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In this video we are going to review the steps to create a force diagram. A force diagram
is our diagrammatic representation of the interactions between an object of interest
and everything else in the world. So in this assignment, in this situation we are going
to work with this: a man pulls a rope attached to a crate across a very smooth surface. Step
1 of making a force diagram is to sketch the situation and circle the object of interest.
So, a man pulls a crate attached to a rope across a very smooth surface. It's important
to indicate in your sketch which is the object of interest. This force diagram is a means
to simplify the whole universe into an object of interest and everything else. This is a
method of analysis that lets us get a lot done. So I have circled my object of interest,
I usually use a dotted line for that. That's step 1 we've sketched the situation and circles
the object of interest. List interactions. What is an interaction? An interaction is
a push or a pull and with only one exception an interaction is between two objects that
are touching. So, the full version of this instruction is "list interactions with the
object of interest."
Let's see, what's touching or pushing or pulling on the object of interest? Well, the rope.
Is the floor touching? Yeah, the floor is definitely touching. Is the man interacting with the crate? Well,
strictly speaking, no. Because the man is not touching the crate, and when we say interaction
we mean a push or a pull of something that is touching. So the man is not interacting
with the crate. What is? There is one thing that can interact with this crate, exert a
push or a pull on it, without touching. It's the earth. Anywhere near the surface of the
earth the force that we often talk about as gravity exerts a pull on that crate downward.
Let's see, anything else pushing or pulling? No. Step 2 is complete. We've got three interacting
objects
Now we're ready to make the actual force diagram. Step 3, dot and label. Dot. Label. The dot
reminds us that in a force diagram we are treating the object of interest as a point
particle. The size and the shape and orientation of the crate is not important, we are treating
it as a simplified type of object. Now let's go through each of our interactions and draw
an arrow with the tail attached to the dot to show the direction of the interaction.
What's the direction of the pull of the rope on the crate? Well, it's important to make
our force diagram match the sketch. So let's show it pulling to the right. And we need
to label each of these interactions following particular rules. It's just a rule, later
on we'll find out why it's useful. This push or pull is a force. So we write a big f for
force and we write in small letters next to the f a subscript describing the two objects
participating in this interaction. This is the force of the rope on the crate. What else
is interacting? The floor. The floor is pushing the crate upward. F floor on crate. We've
got one more interaction, and as we draw these arrows we need to think not just about the
direction of the arrows but the relative lengths of the arrows. So, we need to draw the force
of the earth on the crate and it's obviously going to be down. How long do we draw that
arrow? Well, after we reflect for a moment it seems to be clear that the force of the
earth on the crate down is just as much as the force of the floor on the crate is up.
So we are just about done.
And let's think ahead about our next topic. Are any of the forces on this, exerted on
this crate by other objects, are there any forces that are not balanced by other forces,
and what does that mean for the crate?