Hi. It's Mr. Andersen. And today I'm going to be drawing free body diagrams.
Free body diagrams, they allow us to solve very complex physics problems by just isolating
all of the forces that are acting on one individual body. So this is not true physics. This is
video game physics from the video game portal which is very fun. But if we look at all the
forces that are acting just on this one individual body, in this case the player in the video
game, then we can solve the problem. Now the easiest way to get good at free body diagrams
is to practice them. And so in this podcast we're going to do a lot of practice. So first
of all we have to identify what are all the forces or what are the common forces that
can act on objects? And so if we have an apple sitting right here, there are two groups of
forces that act on the apple. Those that are really distant, and those that are in contact
with the object. And so distant means that they're not actually touching the object.
And so clearly gravity is working on this object. And on this apple that is sitting
here on the table. Now normally when I write gravity or the the force of gravity what I'll
always write is F for the big force and then an abbreviation underneath it. In this case
gravity. And so in which direction is that acting? Well it's acting down towards the
center of the earth. And so there's a force of gravity acting on the apple down. Now forces
are vectors so it's not only should it be the correct size, but it should be the correct
direction. Now there are clearly electromagnetic forces acting on the apple as well. But generally
I'm just going to ignore those because they're pretty small in most of the problems that
we'll do. Okay. So why doesn't the apple just go through this table here down to the center
of the earth? Well the reason why is that we actually have a normal force acting on
it. A normal force is acting in the opposite direction. And so that force is exerted by
the table. And so these two arrows should be the same length because those forces are
balanced or they're equal. Now one thing you should always remember about normal forces
is that they always act perpendicular to the surface that they're sitting on. And so we'll
come back to that on a problem in a little bit. We also have tensional forces. So forces
of tension are going to be when we have an object that for example is hanging from a
rope. And that's suspended from the ceiling. And so if we have an object that's hanging
from a rope, then there's a tensional force that is moving in the other direction. And
so these two, normal force and tensional force, they both counteract gravity in this case.
Okay. Next one I'm going to clear off the screen for a little bit so I can get a little
bit of room. Next would be an applied force. And applied force I'll usually write like
this. Let's say I apply a force to this apple. Let's say I push it in this direction with
my hand. So I'm pushing it in that direction. That would be applied force in this direction.
It means that I'm actually doing work. I'm moving that object. Now if I try and move
the apple in that direction, there are going to forces that oppose that motion. And that
would be the force of friction, so we call that the force of friction. And so the force
of friction is going to be the molecules rubbing right down here. And so it would be moving
in the other direction. And there's also a force of air resistance. And that force of
air resistance is also going to be moving in the other direction. I'm going to draw
that arrow a little bit smaller because it's probably not as big as the frictional force
is. But as it gets faster and faster air resistance is going to be a bigger force. And then the
last one is going to be a spring force. A spring force would be, for example, let's
say we have this apple suspended by a spring. But that spring is in tension. So when we
release the apple the apple goes shooting in the other direction. So that would be a
spring force. And spring force, you have to figure out what kind of, is the spring under
compression or is it in tension to figure out which way that object is actually going
to move. So let me get these out of the way. Those are the basic forces. And there are
other ones, but those are the basic forces that we're going to deal with. And so let's
just go through and do some practice ones. Now with each of these what I would encourage
you to do is after I describe the problem, the best way to learn how to do it is to just
practice. And so first one, what we've got is a bottle of wine that sits on a table.
So we're just looking at the forces that are acting on this bottle of wine right here.
And so you can always pause the video and try and do this force diagram. Or the free
body diagram. So this is how it should look. So there is going to be, all of mine are always
remember going to be a box to represent the object. So I'm going to draw from the center
of gravity, I'm going to draw the force of gravity acting down on that bottle of wine.
Now the bottle of wine is not moving and so there is an equal force in the other direction.
And we're going to call that the normal force. Now I don't see any other forces acting on
the object. And so this would be the correct free body diagram for a bottle of wine sitting
on a table. Okay. Let's try another one. Alright this one we've got a blacksmith sign that
hangs from a beam. And so we've got an object that's suspended by two chains it looks like.
And so let me draw the object itself is going to be a box. We're going to have the force
down of gravity. Now in this case we have a force up, but that force up is going to
be the force of this chain and the force of that chain. And they look to be balanced.
