This lecture is going to be about open and closed systems in energy, which work
a little bit differently than open and closed systems in momentum. You will
remember from the momentum unit that a system is a group of objects, and we can
choose objects to be in our system based on how the concept helps us solve
problems. Because energy works differently than momentum, we're going to
use systems in a slightly different way than we did with momentum. So basically
there's going to be a different rule for when something is an open and closed
system, because there are different rules for when energy leaves the system as
opposed to when momentum leaves the system. Before we go any further we need
to define mechanical energy and non mechanical energy. Mechanical energy is
any type of energy associated with the motion and position of an object that we
can easily calculate and predict. So without even giving you the examples of
mechanical energy here, I think that you could probably predict based on that
definition that kinetic, gravitational potential, and elastic potential energy
are the three types of mechanical energies. They're the only ones with very
specific equations associated with them, and the types of energy associated with
an object's motion and position. Non-mechanical energy is any type of energy
that's not mechanical energy. All of these other types of energy do not
depend on an object's position or movement, and they're a little bit harder
to calculate and predict in general. Using this new definition I can now say
that a closed system in energy is a system where the only energy transfer
and transformation that happens involves mechanical energy, and in a closed system
energy is not transferred to objects outside of the system. The total energy
of the objects in the system stays the same. A very common example of a closed
system in energy is something like a ball compressing a spring. So right now
because of the stretch of the spring, the ball has a certain amount of elastic
potential energy and it doesn't have any kinetic energy or potential energy. So
the total energy of the ball is all elastic potential energy. All of that
energy is stored in elastic potential, which is a type of mechanical energy. So
the idea of a closed system and energy is that if the only transformation of
this energy that happens is from one type of mechanical energy to another
type of mechanical energy. That means that the total amount of
energy in the system will remain the same even if the specific type of
mechanical energy changes. As an example, we could let this ball go and as it
begins to move the potential energy is lost and kinetic energy takes its place.
So the elastic potential energy is being converted into kinetic energy. So it can
roll like that for a little while, and you'll notice that now all of its energy
is kinetic energy, and that's the same amount of energy as it had when it was
compressed into the spring. It's just that before that mechanical energy
existed in the spring, and now it exists in the object itself, but it's the same
total energy. And then if it starts to go up a hill it's gonna rise to a certain
height and as it rises its height, it's gonna lose kinetic energy and gain
potential energy, but again the idea is that if just a transformation of
mechanical energy is happening that means that the total energy in the
system is going to remain the same. The energy is not actually leaving that ball,
it's just being transformed from kinetic energy to potential energy and when it
gets to the top, all of its energy is potential because it's come to a stop. So
it doesn't have any kinetic left, and because the total energy of the ball
remains the same, even in reverse, we know that if we drop the ball from the same
height that it rose to in the first situation, it will actually compress the
spring to the exact same length that it compressed the spring in the other
situation because the total energy is remaining the same. So the total
gravitational potential at the beginning will equal the total elastic potential
at the end. Another example of a closed system is when mechanical energy is
transferred from one object to another. For example, if we have a perfectly
elastic collision between two identical spheres, you can see that one is just
giving the other its kinetic energy but the total amount of kinetic energy
between them is remaining the same. This is actually true for all elastic
collisions. An open system in energy is a system where energy is transformed
from mechanical to non mechanical energy or vice-versa, or mechanical energy is
transferred from or to an object outside the system, and the total energy in the
system changes. So just like in momentum, in an open system the total momentum
changes. Here in energy in an open system the total
energy in the system changes, but the reasons for that are different. An
example of an open system could be mechanical energy being lost to heat due
to friction. Friction on the ground actually performs the function of
transforming kinetic energy into heat energy, so as this box slides across the
floor the friction that it experiences generates heat energy, which is really
coming from its kinetic energy. So after it slides some of that energy is no
longer in the box and can't be retrieved by the box. It's in the air and in the
ground as heat because of friction. So that energy was lost from that system. It
wasn't destroyed, because energy cannot be created or destroyed, it just no
longer exists in the box and it can't be easily retrieved. Now that the heat is in
the air it's just going to be lost to the atmosphere as a whole. Another
example of an open energy system is whenever you hit the gas on your car
the non-mechanical energy is transformed into mechanical energy. So
this car starts with no kinetic energy and no mechanical energy in general, but
as you hit the gas it begins to get more and more kinetic energy.
So that's an example of a change in the mechanical energy of a system. So we
would say that that's an open system and that energy still has to come from
somewhere. It can't be created or destroyed. Here specifically that energy
is coming from the chemical energy in the gas in the car, so you can gain as
much kinetic as you lose in chemical energy. So that's the basic idea of an
open and closed system with energy. The big idea of this unit is that energy
cannot be created or destroyed, which is going to help us solve problems with it.