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Mr. Andersen explains the difference between potential and kinetic gravitational energy. He also uses physics to calculate the energy in various objects.
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Transcript Provided by YouTube:
00:04
Hi. It’s Mr. Andersen and today I’m going to talk about potential and kinetic
00:09
energy. Remember from the last podcast that energy is the ability to do work. And work
00:14
is a force times times the distance. So we measure work and energy both in joules. Now
00:20
there a law of the conservation of energy. In other words that law states that energy
00:24
can neither be created nor destroyed. Now it can be converted into mass according to
00:29
E=mc2. But we’ll get to that later. And so since energy can neither be created nor destroyed,
00:35
it can be converted. And so the two terms that we generally talk about when we talk
00:39
about storing or using energy are potential and kinetic energy. Now I’m talking about
00:44
potential gravitational energy and kinetic energy. And so we also have potential energy
00:50
for example in the chemical bonds of a molecule, but I’m not talking about that. And so the
00:56
two types of energy that we have are potential energy. And that’s energy due to position.
01:02
And kinetic energy. And that’s energy due to motion. And we have equations for each
01:06
of these. Potential energy is mgh, where m is mass, g is gravitational acceleration and
01:13
h is the height. And then kinetic energy is one-half mv squared, where m is mass and v
01:19
is the velocity of the object. Now the best place to look at how energy is converted from
01:24
potential to kinetic energy is in a pendulum. A pendulum is simply a weight attached to
01:30
a string. And so if I hold a pendulum at one side and don’t let it go it has a certain
01:35
amount of potential energy. When I let it go the pendulum will swing back and forth.
01:40
That energy is converted from potential to kinetic and then back to potential energy.
01:45
And then to kinetic and then potential over and over and over again. And so when that
01:49
ball is sitting at the top it has all potential energy. When it’s at the bottom it’s converted
01:54
all of that energy into energy of motion. And so when it’s half way down we would say
01:59
that it has a combination of potential and kinetic energy. And it’s just converted. Now
02:04
will a pendulum swing forever? No. Because we’re going to lose a little bit of that energy
02:08
in friction, in heat, in sound as it moves. And so eventually that pendulum is going to
02:14
come to a stop. And so let’s do a couple of problems with potential energy and kinetic
02:19
energy. Potential energy remember is measured as mgh, where m is mass, g is gravitational
02:26
acceleration and h is height. And so let’s say for example that I climbed to the top
02:31
of a ten story building. And so first of all we have to know my mass, which is 78 kilograms.
02:38
We have to know the acceleration due to gravity or g which is -9.81meters per second squared.
02:45
And then we have to convert that ten story building into meters. And so a ten story building
02:49
is roughly 32 meters high, or that’s our h value. And so if we simply multiply those
02:56
all together, we get 24,485.76 joules. And if we do significant digits that’s 2.4 x 10
03:05
^4 joules of energy that my body has at the top of a building. And as long as I stay at
03:11
the top of that building I can use that on the way down. I don’t want to jump off the
03:16
top because I don’t think I would be able to make it. The next type of energy is called
03:20
kinetic energy. Energy of kinetics or motion is 1/2mv^2. And so that’s energy due to motion.
03:28
And if I jumped off a ten story building I would convert all of that into kinetic energy
03:32
at the bottom of my fall. But I don’t want to do that. And so let’s do one dealing with
03:35
a baseball. Let’s say I pitch a baseball. And there are two different pitches. When
03:40
I throw a baseball I probably throw it around 20 miles per hour, if I were to throw it.
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I’m not a very good thrower. But a really good major league pitcher will throw it at
03:49
100 miles per hour. And so let’s figure out how much kinetic energy would be in one of
03:53
my throws and then those of a pitcher in the major leagues. First of all we have to figure
03:58
out the mass of the baseball. The mass of a baseball is 0.145 kilograms. And since we’re
04:04
doing kinetic energy, the only other value that we need is the speed. And so if you throw
04:10
a 20 mile per hour pitch, that’s roughly 9.0 meters per second. Remember on all of these
04:16
we always have to convert it to meters, or meters per second excuse me, it if’s a velocity.
04:21
A 100 mile per hour pitch then is roughly 45 meters per second. And so first of all
04:27
let’s figure out how much kinetic energy my pitch would have. A 20 mile per hour pitch.
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We use the equation 1/2mv^2, where m is 0.145 kilograms and v is 9.0 meters per second.
04:41
We then take that times 1/2 and square the velocity and I get, using significant digits,
04:47
5.9 joules of energy. Now let’s try the faster pitch. It’s 100 miles per hour so that is
04:53
45 meters per second. So we’re going to use 1/2mv^2. Our mass remains the same, or it’s
05:00
0.145 kilograms. Except our velocity now is 45 meters per second. If I multiply that across
05:08
using significant digits, I get 150 joules of energy. Again when I pitched it 20 miles
05:14
per hour it was only 5.9 joules. And so even though that pitcher is throwing it 5 times
05:20
as fast, he’s getting roughly 25 times the amount of energy out of that pitch. And that’s
05:26
why if you look at the equation, the velocity being squared is super important to understand
05:30
that. And so you can solve complex problems now that you know the equation for potential
05:34
energy and kinetic energy. For example in class we figured out, based on the speed of
05:40
a sprinter and the mass of the sprinter, you should be able to figure out how high they
05:43
could pole vault if all of that kinetic energy were converted into potential energy at the
05:48
height of that fall. But that’s it. That’s in summary again the ways that we can measure
05:53
energy in joules. And it’s the ability to do work. And remember it’s always converted
05:58
from potential or energy due to position to energy of motion or kinetic energy. I hope
06:04
that’s helpful.
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This post was previously published on YouTube.
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Photo credit: Screenshot from video.