Hi. It's Paul Andersen and welcome to disciplinary core idea PS2C. This is on
stability and instability in physical systems. Remember a system can be as large as our solar
system. But it could be as small as this hour glass clock right here, which is in a pretty
stable state. But if I were to flip it upside down, sand would start pouring through it
until it returned to a more stable state. And so that's what this video is really about.
If we look at a system, a system is in it's current state based on all the interactions
going on around it or with it. And so the the stability or instability of that system
is based on the sum total of all of those interactions. And so we can have stable systems,
like this pendulum hanging from the ceiling. We could have objects or systems that are
static. In other words they look stable, but they're relatively unstable. This ball is
ready to slide down this hill. We could have systems that look unstable or are changing,
but they have stable cycles, like the moon moving around the earth. And we can even have
systems that are stable even though they are changing rates. And so this person right here
might be 77 kilograms, but that's based on eating, eating, eating and waste that is coming
out of them over a long period of time even though their weight may remain the same, they
have varying changing rates that are coming in and out. And so how do we maintain stability?
So how do we maintain for example an internal body temperature of 98.6? We use something
called a feedback loop. In other words if it gets too hot inside our body we're going
to start to sweat. And as that sweat evaporates it's going to pull heat away with it. That's
going to lower our temperature. Let's say we start to get too cold. We're going to start
to shiver. And that shivering of the muscles generates heat. And that's going to increase
the temperature. What's interesting about this system here is it requires energy. In
order for us to maintain a stable temperature on the inside, it requires a huge amount of
energy. And if we take away that energy we take away that feedback, it's going to return
to stable, naturally stable state. And so an example of that. Let's say we have two
objects. One is hot and one is cold. It's unstable at this point, especially when we
put them next to each other. We're going to have movement of all of the molecules inside
it and it's going to eventually return to a more stable state. They're both going to
be warm. And so over time in our universe things tend to return to a more stable state
over time. And so sometimes systems are too complex. And so if we're looking at the molecules
that are moving around in those objects that are warm or cold, it's impossible for us to
predict the motion of all of those molecules. They're all moving. They're bouncing. They're
hitting each other. And so the future at this point is unpredictable in a system that's
incredibly complex. But what we can do is we can make predictions about the average
future. In other words when we're measuring the temperature of an object, what we're really
looking at is the average molecular motion. Or if we're measuring a reaction rate we don't
know when all of those specific atoms are going to combine and break apart. But we do
know what's going to be the average rate of chemical change. And so how do you teach this
in schools? Well in kindergarten and first grade you want to talk about objects moving.
And why objects are moving or not moving. And so if we watch this basketball right here
as it moves across the screen, you might ask kids, like why is it moving? And the good
answer might be that it's being pushed. And so we could start to look at all the pushes
and the pulls. Maybe the basketball is being pulled in this direction. Maybe we have gravity
pulling it down. And friction pushing back on it. And so I can imagine in an elementary
class if you have you know an object and a bunch of ropes around and we're all pulling
in different directions, it's the sum of all of those interactions which are going to cause
that motion of the object. And a really good question might be let's say we have a basketball
that's not moving. Are there any interactions on it? Is anything pushing on it? Well they
hopefully start to understand that you could have a bunch of things pulling on it or pushing
on it. But if they all add up to each other then you're not going to have movement. And
so even though there's no movement, it doesn't mean that there's no forces or no interactions
on it. As you move into the upper elementary grades you want to start talking about the
forces, all of the forces they can identify within a system. And so if we're looking at
this pendulum right here, what are the forces acting on that pendulum? It's not moving.
But you could ask them, what are some forces acting on it? Well, one would be gravity pushing
down. And then it's not going down so there must be another force. And that would be the
tension moving up. But if we were to look at this ball now on the top of this pyramid,
what are the forces acting on that? Well they could see gravity. But there's no force holding
it there. So if we let it go, it's just going to move down. Or you could even ask them you
know, the earth and the moon, as the moon orbits around the earth, what are some of
the forces and interactions between those two? Well they might think, I don't know what
there is. But you know that there's got to be a gravitational force otherwise the moon
would just fly off into space. And so this is a good place to start in elementary. And
then you also want to get at this idea that sometimes systems will remain the same even
though things coming in and out of it are going to be changing. And so this is Lake
Mead. And so Lake Mead seems to be stable. This is a picture taken, a satellite image
of that. But you could say is there anything coming into a reservoir like this? And is
there anything going out? And they'll quickly realize that the Colorado River is flowing
into it. And then a lot of water is flowing out of it. And if the sum amount of those
coming in and those going out are equal, then it's going to remain stable even though there
are different rates coming in and out. Also you want to get to the idea of speed. And
so if we look at flower like this that is flowering, you might say this is a pretty
stable system. But they have to realize that speed is putting us at one point in time.
And so if we speed it up a little bit and do a little bit of time lapse photography,
now they wouldn't say that this is stable at all. And so certain things occur so slow
that we can't see them. And so fast that we can't see them as well. And then a lot of
those interactions, so if we're talking about something fast, as hot touches cold, there's
going to be a transfer of energy. That's going to be heat between the hot and the cold object.
But the amount of that heat is going to be based on what those objects are made up of.
And so a really good thing I do in class is ask the students to hold on to the metal on
their desk. And then hold on to the top of their desk. And I ask them which one of these
is warmer? And they'll always say the desk is warmer because the metal on my desk is
cold. And the right answer is that they both are the same temperature. They're room temperature.
And so why does the metal feel colder? It's what it's made up of. It's going to allow
more of that heat to be lost quickly to your hand or vice versa. Next as you get into middle
school you want to talk about stable and unstable systems. And now would be the time where you
could talk about feedback loops. In other words stable systems are going to stay the
same as long as we have constant feedback going into it. And so a great example in us
is going to be the temperature inside our body is going to stay the same because as
we get warm we're going to start to sweat. And as we get cold we start to shiver. But
you might ask them to apply that in their classroom. And so where do we see a stable
system in their classroom? Well the temperature in the classroom stays the same. And why is
that? Well as the temperature starts to get too hot, it's going to turn off the heater.
And maybe if you're lucky enough you've got air conditioning and it's going to cool it
down. And likewise when it gets too cold, then it's going to start to increase the heat
and turn that heater on. And so what happens if we remove all of that energy? It's going
to quickly go to a relatively stable environment. It's going to be the same temperature as its
surroundings or its environment. And then as you get to high school you want to talk
about this idea that sometimes the future is unpredictable. In other words in real complex
systems, and two great examples of that are when we're looking at temperature and the
average, the movement of all of the molecules inside which cause that temperature. And then
also in chemical reactions. And they should understand that even though we can't make
great predictions about all the individual parts of that system, we can make predictions
about the average. And so what is temperature? Temperature really is just an average of the
molecular motion of all the molecules inside that. And so that's where you want to get
to when you get to high school. Those are systems. They're sometimes stable. Sometimes
unstable. If we pull around energy, pull out that energy it's eventually going to be stable
over time. And I hope that was helpful.