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>> In this video, we're going
to take a Closer Look
at Free Body Diagrams.
We know from Newton's second law
that the forces acting on
an object are what make it
accelerate or decelerate,
move at a constant velocity,
or stay still.
To show what forces are acting
on an object, we draw what's
called a free body diagram.
A free body diagram shows
two things-- the outline
of the object or body,
and the relative magnitude
and direction of any forces
acting on that object.
Here are the steps to draw
a free body diagram.
Draw the outline of the object
being considered.
Draw a dot in the center of
the object.
All forces in
our free body diagrams
will start from this dot.
Starting from this dot,
draw an arrow for each force
the object experiences.
The arrow points in
the force direction,
and the stronger the force,
the longer the arrow.
Label the arrows with the name
of the force they represent.
If the force is gravity,
write f sub g.
If it's friction,
write f sub f, and so on.
If forces are equal
in magnitude, draw two
hashmarks on both arrows
showing that they are the same.
Now that you know the process,
try a few examples of drawing
free body diagrams yourself.
In this first example,
a shoebox is sitting still
on the floor.
What forces are acting on
the shoebox, and what is their
relative strength?
Let's follow our
five step process to answer
these questions.
First, draw an outline
of the shoebox.
Then put a dot in the center.
Now what forces does
every object on the surface
of the earth feel?
That's right, gravity.
Gravity is acting down, so we
draw that arrow in our diagram,
and label it f sub g.
Is gravity the only force
acting on the shoebox?
We know the box isn't moving,
so there must be
another force involved,
one that counteracts gravity.
That force is normal force,
the force of the floor
pushing up on the shoebox.
The normal force points up,
and is exactly as strong
as gravity.
We draw an arrow starting at
the center dot and pointing up,
equal in length with
the gravity vector.
We know it is equal in length
because our box is at rest.
The force acting up on the box
equals the force acting down.
We then will put a hash
on each line to show
that they are equal.
The free body diagram
is now complete.
There are two forces acting
on the shoebox,
gravity and normal force.
Each is as strong as the other.
They cancel each other out,
and the box doesn't move.
Now let's try an example
where the object is in motion.
After building a model rocket,
you launch the rocket
and watch it turn sideways
while the fuel is still burning.
At that moment,
with the rocket pointing
horizontally, and the engine
pushing it forward,
what are the forces
on the rocket?
In what direction, up, down,
forward, backward,
is it accelerating?
We start with a sketch
of the rocket outline,
and place a dot at its center
of mass. What forces
are acting on the rocket?
Gravity.
We draw a downward arrow
and label it f sub g.
The thrust from
the rocket engine is pushing
the rocket forward, so we draw
a forward-facing arrow
and label it f sub thrust.
Are there any other
forces involved?
There is one more,
the resistance of the air
to the rocket's motion.
Air resistance is caused
by friction, and points
opposite the rocket's motion,
so we draw one last arrow
in our diagram facing backward,
and label it f sub f
for friction.
The air resistance is not
as strong as the force
of the rocket thrust.
That means the arrow we draw
for the friction force is
shorter than f sub thrust.
Using our completed
free body diagram, we know
that the forces acting on
the rocket are gravity,
engine thrust,
and air resistance,
or friction.
We also know the rocket
is accelerating down because
of gravity, and forward
because the thrust force
is stronger than fiction.
For our last example,
let's look at an object
accelerating at an angle.
You set a wooden block
on a steep ramp, and the block
starts to slide down the ramp.
What forces are acting on
the block, and in what direction
is the net force?
Following our method, we first
draw an outline of the block,
and put a dot in the center
of it.
Now for the forces involved.
We know gravity pulls
the block down.
We draw that arrow in first
and label it f sub g.
What other forces act
on the block?
There's the normal force,
but which way does it point?
Remember the normal force
always points perpendicular
to the surface
exerting the force.
In our case, that's the ramp,
which pushes the block away
from its surface,
so the normal force
points like this.
What other forces are acting
on the block while it slides?
If you said friction,
you're right.
The force of friction acts
parallel to the ramp surface,
and points opposite the motion
of the block.
It looks like this,
and is labeled f sub f.
Are there any other forces?
No. The only forces acting
on the block are gravity,
the normal force, and friction.
Combining these forces
adds up to an arrow
pointing down the ramp.
The component of the force
of gravity pointing down
the ramp is stronger than
the friction force,
and the block speeds up
down the slope.
That's a closer look
at Free Body Diagrams.
For more "Physics in Motion",
go to our homepage,
where the entire series
is available to you.
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