Hi. It's Mr. Andersen and this AP Physics essentials video 41. It is on Newton's third
law which is sometimes referred to as the law of action and reaction. In other words
when you launch a canon ball out this side of the canon you are applying a force to that
canon ball. But the canon ball is exerting an opposite and equal force on the canon.
And that is going to cause recoil. And that is why these giant ropes are on this canon
so it does not just go shooting off across the deck. And so according to Newton's third
law if an object exerts a force on another object, then that other object is going to
exert an opposite and equal force on the first object. We call these action reaction pairs.
This is the action and this is the reaction. And those action-reaction pairs remember will
always be equal in magnitude and opposite in their direction. Now these are sometimes
hard to identify. So you can use a free body diagram to identify where those forces are
and figure out the overall acceleration. So you can use a free body diagram to identify
where those forces are and figure out the overall acceleration. So imagine pushing into
a wall, applying a force to a wall. You are applying an action into the wall and the wall
is actually applying an opposite and equal reaction on you. The wall is pushing on you.
Now you do not go anywhere and the reason why is you apply a force into the ground,
an action, and there is a reaction force from the ground pushing back on you. And so the
net force on you is the same. You do not accelerate anywhere. Now let's put you in space and do
this same scenario. And let's just make sure that that wall does not move anywhere. So
now you apply a force into the wall. It applies an opposite and equal force back on you. And
so now since there is no ground to push against you are simply going to quickly accelerate
and then coast away from the wall. Let's look at another scenario. You are pushing a bowling
ball. And we have kind of frozen it at the point at which you are going to push the bowling
ball. So you apply a force into the bowling ball and it is going to apply an opposite
and equal force back on you. Now the ball is going to move, but you are not going to
move. And the reason why is you are applying a force against the floor and it is applying
a force back on you. So you do not move but the ball is going to quickly move away like
that. Let's do this scenario again in space. What happens? You apply a force into the bowling
ball. It applies an opposite and equal force back on you. What is going to happen now?
There is no ground. You are both going to accelerate away from each other. Why do you
not move as fast as the ball? Because you have a greater mass. Let's look at just walking
or running across the floor. You apply a force into the floor and it applies an opposite
and equal force back on you. That does not make sense. But if you think about it in space,
what happens if there is no floor? There is no action. You apply an action, nothing happens
and there is nothing to push against. You are simply stranded there. And so these action-reaction
pairs make sense but it is sometimes hard to figure out what is pushing on what. And
so we use a free-body diagram to do that. And this is the first time we have used free-body
diagrams. And so what do you include in a free-body diagram? The body. So you are going
to include the body. And you are going to include any external forces on that body.
Now it is more important what you do not include in a free-body diagram. You do not include
any other bodies. No internal forces, you for example pushing on yourself or parts of
an object pushing on other parts of the object. And also you do not include any of the forces
exerted by that body. That is the one mistake that students tend to make. And so let's do
a free-body diagram, a few examples. And so if you have a person standing right here on
the ground, let's do a free-body diagram of their body. So we start, I usually draw a
square like this. This represents the body. What are the forces that are acting on that
body? Well gravity is clearly acting on it in the vertical direction. But the person,
since they do not accelerate down into the earth, there must be an opposite and equal
reaction. That is the ground pushing back on the person. And we call that the normal
force. And so those are going to be the two forces in our free body diagram. Let's change
it up. Now we are looking at this person pushing on the wall. So what did we start with? It
is always easiest to start with the gravity. So gravity is pushing down. That is an external
force. What else do we have? We have that normal force, it is not moving down into the
ground. What else do we have? Let's start identifying those action-reaction pairs. So
you are pushing against the wall. But that is a force exerted by the bodies. So we do
not include it. We do include the wall pushing back on you. The force of the wall. If we
look at the ground, remember, you are applying a force on the ground, but there is a frictional
force in the opposite direction. And so there is no net force there. No net acceleration.
If we put this scenario in space what do we have? Now we have the body again. But there
is no gravity. And therefore there is no normal force. We are applying a force into the wall
and it is applying a force back on us. So that is going to be your free-body diagram.
Where is the net force? It is to the left. And so where is the acceleration? It is to
the left as well. Let's breakdown these two objects. So we have object one which is the
person. And so we have gravity and normal force. Those are easy to do right away. You
are applying a force to the bowling ball and so it is going to apply an opposite and equal
force back on you. And then again you are applying a force in the ground and it is applying
an opposite force on you. And so we have that frictional force. Now let's shift to the bowling
ball itself. So we again have the object. Is gravity acting on the bowling ball? For
sure. But you are quickly going to let it go. There is no normal force. You have just
kind of frozen this picture in time so there is no normal force. What is the only other
external force? It is going to be the person pushing on it. And so where is the acceleration
going to be? Down and to the right. And so did you learn to construct explanations using
Newton's third law or action-reaction? Can you identify those action-reaction pairs?
And then finally, can you use a free-body diagram to identify not only the object but
all the external forces acting on it? I hope so. And I hope that was helpful.