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• 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