Types Of Forces & Free Body Diagrams - Online Tutor, Practice Problems & Exam Prep

Types of Forces

Hey, everyone. So in the last couple of videos, we were introduced to forces and Newton's laws. Now, many problems in this chapter are going to involve multiple kinds or types of forces that are going to be pushing or pulling on your objects. So what I want to do in this video is I want to give you a brief overview of the 5 most common types of forces that you'll see throughout this chapter. Any for every single one of your problems is going to involve some combination of these 5. So we're going to introduce them really quickly, and then we'll go into more detail later on. So let's get started here. The first one is called the applied force, and we've actually seen this already before. This just happens anytime someone or something directly pushes or pulls on your objects. The letter that we'll use for this is going to be f_a. And the simplest example is if you put your hand on a box and you push it. Right? So how do we draw this force? You might be tempted to draw it like this because that's where your hand is coming from, but what I want you to remember is that we're always going to draw forces as a pull arrow, not a push that is acting away from the object's center. So we're not going to draw our forces like this. We're going to go to the object center and they're always going to be sort of as if they were pulling forces away from the center. Alright? So this just gets a symbol f_a, and what you need to know is that it's always in the direction of the push or pull. You're pushing the box to the right, your force is to the right. Let's move on to the second one here, which is called tension. Tension happens whenever you have some kind of a rope or a string or a cable or something like that that's being pulled. The letter that we'll use for this is capital T. So here's what's going on. You take your hand and you put it on a rope and instead of pulling the box directly, you're going to pull in the rope which is attached to the box. So imagine you start pulling on this rope with an applied force of 5 newtons. What's happening here is your 5 newtons sort of gets transferred throughout the rope like this, and it's kind of acting as if you were pulling on the box directly. It's just a way to sort of transfer your 5 newtons and then it just becomes a different force. But remember, we're always going to draw them from the object center. So basically, this is how you draw your arrow. And this is T and it would also equal 5 Newtons. Alright. So you pull with 5 and the tension becomes 5. It's always in the direction of the pull. So if I'm pulling on the rope to the right, then the arrow goes to the right from the object's center. Let's move on to the 3rd force which is called the weight force. And this is really just the force of gravity, presumably by the Earth or whatever planet you happen to be standing on or near. The symbol or the letter that we use is going to be a W, and what you need to know is that we're always going to assume that there is a weight force. Unless the problem explicitly tells you that there is none or that you're out in space or you're way far away from any planets or something like that. So here's how this works. This weight force is always going to act towards the Earth's center. So in your problems, you can usually assume that it's going to be straight downwards of the object's earth centers like this. So in this case here, when you have this object that is sort of near the Earth's surface, you're going to draw an arrow that goes straight down towards the earth's center. Alright, so what about this object over here that's on the surface? It actually doesn't matter whether it's on the surface or whether it's inside of the Earth or floating out near space. Basically, it's always going to have a weight force, but in this situation, it's not going to be acting straight down. It's actually going to be going in this direction. That is your weight force. And finally, for this object here, it's inside of the Earth, but it also still has a weight force that acts in this direction over here. Alright. So that's the weight force. Let's move on to the 4th and 5th one, which are kind of similar related to each other, which are the normal and friction force. The normal force is just a reaction to whenever you have one surface that's pushing on another one. So basically, whenever you have 2 surfaces that are in contact. The letter that we'll use for the normal force is big N. And so here's what's going on. Right? So if you have a box that's resting on the ground, you have 2 surfaces that are in contact. They're touching each other. What's going on here is that this box has a weight force that's acting presumably straight down. Right. We can assume that the Earth's center is straight down, and the reason it doesn't go crashing through the ground is because there is a reaction surface to that surface push that's sort of helping support that box. So that's what the normal force is. It's that reaction force. What you need to know is that this reaction force is always perpendicular, meaning it's always 90 degrees to the surface to the surfaces that are touching. So for example, the surface here is really just the ground. So notice how the ground here and this normal force make a 90-degree angle. Alright. Now you can have other directions. The normal force isn't always going to be acting straight up. It could be going at an angle like this or even sideways. So for example, imagine that you're pushing a box against a vertical wall. In this case, we have a weight force that's acting straight down, right, because it's a vertical wall, but you also have an applied force from your hand that's pushing the box into the wall. The reason it doesn't go crashing through the wall or break through the wall is because there is a reaction to that surface push and it points outwards like this against your hand. That's the normal force. In this case, notice that the surface is the vertical wall like this and this also makes a 90-degree angle with the normal force. Alright. So it's always going to be 90 degrees. Now last but not least, we're going to talk about the friction force, which is really whenever you have a rubbing of 2 rough surfaces that are in contact with each other. So whenever you have something like this where the two surfaces are sliding against each other, the symbol that we use for this is going to be a little capital curly F like that. That's the symbol I'll use. What you need to know is that usually it's going to be opposite to the direction of motion. So imagine a box that's sliding across the ground like this with some velocity. There's actually a couple of forces here at play. There's a weight force that's acting straight down. The reason it doesn't go crashing through the ground is because there's the normal force that's acting straight up, but now there's an extra force because these two surfaces are rough. That's the keyword that you need to know. That rough is usually going to be told, you're going to be given that information in your problem whether the surfaces are smooth or rough or something like that. And now what happens is that there is a friction force and it's going to want to oppose the direction of motion. We'll talk about that more a little bit later on. So basically, you know, if you've ever slid a box across the table or something like that, it eventually comes to a stop and that's because of friction. Anyways, folks, that's it for this one.

