Good morning. Today we are going to draw our first of many free body diagrams. Hey guys.
Hey Bo. Hi Bo. Flipping Physics Mr. Almost every time you do a problem that has forces in it, you will need to draw a free body diagram. A free body diagram, or FBD, is also called a force diagram because it is a diagram of all the forces acting on an object, or body, that has been singled out, or freed, from a group of objects.
Our first free body diagram is going to be of all the forces acting on a book. Start by drawing a rudimentary picture of the object and pertinent surroundings. So we draw the book and the table on which it is resting, and we add a dot near the middle of the object. Technically, the dot is called the center of mass of the object.
Right now, you can think of the dot, or center of mass, as the location at which we consider all the mass of an object to be concentrated. For an object with a constant mass density, the center of mass is in the middle of the object. We will spend more time specifically defining center of mass in later lessons. Alright, let's start adding some forces to the free body diagram. Billy, tell me the name of a force that is acting on the book and give me its direction.
Gravity. Actually, Billy, we need to be careful now because we have two variables that contain the word gravity. Yep, the weight or force of gravity acting on an object which equals the mass of the object times the acceleration due to gravity.
Yeah, so just saying gravity isn't clear. You could mean the force of gravity or the acceleration due to gravity. Yeah, I mean the force of gravity which is straight down.
When you are drawing your force diagrams, you need to think about what is actually happening to the object. For example, Bo, if this were the complete free body diagram of all the forces acting on the book, what would be happening to the book? The book would be accelerating downward. Actually, wouldn't the book be in free fall? Mr. P.: Yes.
If the only force acting on an object is the force of gravity, then the object is accelerating downward and is in free fall. Bobby. Mr. Is the book currently accelerating downward?
No. Therefore, there must be a force acting on the book preventing it from accelerating downward. That force is the force normal.
The symbol for force normal is F sub n. Who can tell me what the word normal means in your math class? Oh, normal means perpendicular.
Mr. Correct, which means that the force normal is the force normal to or perpendicular to to or at a 90 degree angle to a surface and caused by that surface. One other thing you need to know about a force normal is that a surface can never pull on an object, it can only push. Therefore, a force normal is always a push.
That is a complete free body diagram of all the forces acting on the book. Mr. P? Yes, Billy? Does it matter how long we draw those arrows?
Mr. Absolutely. Remember, these arrows represent the force vectors, and therefore the length of these arrows represents the magnitude of those force vectors. And the reason these two arrows have the same length is because in this particular case, the magnitude of the force normal and the magnitude of the force of gravity are the same.
Now, let's change our free body diagram using this force sensor. Bo, please tell me how the free body diagram of the forces acting on the book is different now. You are pushing on the book, which means you're applying a force to the book. Mr. Yes.
And the name of the force I applied to the book is the force applied. So F sub A is the force applied, the definition of which is it's the force applied on an object by a different object or person. Now, Billy, explain to me why why this can't be the complete free body diagram for all the forces acting on the book while I'm pushing on it.
Because if those were all the forces acting on the book, it would be moving to the right. Mr. If this were the complete free body diagram of all the forces acting on the book, the book would be accelerating to the right, which it's not doing. Therefore, there must be a force preventing the book from accelerating to the right. Mr. That force is the force of friction, F sub F.
And we're going to leave a detailed description for the force of friction for a later, actually, set of lessons. Right now, all you need to know about the force of friction is that it is parallel to the surface and it acts to prevent an object from moving or slows down an object. Now, let's change the free body diagram again. Now the book is on an incline. Mr. Again, we draw the book, its pertinent surroundings, and the center of mass, dot.
Bobby, please tell me the name of a force that's acting on the book and give its direction. The force of gravity, and it's straight down. Mr. P.: Yes, the force of gravity or weight of an object is always toward the center of mass of the planet or straight down.
And an incline isn't going to change the direction of the force of gravity. Bo, please tell me another force that's acting on the book right now. The force normal and it is perpendicular to the incline.
Right. The force normal is always a push, so it is perpendicular to the incline and up. Sorry, Bob, I had to.
That force normal I drew, it is perpendicular to the incline, but it's wrong. Bobby, please tell me why this cannot be a complete free body... of all the forces acting on the book while it's on the incline.
Oh, because the book would be sliding down the incline. Mr. Right. A component of the force of gravity is actually down the incline, and therefore, without another force to balance that out, the book would actually be ex- accelerating down the incline.
Billy, is the book accelerating down the incline? No. Bless you. Thanks. Mr. Correct.
So there is a force of friction preventing the book from sliding down the incline. The force of friction is up and parallel to the incline to prevent the object from sliding down the incline. Again, almost every time you work with forces in a problem, you will need to draw a free body diagram. Thank you very much for learning with me today.
I enjoyed learning with you.