Hi. It's Mr. Andersen and today I'm going to talk about potential and kinetic
energy. Remember from the last podcast that energy is the ability to do work. And work
is a force times times the distance. So we measure work and energy both in joules. Now
there a law of the conservation of energy. In other words that law states that energy
can neither be created nor destroyed. Now it can be converted into mass according to
E=mc2. But we'll get to that later. And so since energy can neither be created nor destroyed,
it can be converted. And so the two terms that we generally talk about when we talk
about storing or using energy are potential and kinetic energy. Now I'm talking about
potential gravitational energy and kinetic energy. And so we also have potential energy
for example in the chemical bonds of a molecule, but I'm not talking about that. And so the
two types of energy that we have are potential energy. And that's energy due to position.
And kinetic energy. And that's energy due to motion. And we have equations for each
of these. Potential energy is mgh, where m is mass, g is gravitational acceleration and
h is the height. And then kinetic energy is one-half mv squared, where m is mass and v
is the velocity of the object. Now the best place to look at how energy is converted from
potential to kinetic energy is in a pendulum. A pendulum is simply a weight attached to
a string. And so if I hold a pendulum at one side and don't let it go it has a certain
amount of potential energy. When I let it go the pendulum will swing back and forth.
That energy is converted from potential to kinetic and then back to potential energy.
And then to kinetic and then potential over and over and over again. And so when that
ball is sitting at the top it has all potential energy. When it's at the bottom it's converted
all of that energy into energy of motion. And so when it's half way down we would say
that it has a combination of potential and kinetic energy. And it's just converted. Now
will a pendulum swing forever? No. Because we're going to lose a little bit of that energy
in friction, in heat, in sound as it moves. And so eventually that pendulum is going to
come to a stop. And so let's do a couple of problems with potential energy and kinetic
energy. Potential energy remember is measured as mgh, where m is mass, g is gravitational
acceleration and h is height. And so let's say for example that I climbed to the top
of a ten story building. And so first of all we have to know my mass, which is 78 kilograms.
We have to know the acceleration due to gravity or g which is -9.81meters per second squared.
And then we have to convert that ten story building into meters. And so a ten story building
is roughly 32 meters high, or that's our h value. And so if we simply multiply those
all together, we get 24,485.76 joules. And if we do significant digits that's 2.4 x 10
^4 joules of energy that my body has at the top of a building. And as long as I stay at
the top of that building I can use that on the way down. I don't want to jump off the
top because I don't think I would be able to make it. The next type of energy is called
kinetic energy. Energy of kinetics or motion is 1/2mv^2. And so that's energy due to motion.
And if I jumped off a ten story building I would convert all of that into kinetic energy
at the bottom of my fall. But I don't want to do that. And so let's do one dealing with
a baseball. Let's say I pitch a baseball. And there are two different pitches. When
I throw a baseball I probably throw it around 20 miles per hour, if I were to throw it.
I'm not a very good thrower. But a really good major league pitcher will throw it at
100 miles per hour. And so let's figure out how much kinetic energy would be in one of
my throws and then those of a pitcher in the major leagues. First of all we have to figure
out the mass of the baseball. The mass of a baseball is 0.145 kilograms. And since we're
doing kinetic energy, the only other value that we need is the speed. And so if you throw
a 20 mile per hour pitch, that's roughly 9.0 meters per second. Remember on all of these
we always have to convert it to meters, or meters per second excuse me, it if's a velocity.
A 100 mile per hour pitch then is roughly 45 meters per second. And so first of all
let's figure out how much kinetic energy my pitch would have. A 20 mile per hour pitch.
We use the equation 1/2mv^2, where m is 0.145 kilograms and v is 9.0 meters per second.
We then take that times 1/2 and square the velocity and I get, using significant digits,
5.9 joules of energy. Now let's try the faster pitch. It's 100 miles per hour so that is
45 meters per second. So we're going to use 1/2mv^2. Our mass remains the same, or it's
0.145 kilograms. Except our velocity now is 45 meters per second. If I multiply that across
using significant digits, I get 150 joules of energy. Again when I pitched it 20 miles
per hour it was only 5.9 joules. And so even though that pitcher is throwing it 5 times
as fast, he's getting roughly 25 times the amount of energy out of that pitch. And that's
why if you look at the equation, the velocity being squared is super important to understand
that. And so you can solve complex problems now that you know the equation for potential
energy and kinetic energy. For example in class we figured out, based on the speed of
a sprinter and the mass of the sprinter, you should be able to figure out how high they
could pole vault if all of that kinetic energy were converted into potential energy at the
height of that fall. But that's it. That's in summary again the ways that we can measure
energy in joules. And it's the ability to do work. And remember it's always converted
from potential or energy due to position to energy of motion or kinetic energy. I hope
that's helpful.