 Hello, everyone. My name is Brad Langdell, and I want to talk to you today a little bit about Forces and free body diagrams and how to make them and what they are and what they look like. So here. Let's start So it's a free body diagram a free body diagram is just a diagram that shows all the forces acting on an object So what do you need to know about forces? They're vectors That means they have magnitude means they have direction and you can break them apart into x and y components Just like we did back in the first unit You can also move your vectors around after you've drawn them out because sometimes that helps you to solve a problem Just because you draw them in one spot doesn't mean you have to leave them there the whole time Let's use some examples So here I've got a free body diagram example. It's just a box on a line It's a box is that rest on a horizontal surface now that word at rest means a lot It means that it's not speeding up or slowing down So there's still forces acting on it though, which is sort of weird You'd think that if there's forces on an object and that's moving. That's not always the case So let's put some forces in the first force. I start off here with the force of gravity Pointing down towards the center of the earth nice and easy now if that was the only force acting on the box It would have to accelerate down. That's Newton's second law. So there must be a balancing force And this is called the normal force so f n stands for normal force That force is called the normal force Because of the fact that it is at a right angle to the surface here I can even put it down against the surface if you want So you get a normal force when you're on a nice Surface like a tabletop or the ground or something like that And that is the force which counter acts gravity keeps the object from falling down So there we go There's only two forces on that box and because in total those forces add up to nothing We could say that the net force acting here is zero So our net force is Zero so we get uniform motion All right, let's take a look at another situation where something's moving, but it still has a net force of zero Here we've got a car. We're gonna make the free body diagram and it's moving with uniform motion again That means something we have to read that word carefully. It's on a horizontal Frictionless surface. So what do we have happening here? Well, it was already moving It's got a force of gravity acting on it and Just like before it'll have a normal force acting on it as well There's no friction. So I don't have to put that force in So there's no force needed to keep the car moving as well If it's already going with uniform motion, then that's it It'll continue going with uniform motion forever until something like a net force acts on it to slow it down and stop it So those two diagrams are the same whether you've got an object moving with uniform motion or It's at rest What about something that's speeding up or slowing down? Okay, here's a one-dimensional example for a free-body diagram of an object that's speeding up Sorry object that's slowing down We've got a kid being pulled by a sled rope and through the deep snow and as the rope pulls the sled is going to slow down Okay, let's make a free-body diagram. I got a force of gravity Pulling down on this kid. I also have a normal force pulling up on the kid same as before and Those two forces are balanced Which means that they'll have the same magnitude Opposite directions and there'll be no net force over all upwards or downwards vertically in the y dimension on the kid So the kid won't accelerate up or down There's also a force here from the rope and we have a name for that We call that force from a rope force of tension and I use FT to symbolize that FT is force of tension and There's another force slowing the kid down and I'm thinking I'm going to call that the force of friction Now friction is going to try to slow the object in this case the sled down So I'm going to have it going in the opposite direction as the sleds movement and I'm going to make it a little bit Bigger I want a bigger than the force of tension if you're wondering Why do we want that force of friction larger than the force of tension? It's all because of this keyword slows Down that phrase tells us there has to be an imbalance in forces There's an acceleration and you get that whenever there's one force bigger than the other Now in this case the forces we're looking at are the force of friction and the force of tension They're the two forces that are going to cause that guy to slow down Because they're co-linear forces. They're both in the x direction and the force of friction is bigger than the force of tension So as the sled or moves forwards, he slows down Let's look at another example where we have some co-linear forces causing acceleration here We have a crane lowering a box and it is going to Slow down as it drops. All right. Well, our box does a force of gravity acting on it. Not bad and the force of Tension is also acting up on this box I'm not going to put a normal force on the box because it's not resting against a surface This box is hanging in the air. So it doesn't get a normal force But if this is the free body diagram, then my crane wouldn't slow down It would keep going at the same speed the whole time So I'm going to make this force of tension a little larger than the force of gravity All right. So now the force of tension Going upwards while the force of gravity goes downwards means that this box is slowing down There's more force pulling up on it and there is going downwards pulling down on it So notice how we're careful about what the magnitudes of our vectors look like we're going to make some of them bigger Some of them smaller based on the wording and scenario Here is an example of a 2d situation So now we've got that sledder again But he's being pulled with a force of 15 newtons at 30 degrees. So what does that look like? How was this going to change things? Well, it's not going to change anything with our force of gravity really and At this point in time, it's not going to change much with our normal force either But it is going to change what our Force of tension looks like so now I'm going to make that force of tension go at an angle an angle would be 35 degrees If I was to kind of measure it in here, it would be 35 degrees from the positive x-axis right in there There's my angle theta Now there's still a force of friction pulling back on This guy so we can put that in as well We don't have a lot of info in terms of whether he's accelerating or decelerating But we'll eventually be able to figure that out eventually We'll learn how to add all these vectors together to figure out whether this guy's accelerating or not But that'll be in another video later on So this shows us that we can have Free body diagrams where you have forces at vector vectors at angles and One of the things we can start to do with these eventually is we can move these around to sort of make them into triangles or other shapes That are more helpful. Here's an example of of that. All right, let's take a look at this one here Now we've got another free body diagram But this time we've got two dimensional forces that we're going to see if we can kind of put here into a different shape So I've got a force of 750 Newton's north There we go. That looks pretty good 500 Newton's west. So I don't know. I'll use this green one here. Maybe I Don't really have names for these forces. Maybe these are both applied forces I guess I could put that in there if you wanted to from two different You know, maybe trucks or something that are are pulling that stuck truck out. All right So that might be your sort of free body diagram for that now because in this case We have like a bird's-eye view. We're looking down at the truck I'm not going to include the normal force and force of gravity because I'm kind of looking at this from above So I couldn't see that force of gravity vector going into the page or the normal force coming out of the page So that's kind of my free body diagram other than I need to adjust the the sizes a bit I got to make that 500 Newton west vector a little bit smaller Watch what I can do here. I can also now kind of take those same vectors and I can just move them around what if I just kind of close vectors and Redrew them over here see what I could do I could end up having a free body diagram changing into a vector Diagram and I could go and start to think about What the net force would look like on this truck because the truck wouldn't just go up and it wouldn't just go north And west it would have a net force acting on it north and west This is an idea we're going to work on when we solve problems with net force diagrams is we're going to draw out these pictures Our net our free body diagrams, but then we're going to take those vectors We're going to rearrange them into shapes so we can start to do some math and solve things with them So for more on that check out some more of the videos on my youtube channel or check out ldindustries.ca