6. Intro to Forces (Dynamics)
Types Of Forces & Free Body Diagrams
Types of Forces
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Hey guys. So in the last couple videos we were introduced to forces and Newton's laws. But a lot of times in your problems in this chapter, you're gonna have multiple kinds or types of forces that are gonna be acting on an object. So what I want to do in this video, I want to give you a quick introduction to the five most common types of forces that you're gonna see in all of your problems as chapter All of your problems are gonna involve some combination of these five forces. And then later radios were gonna cover them a little bit more deeply. So let's go ahead and check it out. The first one is called an Applied Force and it's one that we've already seen before. The symbol for the applied force is F. A. And this happens anytime you have someone or something that is directly pushing or pulling on this object. For example, I take my hand and I put it on the box and I want to push it. Now, the way that you're going to draw this arrow, we know that forces are arrows. You might be tempted to draw it like this with a force like that. But what I want you to do is remember that we're always gonna draw forces as a pull arrow that acts from the objects center. So we're gonna pick up this arrow and instead of drawing it like this, we're gonna draw it from the objects center like that. So that's gonna be our applied force. And there's always going to be an acting in the direction of your push or your pull. So I'm pushing to the right, so the arrow goes to the right, let's move on to the second one, which is called tension. Now the symbol we use for tension is is called T. And this happens any time you have a rope or a string or a cable that's being pulled. And the way this works is that instead of you pulling your hand directly on the box, you're gonna be pulling on a rope that's attached to the box. So, for example, if I'm pulling on this rope like this and the applied force by F. A. Is equal to five, then what's happening is my five newtons is actually getting transferred through the rope and acting as if we were actually acting on the box itself. So if I'm pulling on the rope with five newtons, that five newtons gets transferred through the rope. And it's basically as if I were actually pulling the box itself with a force of five newtons, that's the tension. And the tension is always going to be in the direction of the pull. And again, it always happens whenever there's some kind of rope or a string that's attached to an object, let's move on to the third one, which is called the weight force. The symbol we use for the weight force is a capital W. And really, this is just the gravitational pull of the gravitational force from the Earth. You're always gonna assuming your problems that there is a weight force unless you explicitly told that there is none. And the direction of the weight force is always going to act towards the objects towards the Earth's center or whatever planet that you're on. So for example, I have this box here that's floating way above the surface of the earth. And what we're gonna do is we're always going to draw an arrow that acts towards the Earth's center, which is right over here. Now, if you had a box that was on the surface, you just draw another arrow that acts towards the object center and it doesn't matter whether the object is on the surface inside the earth or above the earth. There's always going to be a weight force and act towards the objects. The acts towards the earth center. So all of these forces here are the weight force. Now let's move on to our 4th and 5th force, which is called the normal. So the normal force is our fourth force. And this is really just a reaction to some kind of a surface push. It happens whenever you have two objects or two surfaces that are touching each other or in contact. So the normal force, the symbol we're gonna use for this is called N. And really what's going on here is you have these two surfaces that are in contact. And this box is being pushed against the surface by the weight force. Remember we said that the box is gonna have some weight force. You're always gonna assume that there is one. So this box is being pushed against this tabletop or whatever the surface it's lying on. And so the reaction to that surface pushes that the surface pushes back on this object. And that's called the normal force. Now the direction of the normal force is always gonna be perpendicular, perpendicular means 90° to the surface. So for example, our surface here is the horizontal line and our normal force makes a 90° angle with that are normal is always gonna be perpendicular. You can also have normal forces that are horizontal, not only vertical. For example, in this other example here we've got this box. I know that I have a weight force like this that acts straight down. But with this force here my hand means that there is an applied force. So I have an applied force that pushes to the right against this surface. Here we have these two surfaces that are in contact with this is my F. A. And the reaction to that surface push is that I have a normal force that pushes back against me to the left. Notice how now the vertical line is kind of like the surface and I have this normal force that is perpendicular 90 degrees to that surface. Alright. And last but not least we're gonna cover the friction force. The friction force is given by the simple little F. And whenever it happens, whenever you have two surfaces that are in contact, but those two surfaces are rough. So every time you have to surfaces that are rubbing against each other, that's when you're gonna have friction. So, for example, here we've got these two surfaces and I have them in contact. Let's say that the surfaces are rough. Now the force of friction is usually going to be opposite of your direction of motion. So, for example, if I'm moving this box to the right, then that means that there is going to be a friction force of resisting force that's going to try to bring this box to a stop and it's going to act to the left. That's how the friction force works. We're gonna cover that a little bit more deeply in a later video. That's it for this one. Guys, let's go ahead and take a look at some examples
A tire swing hangs from a tree branch by a rope. Identify all the forces acting on the tire.
