Hi. It’s Mr. Andersen and this AP Physics
essentials video 79. It is on kinetic energy.
And you might be thinking to yourself why
are we just talking about kinetic energy right
now so far into the course? Well there are
some subtleties of kinetic energy and potential
energy that we really could not talk about
until we get to this point. And so remember
kinetic energy is energy of motion. If an
object has mass and velocity it has kinetic
energy. But let’s say that object is simply
rotating. Does it have kinetic energy right
now? Sure does. It is rotational kinetic energy.
Instead of using its velocity we are using
its angular velocity. But a good question
might be does it have potential energy at
this point? And the right answer is no. In
a classical sense an object can not have potential
energy because an object is isolated from
the system. We have to add another object.
So if I add the earth to it, now it has potential
energy. The earth is producing this gravitational
field. And there is potential energy stored
in that baseball. And so if an object is in
motion then it has kinetic energy. It has
to have mass and velocity. And remember the
equation for kinetic energy is one-half m
v squared. We know the mass and velocity,
we can calculate its kinetic energy in joules.
Now that object could also be rotating. And
if it is rotating like that, it still has
rotational kinetic energy. And instead of
using mass and velocity we are using its moment
of inertia or rotational inertia instead of
mass. And then we are using its angular velocity.
You can see the equation is essentially going
to be the same thing. Now kinetic energy is
one-half I omega squared, where I is rotational
inertia and omega is going to be the angular
velocity. But remember this does not have
potential energy if it is in motion. We can
only have potential energy if there is another
object or another part of this system. Then
it can have potential energy. It could be
gravitational. It could be electric potential
energy. But remember objects by themselves
do not have potential energy. And let me us
a little PHET simulation to get at that. So
if we take a pendulum and we lift it up on
the earth, it is going to have potential energy,
which becomes kinetic and then it becomes
potential energy again. It is going to oscillate
back and forth. But watch what happen if I
get rid of the earth. Now there is no gravity
and I simply let it go. Well that object is
going to stay there. It has no potential energy.
There is no stored energy because it is a
single object. Let me add the earth again.
Now we have added that potential energy back.
Let’s say I pull the earth away what happens?
Now all that energy is just in kinetic energy
or energy of motion. It is just going to keep
moving like that. And so this is gravitational
potential energy. But remember we could have
electric potential energy as well. If we have
a single charge like this, then there is going
to be no potential energy. But if I add other
objects, then we can convert some of that
electric potential energy into kinetic energy.
If we know the mass of the object and the
velocity we can figure out its kinetic energy.
And the equation is pretty straight forward.
If I were to pitch a baseball at around 90
miles per hour, a baseball weighs around 145
grams, 90 mile per hour pitch is going to
be 41 meters per second. So I essentially
add those values to my equation. And so that
is 1/2 times .0145 kilograms, remember we
have to convert grams into kilograms, and
then I have my meters per second. So I could
figure that out to be 120 joules of kinetic
energy. So that energy is in the motion of
the object. But it does not have to be moving
to have kinetic energy. It could be simply
rotating. And our equation is like this. It
is one-half I omega squared. So let’s say
that is rotating like this. All I need to
know is what is its rotational inertia. And
let’s say that is 0.047 kilogram meter squared.
And then I have to know how fast it is rotating.
So its angular velocity is 6.1 radians per
second. And then I simply plug in those values.
So I put those into my equation and I could
figure out using significant digits that it
0.87 joules of energy just in that spinning
baseball. And so as you pitch a baseball it
is going to have that translational kinetic
energy but also the rotational kinetic energy
as well. And so did you learn to use a model
to represent a single object that has kinetic
energy? And can you calculate that? And then
finally do you understand that an object by
itself cannot have potential energy? We have
to have another object or system in order
for us to store that energy due to position.
I hope so, and I hope that was helpful.
