Hi. It’s Mr. Andersen and this is AP Physics essentials video 76. It is on the conservation
laws. And conservation just means stays the same. So let’s say I have seven Legos and
I put them together, the number of Legos is going to be conserved. If I take them apart
we still have seven Legos. If I put them together in a different arrangement we still have seven
Legos. The Legos are being conserved. Now let’s say we put them together like this,
well the number of Legos is not conserved. What does that mean? Legos came from somewhere
else and that means I have not defined my system correctly. I have what is called an
open system. And so we are having new Legos come in or we could have some of those Legos
leave. And so once we define a proper system with the correct objects inside it, then whatever
is inside is going to be conserved. Now what could be inside? It could be the amount of
energy. It could be the charge. It could be the momentum, either linear momentum or angular
momentum. It is essentially everything within that system that is going to stay the same.
A system is a series of two or more objects that are separated from their environment.
And if there is no boundary between the system and the environment we call that an open system.
In order for the conservation laws to apply we have to have a closed system or an isolated
system. Once we have an isolated system then we have these conservation laws. In other
words inside that system we are going to maintain the same amount of energy. Or we could have
the same amount of charge. Or the same amount of momentum, either linear momentum or angular
momentum. And so an open and closed system is important to understand. An open system
is any system where we can have matter and energy either entering into that system or
leaving. And so for us to have an isolated system we have to have no movement of energy
or matter into or out of that system. But once we do, once we define that closed system
then the conservation laws apply. So imagine that I have an electric circuit like this
and the dotted line represents a system. So this system right here, would you say that
that is an open system or a closed system? Well it is going to be an open system, because
we clearly have energy coming in and energy going out. And so that is going to be an open
system. What would a closed system look like? It would be the whole circuit itself. Now
it is hard to make something that is totally separated from the universe. But for solving
a physics problem like this, this would be a great closed system. Let’s say we were
looking at a closed system here. So I am going to drop a basketball with an apple on the
top of it. Let’s watch what happens. So if I were to give you that as a problem to
figure out what is going on, if you were to identify the systems, so that apple itself,
would you call that open or closed? I would call that an open system. So we are clearly
not accounting for the basketball. So if we were to make this our system that also would
be an open system. Because I am not taking into account the floor, or if you watch this,
that apple actually bounces off of the roof. And so this would be a better representation
of this closed system. And so once we have defined what that closed system is, then we
have conservation of energy charge and momentum. And so if we say this is a system, me and
a bowling ball on a string, if we say that is the system, we are going to have conservation
of energy within that system. So that bowling ball has a certain amount of potential energy.
And when I let it go, it is going to be converted to kinetic, back into potential energy, but
it is not going to hit me in the head because we are not going to have energy coming into
the system. Now if somebody were to push on that bowling ball then we would have an open
system, we would add energy to it, and it would not be conserved. If we were to look
at conservation of charge. Here I am going to charge up this glass rod. Bring it close
to an electroscope. And you can see what is going on there. As we discussed in previous
videos, what is going on is the electrons are actually moving within the conductor of
the electroscope. Now it is not like we are adding new charges to this system, or we are
pulling them away, they are just moving from one location to another. The charge is being
conserved. Same thing applies to momentum. And so in this famous Newton’s cradle what
is being transferred between each side of that swinging pendulum is going to be linear
momentum. And same thing when I dropped the basketball. We are transferring some of that
momentum of the basketball into the apple, but the total amount of linear momentum is
conserved. And the same thing applies to the angular momentum of this gyroscope. And so
once you understand what that closed system is, all of those energy, charge or momentum
is going to be conserved within that system. And so did you learn to define open and closed
or isolated systems and apply the conservations of energy, charge and momentum? I hope so.
And I hope that was helpful.