Hi. It’s Mr. Andersen and this AP Physics
essentials video 82. It is on kinetic and
potential energy. Remember kinetic energy
is energy of motion. And potential is due
to its position. And so if you take a wrecking
ball like this, a wrecking ball has a huge
amount of mass, and so you can connect it
to a big crane and as you move that crane
back you are storing some of that gravitational
potential energy. And so when we release it
by moving the crane it has a huge amount of
kinetic energy. We can use that kinetic energy
to do work, in this case we are breaking down
a building. And so if you have a system with
internal structure then there is going to
be internal energy within that system. And
that energy could be kinetic energy, that
is energy due to its motion or it could be
potential energy, which is due to its position.
Now if we set up this system in such a way
that it is a closed system, in other words
we are not losing energy out of the system
and we are not taking energy in, then as we
convert kinetic energy into potential energy
there is the conservation of energy. The amount
of internal energy is going to stay the same.
So if I knew how much potential energy that
wrecking ball had, I could have figure out
how fast it is going when it actually crashes
into the building. And so here is a little
PHET simulation that gets at that internal
energy. So I have a skateboarder here. And
if we just let him go, when he is at the top
it is all potential energy and when he is
at the bottom it is going to be kinetic energy.
And so if we quantify that, this is what it
is going to look like. And so total energy
is going to be 3448 joules. And you can see
that the total energy across the top is going
to remain constant. It is never going to change.
But what is happening is we are oscillating
from energy being potential energy, so you
can see right now that almost all of that
energy is potential energy. And then if I
let it play forward a little bit, what is
going to happen to that energy? It is now
going to be converted to kinetic energy, when
the skater is at the bottom of that ramp.
And it will just be converted back and forth
from potential to kinetic to potential to
kinetic. And so if you watch the skateboarder
what is going to happen, he will just keep
going forever. And you know that is not how
nature works. And so what we can do is we
can increase the coefficient of friction.
Now we are converting some of that energy
into thermal energy and then he quickly comes
to rest. And so knowing this idea of conservation
of energy, what you can do is figure out,
okay, what is the potential energy. And how
much of that is converted into kinetic energy?
And you can solve problems like this. So let’s
say we have a skateboarder who is at the top
of the ramp at 8 meters in height. So how
much potential energy would he have? Well
we know the height. We know since he is on
the earth that we know the gravitational field
strength. So the only thing we have to figure
out is what his mass is. And so he is going
to be 75 kilograms. And so here is our equation.
It is m g times the change in y, where y is
how far we are in that gravitational field
strength. So we have 75 kilograms. Gravitational
field strength of 9.8 and 8 meters. And so
how many joules of energy does he have? Using
significant digits, 5900 joules of potential
energy stored there. So a simple problem you
could have in AP Physics is let’s watch
this skateboarder go. And so where is all
of the energy now? It is in kinetic energy.
And so since we knew how much energy is potential
before, we could figure out that there is
going to be 5900 joules of kinetic energy
at the bottom. And we could also, if we know
what that equation for kinetic energy is,
which is 1/2 m v squared, we could even figure
out like how fast is that skateboarder going
at the bottom. If we say that all that energy
is converted from potential into kinetic,
so we could set that up to our equation right
here, where we put in the mass of the skateboarder
and then we simply solve for v. And so using
significant digits at the bottom, that skateboarder
is going to be going 13 meters per second.
And so what happens at this point? If we let
it go, the skateboarder is going to go up
like that. So we could figure out how much
energy he has there. So how would we figure
that out? Another way to think about that
is the energy from here to here is going to
be converted into the kinetic energy that
he has at this point. And so is he going to
keep going? For sure. He is going to go flying
off the edge of the ramp like that. And so
did you learn to describe and make predictions
about internal energy? Remember if it is a
closed system the internal energy is going
to stay the same it is just going to be converted
from potential to kinetic and then back again.
And then finally could you calculate kinetic
energy if we know potential energy or vice
versa. I hope so. And I hope that was helpful.