A tire swing hangs from a tree branch by a rope. Identify all the forces acting on the tire.

• A. Weight and Normal Force
• B. Weight and applied force
• C. Weight and tension
• D. Weight, tension, and normal force

You push a couch across a carpeted (rough) living room floor. Identify all the forces acting on the couch.

• A. Weight, normal, friction
• B. Weight, applied, friction
• C. Weight, applied, normal, tension
• D. Weight, applied, normal, friction

Free-Body Diagrams

Hey, everyone. So now that we've covered Newton's laws and the main types of forces that we'll see in this chapter, in this video, I want to cover how to draw what's called a free body diagram. Free body diagrams are really important because they're simple diagrams that help you organize all the forces that are acting on objects in a problem. And more importantly, some problems will ask you explicitly to draw one of these diagrams as a first step. So even if your problem doesn't ask you to do it, you should do it anyway because it will always help you solve the rest of the problem. So let's go ahead and check it out here. A free body diagram, which is sometimes written as FBD, shows only the forces that are acting on a single object, and that single object is usually drawn as a dot or a box. Right? So some professors will just do dots. Some of them will do boxes. If they have a preference, you should stick to it. If not, you should just pick one and choose. I'm just gonna go ahead and draw mine as a dot here. Alright? So we're gonna draw basically this diagram of, you know, a hand with a rope pulling the box. We're gonna turn that into a very clean diagram showing only the forces acting on an object. And we're gonna do it by drawing all of the forces as arrows from the object's center, which we always do, but what I've done here is I've given you a particular order in which you consider all of the forces here. You won't see this in your textbook, but this is just an order that I think is the easiest so you don't lose track or forget any. Alright? So let's just go ahead and get started here with this example. I've got a rope that's sort of pulling this box across a rough surface like this. So the first force we're gonna consider is the weight force. And remember the weight force is always acting on any object whether it's resting or in the air unless otherwise stated. Right? So we're gonna draw this arrow here and we're gonna draw it basically straight down. Right? Presumably that's towards the Earth's center. So that's my weight force. Alright? So let's see. So the next pair we're gonna consider are applied forces and tensions. Remember applied forces happen anytime you have something that's directly pushing or pulling the object. Do we have that? Well, you might think that this hand here means that there is an applied force, but actually there isn't because, remember, this hand here isn't applying a force on the rope. But remember that this applied force sort of gets transferred through the rope, and really what happens is that there is a tension that's caused on the box. So really there is a tension force, but there is not an applied force here. Be very careful when you consider these kinds of forces because you don't want to double count them. Alright? There is an applied force, but that's sort of getting transferred through the rope and it's appearing as a tension. That's what's directly pulling on the box. Okay? So no applied force, but there is a tension. And so the next we're gonna consider is a normal force. This happens anytime you have two surfaces in contact. Do we have that? Well, yes, we do because the box is sort of resting on the ground like this, so there is a normal force and it's gonna be perpendicular to that surface. So in this case, it's gonna point straight up because we have sort of a 90-degree angle like this between the surface. The last thing we're gonna do is consider any friction, right? We have a normal force. So when this happens, when you have the two surfaces that are in contact that are rough. So for the sake of argument here, we're just gonna assume that this surface here is rough. So you're trying to pull this box over here, but there's gonna be some friction. Now what happens is the box wants to slide over