Hi. It’s Mr. Andersen and this AP Physics essentials video 63. It is on total energy.
So total energy of a system. And in this simulation right here I have this red sphere. I have
given it velocity. So it has kinetic energy. Due to its position it has potential gravitational
energy. And since I have stretched this spring right here it is also going to have elastic
potential energy. So we could figure out the total energy of this system. But watch what
happens when I let it go. You can see that all those energies are being converted back
and forth. So some of that elastic potential energy becomes kinetic energy. And then we
lose that. And if I were to keep letting it go, if this was a perfect system with no friction
it would go forever. But since we have friction we are converting that into other forms of
energy. Now the total energy is not changing. But due to frictional changes and positional
changes, eventually this would come to rest. And so if we have a system that is filled
with objects, we can calculate the total energy by just adding up all the energies of the
parts. So the kinetic energy, the potential energy, and even the internal energy. Now
what is internal energy? That would be energy of the particles themselves, within an object
as they move around. For example a glass of water has a certain amount of internal energy
as the molecules move. Now it is hard to calculate that. And in AP Physics you will only be expected
to calculate kinetic and potential energy. And so kinetic energy, again energy of motion,
but potential energy could be either due to gravitational position or it could be elastic
potential energy. And again, these can be converted from one form to another. You should
also be able to predict what is going to happen to that system over time. And so as position
changes and as friction affects the objects inside that system, it is going to convert
the total energy of the system. And so remember energy and work are interrelated. Work is
a force applied over a given distance. And so for something to have energy it has to
have the ability to do work. So let me show you what that might look like. If we have
a car that has kinetic energy, so it has speed moving from left to right, it could apply
a force on that object and move it a given distance. So it clearly has kinetic energy.
It also at this point has potential energy due to its gravitational position. So as it
falls in could also apply force over a given distance. And so let’s go through how you
would calculate all these different forms of energy. I would encourage you at each of
these questions to pause the video and try to calculate the kinetic energy on your own.
So how do we figure out the kinetic energy of a baseball pitch? We first would have to
know the mass of the baseball and then the speed of the baseball or the velocity of the
baseball. A typical baseball weighs 145 grams. And let’s say I could throw it at 90 miles
per hour. That would be 41 meters per second. So could you calculate the kinetic energy.
All you are going to do is plug in the those two values. So it would be equal to our mass,
which again we have to convert it to kilograms times our velocity and then we would square
it. And so I would get a value like this. Again this would be too many significant digits.
And so we would have 120 joules of kinetic energy in that baseball. That ability to do
work. That many joules of energy. Now if we look at that equation, what happens if we
double the mass of the baseball? We double the amount of kinetic energy. What happens
if we double the velocity? Since we are squaring that, then we would have 480 joules, so it
would be 4 times the amount of energy. Now this is just translational kinetic energy.
And we will get to, in future videos, how you would figure out the rotational kinetic
energy as well. So that baseball is going to be spinning. And so if we want to calculate
its rotational kinetic energy that would be 1/2, instead of mass, it would be its rotational
inertia times its angular velocity squared. Now let’s look at gravitational potential
energy. Remember that is going to equal m g h, where m is the mass of the object, g
is the gravitational field strength and h is its position in relation to ground we will
say. And so let’s say that baseball is 145 grams again. But I am holding it right at
head level. And so I am holding it about here and then I am dropping it, so it is about
1.8 meters off the ground. How much gravitational potential energy would it have? Again we are
just going to fill in those values. And so here is my mass. Here is gravitational field
strength and then here is going to be my height. So again we are going to have 2.6 joules of
energy just due to its position above ground. Now we could also have elastic potential energy.
And so if we take this baseball attached to a spring and I simply push it in, we are going
to store potential energy in that spring itself. How do we calculate that? Our equation is
right here. It is 1/2 k x squared. And so we have to kind of know where it was to begin
with. So here is the initial position of that baseball. And so what is k? That is going
to be our spring constant. Let’s say it is 120 newton meters. And then what is going
to be x? x is going to be the change in position. And so it is going to be moving, we will say
from here to here, so 22 centimeter difference. And so how could I calculate the elastic potential
energy? I simply plug in those values. I put my spring constant right here. And then I
have to convert this to meters. So this would be my change in position. And then I would
simply solve that value. So I would have 3.0 joules of energy just as I squeeze that in
due to its elastic potential energy. And so again, you have to be able to predict what
is going to happen over time. If there is no friction it is simply going to keep moving.
But due to changes in position and due to frictional changes, that is going to be converted
over time and we are going to lose a lot of that energy into the surroundings. And so
did you learn to calculate the total energy of a system? Again you should be able to calculate
all forms of kinetic and potential energy. And then finally can you predict changes in
the total energy of a system due to changes in position and frictional interactions? I
hope so. And I hope that was helpful.