Weight and Normal Force
Weight and applied force
Weight and tension
Weight, tension, and normal force
You push a couch across a carpeted (rough) living room floor. Identify all the forces acting on the couch.
Weight, normal, friction
Weight, applied, friction
Weight, applied, normal, tension
Weight, applied, normal, friction
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Hey, guys. So now that we've covered Newton's laws and the most common types of forces that we're gonna see in this chapter in this video I want to show you how to draw a free body diagram. Free body diagrams are really useful because they help us organize what's happening inside of a problem. And more importantly, a lot of problems are going to ask you specifically to draw this diagram as a first step. So even if a problem doesn't ask you to draw when you're still gonna do it anyways, because going to help you solve the rest of the problem, So let's get started. What is a free body diagram anyways? Well, the symbol or the abbreviation is F B D, and it's really just a diagram that shows the forces that are acting on a single object, and we're gonna draw that single object as either a dot or a box. Some professors will do dots and we'll do boxes. Just pick one double check with your professor and then stick to it. So we're gonna draw all the forces that are acting on this object, and we're gonna draw all those forces as arrows that are coming from the objects center. Now remember, there's a lot of different types of forces. What I've done here is I've given you an order. So we should go down this order here so that you're not forgetting any of those forces. So we've got this complicated diagram here and we're gonna We're gonna go ahead and navigate through all of those forces and come up with a free body diagram. So let's get started. The first one is the weight force. Remember that the weight forces just the gravitational force from the Earth. It's always acting unless it's otherwise stated. So what happens is this box here has a weight force and that's going to act towards the Earth Center. Now, if you don't know where the earth is, you can usually just assume that it's straight down. So this is going to be the weight force like this. So the second one we're gonna look for is any applied force or tension. Remember, you have an applied force. Whenever you have someone or something that is directly pushing or pulling that object, you're gonna have attention. Anytime you have a rope or string or something like that. So what happens here is we have a hand that is exerting a force on this rope here. Now you might think that's an applied force here. But what happens is this Hand is actually doing is exerting an applied force on the rope. But remember that that applied forces traveling through the rope so that the rope is actually the thing that's pulling the box. So, really, what happens here is there's not an applied force. There's just a tension force because the apply force is actually traveling through the rope, so there's no applied force in this problem. But there is attention, all right, so let's go to the third one. A normal force, remember, a normal force is going to be perpendicular or 90 degrees, and this happens whenever you have two surfaces that are touching each other or in contact. So in this case, we've got this box that is touching the table top or the floor or something like that. So those two surfaces are in contact. And remember that the weight force is pulling this object and it's pulling this box down to the ground, and the normal force is going to be a response, a reaction to that surface push. And it's going to go straight up like this because that makes a 90 degree angle with the surface. So that is our normal force. Now, the fourth one we're gonna look at is the friction. So if those two surfaces are now rough, so if you have these things rubbing against each other, that's when you're gonna have some friction. So what happens is this tension right? This hand is pulling this object up into the right. So what happens if this box actually wants to slide this way? Right? It's not actually gonna go up like that, but it is going to go off to the right like this. So you're gonna have some friction for us. It's going to be opposite to the direction that it wants to move for that motion. And so it's going to move to the left like this, so it's gonna be our friction force. So notice how this diagram is kind of messy. So one thing we can do is we can actually just say we're going to just draw this object as a dot like this, with all the forces that are acting on the object. So we've got the weight force like that. Then we've got the tension, and then we've got our normal force. And then finally, we've got our friction force like this. So this is a free body diagrams, just a dot with all the forces in the directions labeled like that. So that's what we're gonna do here. All right, That's really all there is to it, guys. So let's go ahead and take a look at these examples. We're gonna draw a free body diagrams for both of these situations because we're gonna have to calculate the acceleration for the following scenarios. So let's go ahead and get to problem A which is we're gonna push our 2 kg physics textbook to the right with the force of 20 Newtons. And then we also have some kinetic friction. So remember, the first thing we're gonna do is draw a free body diagram, and we're just gonna draw it as a dot or a box. I'm just gonna go ahead and draw this as a dots. Now, we're just gonna go ahead and stick to the order, remember that there is a weight force, so we're gonna label that weight force like this that's gonna be weights. And we also have a A force to the rights. So we're gonna look for anything. Applied forces or tensions were told that you are pushing the textbook to the rights with a force of 20 Newtons, so we know that that's going to be an applied force. This is my f A. And we know this is equal to 20 Newtons and then we also have the force of kinetic friction. Now there isn't any tension or anything like that because we're not told that there's a rope or string or a cable. So there's no tension. But