16. Angular Momentum

Conservation of Angular Momentum

# Star collapses

Patrick Ford

346

2

Was this helpful?

Bookmarked

Hey guys, So let's check out this example here. So a very common example of conservation of angular momentum questions is that of a star dying and that's because the star spins around itself and when the star dies or collapses it will change. Not only its mass, it will become lighter but it'll also reduce in size and radius and volume. Okay, so this is a good setup for a conservation of angular momentum because momentum will be conserved. So let's check it out. Um when they start exhaust all of its stellar energy, it dies. That's why it's sad, poor thing. Um at which point a gravitational collapse happens, causing its radius and mass to decrease substantially. So it's just telling you what happens. No actual information there. Our son spins around itself at its equator at the middle point right there every 24.5 days. The time that it takes for you to go around yourself, for you to complete a full revolution of any sort is period. So period of the sun is around itself 24. days. If our somewhere to collapse and shrink 90% mass and 90% in radius. In other words, our new mass, the new mass of the sun, I'm gonna call this m prime um is going to be, it's shrinking 90% in mass, meaning my new mass is 10% of the original mass. Okay, in the new radius is 10% of the original radius. I want to know how long would its new period of rotation take in days. In other words, if it's taken 24 a half days for the center spin around itself, how long would it take for the center spin around itself? Once these changes happen. In other words, what is my T. Final? Right? Think of this as as T. Sun initial. I want to know what is my T. Final. And what I wanna do here is um instead of writing L. Initial equals L. Final. Because we don't have actual numbers here, we just have percentages in terms of drop. This is really a proportional reasoning question. What I'm gonna do is I'm gonna write actually like this I'm gonna say L. Initial equals a constant. I'm gonna expand this. Okay I'm gonna expand this. The gonna be I initial omega initial is a constant eye. So a sun. The sun can be treated as a um as a solid sphere even though it's actually like a huge ball of gas. Um So so treating is a solid sphere is kind of kind of bad but it won't matter as I show in the second. So I have let's just do that for now half M. R squared. Um And then omega um I want not omega but I want period and remember omega is two pi over t. So I'm gonna rewrite this as two pi over tea and it's a constant because this is a proportional reasoning question. Um This number doesn't matter and this number doesn't matter. The only thing that matters are the variables that are changing. So even though it was kind of crappy to um to model the sun as a solid sphere, it doesn't matter because that fraction goes away anyway. So just just write em are square and you're good. It's sort of what I did earlier, what I had like box M r square right omega. So something like that, because the fraction doesn't matter. So here's what's happening um this guy here is decreasing by by 90%. So basically it's being multiplied by 0.1, right? And then this guy here is being multiplied. Imagine that you're putting a 0.1 in front of the M and a 0.1 in front of the are now the r is squared, so if you square 0.1 you get 0.1. So think of it as the left side of the equation here Is being multiplied by a combination of these two numbers, which is 0.001. Basically the left side of the equation becomes 1000 times smaller. Therefore the right side of the equation has to become 1000 times greater. Okay, so the right side of the equation has to become 1000 times greater. So this side here grows by 1000. Now the problem here is it's a little bit complicated because t is in the bottom. So if the whole thing grows by That means that T, which is in the denominator actually goes down by a factor of 1000. Okay if your fraction goes up, your denominator went down and that's how your fraction goes up, right? Imagine for example you have 100 divided by 10, that's 10 Um 100 divided by two. That's 50. Okay. Your entire thing went up because your denominator went down. So if this goes down by a lot, right side right here goes up by a lots. Uh and then the denominator goes down by a lot. So basically your new period is 1000 times smaller than your old period. So I can write T. Final is um one over T. Initial. So it's basically 24.5 days divided by 1000. And then what you wanna do is you want to convert this into ours? Okay Um or or some some measure of that sort. So we're gonna do here that one Day has 24 hours. This canceled his cancels and you multiply some stuff. Um you multiply some stuff and I actually have this in minutes. Okay I actually have this in minutes. So let me change that in two minutes. So it's gonna be um 24 hours times minutes. And when you do this you get that the answer is about 35 Minutes. I did minutes. Because otherwise you end up with like .55 hours and minutes makes more sense. It's easier to make sense out of it. Okay? So imagine this, this the sun takes 24 days to go around itself, and after it collapses and it shrinks significantly. It's gonna spend 1000 times faster, So it's gonna make a full revolution around itself in just 35 minutes. Okay, so that's it for this one. Let me know if you have any questions and let's keep going.

Related Videos

Related Practice

Showing 1 of 12 videos