– 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

63 thoughts on “Introduction to Free Body Diagrams or Force Diagrams”

  1. The first of many, many free body diagrams we will draw for physics. ย Seriously, we will be drawing free body diagrams for the rest of our physics careers. Yes, that is how important they are. #PhysicsED #flipclass

  2. no matter how leess view u got ..but trust me you are the most interesting phy teacher that a student can have …best of luck .

  3. You made physics quite fun and interesting! It has such a different vibe than a simple, relatively-boring lecture. Thanks for all the help! ^-^

  4. Hey I really enjoy watching your videos to help me learn more in class. Do you have any videos on static tension?

  5. "Try not to have a good time… this is supposed to be educational." -Charles M. Schulz (but the literal motto of most physics professors… except for this guy!) Thanks man

  6. Hidden gem of physics videos, I watch you and Prof Walter Lewin side by side. Why can't physics be taught in the classroom as a source of joy, like it is meant to be, and as you clearly do? Anyway, you are helping me marry my love of pure math to the applied math that is physics ๐Ÿ˜‰ Thanks again!

  7. Right now we're learning about Free Body diagrams.This video helped me understand it better. I thank you so very much. I have a better understanding of it now.

  8. You guys are really doing a greay job.You need more views.Those 3 students really make things a lot fun.I love how they all appear different because of their personalities.

  9. your content is really amazing and the way you present the information is ideal.
    go on and never stop.
    deserve more than that!

  10. ๐Ÿค”๐Ÿค”๐Ÿค”๐Ÿค”๐Ÿค”โœŠ๐ŸฟโœŠ๐ŸฟโœŠ๐Ÿฟ

  11. I donโ€™t fully understand what you reallly mean when you say the normal force is always a push? Does push imply it being a contact force? thatโ€™s what I first thought but that seems redundant. Can anybody explain?

  12. Thank you for helping me understand! I really donโ€™t understand what my teacher has been teaching me and I am always afraid to ask for help because I actually go to a science school and almost everyone is smart there so it would be really embarrassing. :/
    Thank you so much! ๐Ÿ˜

  13. we're like 8 in my physics class and yknow what, no one literally ever says anything apart from teacher unlike these students in the video

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