there is a normal force because if you have this physics textbooks that is on a flat table, then you're going to have a normal force. That is a response to the surface push in this case, the weight force like that. So we know that's the normal. And then finally, we have a kinetic friction. We know that that is going to be eight Newtons Now the box is gonna want to be pushed to the right, so friction is going to be opposite to the direction of motion. It's going to go to the left like this. So we know this friction force is gonna be eight Newtons. So this is our free body diagram. So now that we have a free body diagram, we're going to have to calculate the acceleration for both of these objects. So really, what we're looking for is looking for a Now remember that we have to calculate the acceleration by using f equals m A. So we're gonna start off with the sum of all forces equals m A. And now we just have to pick a direction for positive and then use f equals m A. So I'm just gonna choose the right to be the positive direction. So what I've got here is I've got a 20 Newton force that's positive. And then I've got an eight Newton force that is to the left. So that's actually gonna be a negative eight Newtons. And this is going to be equal to the mass, which is two times the acceleration. So what I've got here is I've got 12 equals two a. And so therefore what I've got is that the acceleration is equal to 6 m per second squared, so this is gonna be our acceleration. All right, so let's go ahead and move on to the second one. We've got a rope. We're gonna pull box upwards with the force of 90 Newton's the boxes, we use 50 and the mask is 5 kg. So we're gonna start off with the free body diagram. We know that That's just gonna be a dot like this. The first thing we're gonna look at is the weight force. So when we have this weight force that's acting straight down, we know this weight force is equal to 50 Newtons, so this is gonna be 50 downwards like this. Now we're going to look for any applied force or tensions, so we don't have an applied force. But we are what we are told. They're using a rope. We are pulling a box upwards with the force of 90 Newton's. So now that means that we have a forced upwards like this. This is going to be my tension because we're using a rope and it's going to be 90. Newton's like this now. We're not told that this that this box is on some kind of a surface like a table or anything like that. So there isn't no there's a normal force because there's no two surfaces that are in contact with each other. And so because there's no two surfaces in contact, there's also no friction. So really, these are just the only two forces that are acting on this object. So now that we have a free body diagram, we're going to calculate the acceleration, just using the exact same steps that we did before. So to calculate the acceleration we're gonna use f equals m A. So we're gonna pick the direction of positive, which is going to say upwards as the positive direction, and so are F equals. Emma says that 90 Newton's upwards plus the 50 Newtons downwards, which is gonna be negative, is equal to the mass, which is five times the acceleration. So we've got 40 equals five a. And so once we divide, we're just gonna get that. Um the acceleration is you go to 8 m per second squared and that is going to be 8 m per second squared upwards. Because this this force is the stronger one. That's it. That's it for this one. Guys, let me know if you have any questions
Drawing a FBD
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Hey, guys, gonna practice problem for you. We're gonna go ahead and draw this free body diagram for the following situation. You're taking a block and you're pushing it up against a vertical wall. So I've got the wall like this. We know it's rough. We're gonna take this block, and we're gonna push it up against the wall. And we know the force that we're pushing on it with with this force here makes a 45 degree angle like this. We also know that the book is gonna be sliding upwards, which means the velocity is gonna go up like this. So we have to draw a free body diagram. Remember, this isn't a free body diagram. And instead, we're gonna have to go ahead and look and look for all of the forces here in this particular order to draw our free body diagram. We draw this as a dot or a box like this and we start off with the weight force. Remember that the weight force always acts downwards, right? Unless you're otherwise told towards the Earth's center. So that means that your weight force is going to be down like this. Next we look for any applied forces. This happens whenever you have direct pushes or pulls, and we know we have one here. This is our push at 45 degrees. So we know that this force here is this f we know that acts at degrees. You don't necessarily have to draw the the angle and the diagram. All right, so that's our force. Now we look for tensions, tensions, captains with ropes or strings. And there are no ropes or strings in this problem. So there's no tension, and then now normal. Normal happens when you already have two services that are in contact. So this this block here is being pushed up against this wall. Those two surfaces are touching each other, so there's a normal force that acts perpendicular to the surface. So if the wall is like this, then your normal force is going to be pointing out like this. It doesn't always point upwards, so that's your normal force. So that means we draw in our free body diagrams. So that's our normal. And then finally, when you have frictions, frictions happens. Whatever you have to rough surfaces in contact. So here we have a rough vertical wall. And remember, friction always acts opposing motion. So in our diagram, here we have The velocity of our block was upwards, Which means our friction force is going to oppose that upward velocity. So we know we have a friction force that points downwards like this. That's our free body diagram. Guys. Let me know if you have any question.
Additional resources for Types Of Forces & Free Body Diagrams
PRACTICE PROBLEMS AND ACTIVITIES (3)
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