- Good morning. Today we're going to draw
our first of many Free Body Diagrams.
- Hey guys.
- Hey Bo.
- Hi Bo.
♫ Flipping physics ♫
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 it's 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?
- 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, 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?
- [Billy] Oh! Oh! Normal mean perpendicular.
- Correct, which means that the force normal
is the force normal to, or perpendicular 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 force acting on the book.
- [Billy] Mr. P.
- Yes Billy?
- Does it matter how long we draw those arrows?
- [Mr. P.] 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 forces acting on the book is different now.
- You are pushing on the book
which mean you're applying force to the book.
- 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 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.
- 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.
That force is the Force of Friction, F sub f.
And we're gonna 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.
Again, we draw the book, it's 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 it's direction.
- The force of gravity and it's straight down.
- Yes, the force of gravity, or weight of an object,
is always toward the center of mass
of the planet or straight down.
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.
- (sigh)
- Right, the Force Normal is always a push so
it is perpendicular to the incline and up.
- Sorry Bo, 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 Diagram of all
the forces acting on the book while it's on the incline.
- Oh, because the book would be
sliding down the incline.
- 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 accelerating down the incline.
Billy, is the book accelerating down the incline?
- No.
- (sneeze)
- Bless you.
- Thanks.
- [Mr. P.] 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.
(space music0