Hi. It’s Mr. Andersen and this is AP Physics essentials video 80. It is on internal energy
which is the energy found in a system due to the internal structure of that system.
And I am going to start with this system right here. We have an egg. And so I am going to
release my hand from the egg and it starts to spin. Which seems kind of magical. If we
treat that egg as an object how could it just start spinning? It seems like there is no
force being applied to it. And so I kind of cheated in this video. I am going to show
you what I had before I stopped the video. So what I did was I spun the egg. Then I held
it. And then I let if go. And so you are probably figuring out what happened. All of the material
inside the egg was spinning. And so when I stopped the outside of the egg I was just
stopping that one object on the outside and we continued to have internal changes in the
structure on the inside. And that is why we got that odd behavior. And so we could think
of that egg now as a system with different objects or different parts. And that internal
structure and how they are arranged is giving us some internal energy. And as we change
those objects inside the system we can change the internal energy. And in physics 1 you
should be able to apply this principle to mass spring oscillators and also a simple
pendulum. And then as we move into physics 2 we should be able to apply this to an electric
charge found inside an electric field. And what is interesting is that energy is sometimes
hard to observe as it moves from one object to another. But what we will learn is this
conservation of energy. If we know that that set amount of energy is not going to change,
we can use that to figure out where that energy actually is. And so what you want to do is
be a detective when you are trying to figure out what is going on with an object and is
it a system. And so you can think of an object as a black box. And you look at its behavior
and try to figure out what is going on. So let’s say this black box just sits there.
And so can we treat it as an object with no other objects outside of it? For sure. And
so it fits. It could that there is no forces applied to it or there are balanced forces
on it. Or it is just an object by itself. So the object model works. Let me give you
another black box. Let’s say we have a box that is moving with a constant rate and so
does the object model fit? Could it be an object or is this part of a system? Well it
could be an object again. It could just have inertia. It could be an object in motion.
It is just going to continue in motion. There does not have to be any other object and so
the object model works. Let’s say we have a black box that looks like this. So what
we are getting is forces that are changing. We are having these restoring forces occur.
Well now that object model simply does not work. And so we have to go to a system model.
And so there are a few examples in physics. The first one is going to be a mass spring
oscillator. So what I am going to do is take this object in the middle, this cart and I
am going to pull it back. And when I pull it back there is going to be a force in the
spring. And so that spring becomes another object. So that restoring force is going to
pull it in that direction. But now once I have gone in that direction there is another
restoring force pulling it back. And so it is going to oscillate. It is going to move
back and forth. And the reason we are doing that, we are having these changing forces
is that we have three objects. We have the object in the middle, the cart and then we
have the two springs on either side. So what we know about this system is that the amount
of energy I put into the system is being conserved. Now we might loose some to thermal energy,
but as we convert that from potential to kinetic to potential energy, the energy is not going
anywhere. And so if we look at the equation of a spring, if we figure out the period of
an oscillating spring like that, the period remember is how long it takes to go back and
forth, there are really two things that affect that. One is going to be the mass of the object.
The heavier the mass is, since this is on the top of our equation, the larger this number
is, the larger the period is going to be. So it is going to move more slowly. And then
we have the spring constant. So the more stiff that spring is going to be, the larger this
number is down here, that is actually going to speed our frequency up. Now lets look at
a simple pendulum like this. So if I start a pendulum oscillating, does that fit our
object model? We would have to say no. And so this is going to be a system. Now what
are the objects in this system? It is the pendulum. So the bob of the pendulum. But
the other object is going to be the earth, since there is a gravitation field, that is
where that potential energy is going to be. Now as we talked about it in an earlier video,
what are going to be the things that affect the period of a pendulum? Well the length.
So the length to the center of the mass, the bigger that number is, since it is on the
top of the equation, that means we are going to have a larger period. And then we also
have the gravitational field strength. So the greater that gravitational field strength
is, the more field strength there is, the shorter that pendulum is going to be. So on
Jupiter for example it would be moving back and forth really really quickly. But on the
moon it would be moving really really slow. Now in AP Physics 2 you should be able to
apply this to a charge inside an electric field. And so does our object model work here?
Let’s watch what happens to this charge. Well it is moving away. Let’s say we put
it here. It is moving away again. Put it here it is moving away even faster. And so there
must be an electric field. So this must be a system and that other object, that other
charge, is producing an electric field. And so if you put a test charge in it then it
is going accelerate away. And we could even figure out the force using Coulomb’s Law
based on what those two charges are and what is the distance between them. And so did you
learn to calculate the expected behavior of a system using the object model? You always
start with the object model first. And if that does not work, if this fails, then you
have to go to a systems model. And what is neat about a systems model is due to the conservation
of energy the amount of energy in that system is going to be conserved, it is just going
to be converted from one object to another. Hopefully you learned that.
And I hope that was helpful.