And so I'm going to draw a force like that and a force like that. And so these two forces
are going to be tensional forces. And these two right here, the distance of that to that,
if I add two of those that should be equal to the force of the gravity down here. In
other words, if I were to add those two vectors up, the force of gravity and then the two
forces of tension, we'd find that the object really isn't moving at all. And so that would
be the correct free body diagram of a blacksmith sign hanging from a beam. Okay. Let's try
another one. So the next one, what we've got is a ball accelerating down a ramp. Now one
thing you want to start listening for is acceleration or constant velocity. If something is accelerating
that means it's getting faster and faster. And that usually means we have unbalanced
forces. If it's constant velocity in these problems, that means that the forces are balanced.
And we'll get to one of those in a second. Okay. So this one, let's first of all draw
the box. So this represents the ball. There's going to be the force down. So that's the
force of gravity. Okay. Now let's look at all the forces around this ball right here.
So first of all the ball is siting on an inclined plane and moving down a ramp. And so there's
a normal force here. But the normal force remember, the normal force is always going
to be perpendicular to the ramp that you're sitting on. And so there's going to be a normal
force in that direction. So I'm going to call that the force normal. Okay. Next the ball
is rolling down the plane. And so we also have a frictional force. Now it's not probably
not very big. And we also have an air resistance force. Now those ones, so frictional force
and air resistance, instead of being perpendicular, they're actually going to be perpendicular
to the or excuse me parallel to the motion. And so if it's a normal force that's always
going to be perpendicular. But in this case it's going in this direction. Motions in that
direction. Our force is going to be parallel. And so I'm going to try and draw and that's
not very good. But I'm going to try a draw a line that's parallel to the ramp. And I
could always represent that as the force of friction. And lots of times I'll just double
these up. And then the force of air resistance. So that represents both of those. And those
are going to be up the ramp. Now I can't see any other forces acting on it. It's the force
of gravity that's causing it to accelerate down the ramp. Okay. Let's do another one.
Next one we've got a box is dragged across the floor with a rope. So it doesn't tell
us much about if it's accelerating or not. But it's being dragged across the floor. And
so let me draw this one. First of all the box looks like this. This is the first box
that's actually a box. We've got the force down of gravity. Again that's one that you
can pretty much get used to drawing in all of these. Now it's being pulled at an angle.
Let me try to get my angle about right. So it's being pulled in that direction. And we're
going to call that the applied force. So we're applying a force. And we're applying a force
at an angle. Now there's also going to be forces that are slowing it down. And so those
are going to be frictional forces. And so that's going to be parallel to what we're
on. And so we're going to have the force of friction. And then we're going to have the
force of air resistance as well. And that's it. Those are all the forces that are acting
on that box. So let's try another one. Next one, I love ones like this. Next we've got
a golf ball. So we're dealing just with this golf ball right here. And it's just been hit.
So the club just swang, just came by, hit the golf ball and now it's kind of in flight.
And so how do we do that? Well let's draw the box to represent the golf ball again.
So we've got the force of gravity that's acting down on the golf ball. Now a tendency I've
seen students do this a lot is since it's moving, or it's been hit, the tendency is
to put some kind of an applied force on that. But remember, once we've hit it then there's
no force acting on it. And so the only other force that's acting on it is going to be the
force of, since it's not on the ground, it's still going to be the force of air resistance.
And so it looks like the ball kind of went in this direction. So it went like that. And
so I draw my force of air resistance in the opposite direction. And those are the only
forces that are acting on it. There's no applied force because we've already applied that force.
And that's what's made it take off. Alright. Last one then. This is the last one. We've
got a climber. And the climber is repelling down a rope. And the climber is moving at
a constant speed. Okay. So it's not accelerating. So let me draw the object again. So we've
got the force of gravity that's acting down on the object. And now we've got a force that's
going up. So if there was no rope that would be the only force except for maybe air resistance
going in the other direction. But what's slowing you down is that you'll run the rope through
a carabiner or through a some kind of a figure eight device. And that slows down the climber.
And so it's friction that's actually slowing down the climber. And so we would have the
force of tension that's holding it there. But it's also the force of friction inside
that rope that is slowing it down. And since that climber is moving down at a constant
speed, the force of gravity and the force of tension and friction are going to be balanced.
Or they're going to be equal. And so if you're ever moving at a constant velocity we can't
tell the difference, at least in a force diagram between that and then just sitting there.
In other words not moving. And so that's one common misconception that we have on free
body diagrams. But that's about it. And so I hope that's helpful.