We're going where few people are allowed to go.
Where tons of containers are loaded and unloaded every day.
Behind the scenes at the Port of Savannah.
♪♪
Free body diagrams help people like shippers,
builders, engineers, and architects
visualize the forces in play.
Today we're gonna show you how to draw them.
So let's get started.
First, you'll have to know that there are all kinds of forces
all around us.
In this section, we'll focus on gravity,
the normal force,
tension, applied force, and friction.
Which forces are acting on all objects
like these giant containers?
On a free body diagram, you will draw the forces
acting on an object like this big container.
It's just sitting on the ground,
so what are the forces and how do you show them?
To represent the container,
you simply draw a square
and put a dot in the center.
In this course, the dot will always go in the center
of the object.
Now keep in mind,
forces are a vector quantity,
so they're written as arrows
that have a magnitude and direction.
When you're drawing these forces,
only draw forces
that are acting directly on the object
and draw all of them from the dot at the center.
Okay, looking at our list,
what forces are acting on this container?
Gravity is one, right?
Everything on Earth has gravity pulling it down
toward the center of the earth.
So that vector points down,
always perpendicular to the ground.
You write that F sub G for gravity
and you draw an arrow pointing down.
The direction of the arrow
represents the direction of the force
and the length of the arrow
represents the magnitude of the force.
Okay, so what are other forces acting on the container?
Any time the object is resting on a surface,
you have normal force pushing up on the object.
It's perpendicular to the surface,
so draw an arrow up.
That's F sub N for normal force.
Now how do you know whether these arrows
are the same length
and how much do each of these forces
exert on the object?
Remember, vectors are magnitudes
of the quantity in a certain direction.
The magnitude is the length of the arrow
in a certain direction.
So here's how to figure out
if one force is greater than another.
If the object is motionless,
the net force is zero.
So these arrows are going to be the same length.
You put a little line--
a little hash across each--
to show they're equal to each other in magnitude.
What forces are in play when I'm pushing a box
and it's gaining speed as I move it?
By pushing the box,
I'm applying a different force
appropriately called applied force.
The symbol for that is F sub a-p-p.
How do you draw a free body diagram
of the forces at work here?
Again, draw your box,
put your dot in the center.
What forces are acting on the box?
There's gravity pulling the box down
and the normal force
acting up from the surface.
Do you know how long the arrows should be?
To figure it out look at the Y-axis
and ask yourself,
are there any forces accelerating
in the Y direction?
If not, the forces are equal
in magnitude, so the arrows
should be the same length.
Next, look at the forces acting on the box
in the X direction.
There is acceleration due to the applied force, right?
On your free body diagram,
you draw applied force with the arrow going
from the center of the box
in the direction of the force.
And there's another force at work as well
between the floor and the box.
Do you know what force that is?
Here's a hint.
What's this force?
(striking match)
The force is friction.
When two objects come in contact with each other,
friction acts opposite to the direction of motion.
You write friction F sub lowercase F.
In the case of the moving box...
it's kinetic friction.
Kinetic because at least one of the objects is moving.
Here's how you show it on your diagram.
If the box is moving to the left,
then we know friction moves to the right
because friction always opposes motion.
In this case, the applied force arrow
is longer than the friction arrow.
Why?
Because we said earlier
that the box was speeding up.
An object will always accelerate
in the direction of the net force.
Even though friction is opposing the motion
acting to the right,
the applied force acting to the left
will have a larger arrow
than friction because we're speeding up.
Now what if the box is slowing down?
The frictional force would be greater
and the arrow would be longer
than the applied force arrow.
What if the frictional force
and the applied force are the same?
The box would either be at rest
or it would be moving at a constant velocity
and the arrows would be the same length.
Okay, here's another one for you.
A container suspended by a crane-
what forces are in play here?
Gravity of course.
Do we have a normal force?
No, because it's not resting on a surface.
But it is suspended by a rope.
What type of force is keeping our object up?
Tension.
The force along a string, rope,
or a chain.
The tension symbol is F sub T.
So how do you draw this?
The object isn't moving
so the net force is zero.
Since the net force is zero,
there must be another force acting on the box.
Any guesses?
Even though its suspended,
gravity in the form of weight
is still working on the container.
So the downward force is weight-
with a downward arrow equal to the length of the arrow
for tension and marked with an F sub W.
So the force moving up is equal to the force down.
Vectors are equal and balanced.
Now that you know how to do free body diagrams,
look around you and test your ability
to figure out what forces are in play
and how they interact.
That's physics in motion.
And for more practice,
check out the closer look on free body diagrams.
For more practice problems, lab activities,
and note-taking guides,
check out the Physics in Motion toolkit.