Hi. It's Mr. Andersen and this AP Physics essentials video 43. It is on free-body diagrams
which are what they sound like. It is essentially drawing a diagram of a body removed from its
environment and all the forces that are acting on that. But before we draw some of those
I want to give you a thought problem. Imagine we have a scale on a platform and you just
simply stand on it. So we figure out your weight. What do you think would happen to
your weight if we angled that platform? Is it going to remain the same? Are you going
to weigh less? Are you going to weigh more? What if we make it even steeper? Well I tried
this. I stood on a scale. And so this is my weight. And then I put it on a platform. And
I used my iPhone to figure out that this is around a 17 degree angle. And you can see
that my weight got less. And so that seems kind of magical. It would be a great way to
lose weight. But what is really going on there, you have to draw a free-body diagram. So I
am the object sitting on a platform. And so we have weight, or excuse me, we have gravity
acting in the vertical. And then we have what is called the normal force. That is the force
of the platform holding me in place. And so we could use that to break our weight down
into its two different parts. We have this, which is going to be the force into the platform.
And then we have the force down the platform. And so you can see that this vector is smaller
than this vector. And that is where it is going. But it is not magic. We could use a
free-body diagram just to figure it out. And so in a free-body diagram you are taking an
object and you are removing it from its environment. And then you are going to label all of the
external forces acting on it. And so on this object, let's say these are the external forces
acting on it. Each of those arrows represents both the magnitude and then the direction
of that force which is acting on the object. Now if you look at it the vector sum of this
one is going to be to the right. So acceleration is going to be to the right. And you will
save yourself huge headaches if you can always set up your coordinate system. And so it is
parallel. One of the axis, either x or y is parallel to the acceleration of the object.
And so let's try some of these. When you are drawing a free-body diagram, we are always
going to draw free-body diagram of this car, you really want to include only two things.
The body and external forces acting on the body. Do not make these common mistakes. Do
not include other bodies. Do not include internal forces, for example the car acting on itself.
Or forces exerted by the body. And so how would you draw a free-body diagram of this
car? Well we are going to have the platform. And let's say that it is stationary. And the
car is not moving. And so let's draw the car. So that is what the car looks like. That is
the object. What forces are acting on it? Well gravity is going to be acting in the
vertical. So the force of gravity. But it is not accelerating down. And so we know there
must be an opposing force and we call that the normal force. Remember that the normal
force will always act perpendicular to the object on which it is sitting. In this case
it is going to act in the vertical. Now are there other forces acting on the object? I
would say no. It is just sitting there. There is no net acceleration. So it would be easy
to setup a quadrant system like this. Now in all of the subsequent free-body diagrams
that I am going to explain, what you should do is pause the video and try to sketch them
out. Then unpause the video and see if you were right. So let's try another one. We have
a car that is accelerating to the right. And I have just paused the video. It did not stop,
I just paused the video at this point. So you are going to draw the free-body diagram
of a car accelerating to the right. I would try this on your own, or at least think it
through. But let me show you what the right answer is. We are going to have the object
obviously. We have gravity which is acting in the vertical. And we have the normal force.
Get used to drawing those. Especially if it is sitting on any kind of a platform. What
other forces are acting on that body? Well you want to start saying it is pushing on
the ground somehow. But again that would be a force exerted by the body so you do not
want to include that. But as it pushes on the ground the ground pushes back on it. And
that it Newton's third law and that is clearly a force on this car. And so we are going to
label that as force traction. That is going to be towards the right. It is the ground
pushing on it going to the right. Are there other forces acting on that car as it starts
to speed up? We are going to have frictional forces, both frictional forces of the ground
and air resistance as well. And so you can see that the net vector sum is going to be
to the right. So acceleration is going to be to the right. And so you would set up a
quadrant system that is going to look like that. Let's try another one. Again, just the
body and external forces. Now we have a car that is just falling to earth. And I paused
it at this point. So what would be the free-body diagram? Let me show you. We are going to
have the object. We are going to have force of gravity down. There is no normal force
here. That is why it is accelerating down. But we do have a frictional force in the other
direction. So we are going to have that air resistance rather that is kind of pushing
or opposing that motion. So where is acceleration? Net acceleration is going to be down. So you
could set up a quadrant system like this. This would be fair. What about this. We launched
a car up and it is just kind of paused at the top of its arch. I paused the video here.
What would you draw? Well again I have the object. And we have gravity acting down. Now
again the one before this, even though it was accelerating down and this one it seems
like it was moving up and then paused, we still are going to have a net force down.
And so our net acceleration is going to be down. It is probably not frictional forces
acting on it, and so that would be easy to set up a quadrant system that looks just like
this. Let's start trying to put it at an angle. So now we have a car that is starting to accelerate
down a ramp. And I have paused it here. So let's draw that. Again try this on your own
first. But we have the body. We have gravity acting down. Where is our normal force going
to act? It is going to act perpendicular to the platform so it will be in that direction.
What other forces do we have? There is going to be some frictional forces and air resistance
at this point as well. And so we could do a vector sum and figure out the acceleration.
But you can see that it is going to be down the platform. So when you set up your quadrant
system you should set it up like this. Let's try another one. We have a car that is hooked
up and it is motionless at this point. So again we have gravity acting down. What else?
Normal force at an angle. And now we have this tensional force that is holding it there.
And we could do a vector sum of these and see that there is no acceleration but it would
be good to set up a quadrant system so it look likes that. And so did you learn to create
and use these free-body diagrams? We can then use them qualitatively and quantitatively
to solve problems. I hope so. And I hope that was helpful.