttal to Socrates, excuse me, Aristotle. Every object remains in a state of constant motion or rest unless a net force acts upon it. And this is also called the law of inertia, because the inertia is that thing that causes the object to want to maintain its current state of motion. We work against inertia when a net force is placed or acted upon an object, or exerted on an object. So we're going to look more at this and his other two laws coming up in the next few lessons. Thanks for watching this one, guys. This is it for lesson one. Look forward to the next video soon. This is Falconator, signing out.