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9. Work & Energy

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Hey guys. So we're gonna be talking a lot about energies in the next couple of videos and works. So in this video, I want to briefly introduce you to the different types of energy that we'll be talking about and also specifically kinetic energy. Let's check this out. So what is energy? You'll be looking through your text books and talking with your professors and you'll notice there's actually not really a good well established definition for energy. The best I can come up with is that energy is just a physical quantity or property that some objects have. Now. Scientists have been, have debated this for years. We don't really know what exactly energy is, but fortunately for us, we do know how it works and that's more important. So let's talk about that. The unit for energy that we use is called the jewel. It's written by the symbol or letter J. And the basic idea is that energy exists in many different forms throughout the universe. We've got things like thermal energy, heat. We've also got light energy. We've got sound, we have electrical, we also have things like kinetic and potential energies. There's many more uh, you know, besides the eight that I've listed here. But the main thing, the main point here is that energy in the universe can't be created or destroyed the way that energy gets transferred and shuffled around the universe. Is it basically just gets transferred between these forms. Let me show you some examples that you're pretty familiar with. You plug a power, you know, you plug a light bulb into a power outlet and you basically are converting electrical energy that's sort of wrapped up and stored inside of the electrons in that power outlet when it gets to the light, the light bulb, it comes out in the form of heat and lights. So you're basically transferring all this electrical energy and then you're using it to generate heat and lights. There's also energy that's associated with motion. So, for example, you push toy box against the spring, you basically stored some energy inside of the spring when you're pushing up against it and then when it gets released, that energy has to get transferred to the speed of the toy. So basically what's happening here is that you are now transferring potential energy that's inside of the spring specifically. It's called elastic potential, which we'll talk about soon. And this is becoming kinetic energy. So, I want to talk about this kinetic energy here, which you'll know you'll notice is associated with an object motion. Kinetic energy, which will write as the symbol K or K. E. Is really just the energy that is due to an object motion specifically, and objects speed. So the idea here, we're just gonna show you the equation for kinetic energy is that it is one half M. V. Squared depends on your mass and your speed. Notice I haven't put any arrows on this formula. So the idea here is that all energy is when we're talking about these energies here are scalar is they are not vectors. So what happens is specifically kinetic energy is always going to be positive and it's never gonna have a direction associated with it. If you're moving to the right and to the left with the same speed, you have the same energy, it doesn't matter the direction. So let's go ahead and take a look at this problem here. We want to calculate the kinetic energy of a five kg box, moving to the right with three m per second in part A. So the idea here is that I have this box like this, it has a massive five and it's moving to the right with a velocity, you're starting a speed of three. So if you want to calculate the kinetic energy we just plug this in one half M. V squared. So this is just one half times the mass, which is five times your speed of three squared. So if you go ahead and work this out, what you're gonna get is you're gonna get um I believe this is 22.5 jewels. Alright, so now let's look at part B, part B. Now you're gonna have the same five kg box except it's moving to the left now with two m per second. So we have this box like this, whoops. So this box like this and now it has a speed to the left. And the speed here is to remember we said the kinetic energy is a scalar, it only just depends on your mass and speed. So we don't actually care about the direction of this speed here. So this kinetic energy one half Mv squared, this is kinetic one half of five times two squared. So we go ahead and work this out. What you're gonna get is 10 jewels. So you have 10 jewels regardless. You know whether you're moving to the left or right, it's always gonna be a positive number that we see. Alright, so that's it for this one guys, let me know if you have any questions.

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Problem

About 50,000 years ago, a meteor crashed into the earth near present-day Flagstaff, Arizona. Some estimates suggest this meteor had a mass of about 1.4×10^{8} kg and released 1×10^{16} J of energy when it slammed into the Earth. Calculate the approximate speed of this meteor before impact.

A

8,450 m/s

B

12,000 m/s

C

1.43×10^{8} m/s

Additional resources for Intro to Energy & Kinetic Energy

PRACTICE PROBLEMS AND ACTIVITIES (3)

- (c) Is it reasonable that a 30-kg child could run fast enough to have 100 J of kinetic energy?
- A mother has four times the mass of her young son. Both are running with the same kinetic energy. What is the ...
- At what speed does a 1000 kg compact car have the same kinetic energy as a 20,000 kg truck going 25 km/h?

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