here in this direction. It's not gonna go up like that in that angle here. So what happens here is that the friction force is usually gonna oppose that direction of motion here. So in this case, what happens is you do have a friction force, except it's gonna point to the left and try to slow the box down. Alright? So notice how this diagram gets really messy. And what happens is, in more complicated problems, if you have lots of errors, you could lose track of them. And that's the whole point of a free body diagram. We're gonna take this and we're gonna turn it into a very clean diagram showing only the forces. So we have the dot, we've got the weight force, we've got our tension like this, we've got our normal force that points up, and then we've got our friction force that points off to the left. This is the free body diagram here. Alright? So, again, really important because it shows only the forces, but that's always how we're gonna draw this. Alright? That's all there is to it. So now what we're gonna do is we're gonna take our free body diagrams and what we know about forces. And for these example problems, we're gonna calculate the acceleration of these following situations. But what we're gonna do first is we're gonna draw the free body diagram. Alright? So let's get to it. So in this first example, we're gonna push a physics textbook to the right with some force. You're gonna know this with also going to be some kinetic friction. Alright? So again, let's just stick to the order of the forces. So what I'm gonna do first is I'm gonna draw a free body diagram. So again, I'm just gonna stick to a dot like this. First one is weight. So there's gonna be a weight force and it's gonna act straight down presumably towards the Earth's center. We don't know. We can just always assume that it's down. Okay? And, let's see. So we've got a "", weight"": "", force"": "". Is there any applied force or tension? Is there anything directly pushing or pulling my object? Well, yes, there is because you're pushing your hand, right? You're putting your hand in a box and pushing to the right. So there's gonna be an applied force that acts over here to the right. We actually know what that applied force is. It's 20. Okay? Now is there a normal force? We have two surfaces in contact. Well, yes, we do because the box is resting along the table. So therefore the normal force just like it did up in the previous diagram is gonna point up. That's gonna be my normal force. And last but not least is there friction? Again, there's gonna be friction because we have explicitly that's told to us that there's a force of kinetic friction. Now what happens here is that the box wants to go this way because I'm pushing it to the right. The friction force wants to oppose that motion, so just like it did above the friction force is gonna point to the left. Alright? Now that's gonna be my friction. What's really important here is that because we know the magnitudes of the forces, we draw the arrows so...

Drawing a FBD

Hey, guys. I got a practice problem for you. We're going to go ahead and draw this free body diagram for the following situation. You're taking a block and pushing it up against a vertical wall. So I've got the wall like this. We know it's rough. We're going to take this block and push it up against the wall. And we know that the force we're pushing with is at a 45-degree angle like this. We also know that the block is going to be sliding upwards, which means the velocity is going to go up like this. So, we have to draw a free body diagram. Remember, this isn't just a free body diagram. And instead, we're going to have to 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, and we know that it acts at 45 degrees. You don't necessarily have to draw the angle in the diagram. Alright? So that's our force. Now we look for tensions. Tensions happen to be 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 surfaces that are in contact. So, 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 it in our free body diagram. So that's our normal. And then finally, when you have frictions, frictions happen whenever you have two 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 had 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 questions.